News

Culture

Essential Pesticide Readings for Beginners (一)

1. Pesticide and its history

1.1 Definition of pesticides

It is very hard to give a fully satisfactory definition for pesticide because of fast development of pesticides.

As we know, FAO (Food and Agriculture Organization) and WHO (World Health Organization) are the most professional and authoritative international organizations in pesticide management. Therefore hereinafter the latest definition of pesticide given by FAO/WHO is adopted:

Pesticide means any substance, or mixture of substances of chemical or biological ingredients intended for repelling, destroying or controlling any pest, or regulating plant growth (Source: The International Code of Conductton Pesticide Management, FAO/WHO, 2014).

1.2 History of pesticide

Pesticides are by no means a new invention. The first recorded intentional use of a pesticide dates back to 2500 BC when the Sumerians (苏美尔人) rubbed foul-smelling(臭味的) sulfur compounds on their bodies to control insects and mites in belief that the stench would repel the pests. Ancient Egyptians also experimented with pesticides. The Ebers’ Papyrus (埃伯斯氏古医籍), the oldest known medical document (dated around 1550 BC) describes over 800 recipes, many containing recognizable substances, that were used as poisons and pesticides.

The following is a chronological list of selected significant events in pesticide history:

· 12000 BC: First records of insects in human society.

· 2000 BC: First reported use of sulfur as a pesticide by pre-Roman civilizations.

· 1200 BC: First reports of nonselective herbicide use as biblical armies salt and ash the fields of the conquered.

· 100 BC: The Romans apply hellebore for control of rats, mice, and insects.

· 300: Earliest recording of biological control – Chinese use predatory ants in citrus for control of destructive insects.

· 900: Chinese use arsenic to control garden insects.

· 1649: Rotenone used to paralyze fish in South America.

· 1690: Nicotine extracted from tobacco for insecticide use.

· 1787: Soap mentioned as an insecticide.

· 1848: Rotenone used as an insecticide in Asia.

· 1850s: Lime and copper mixture used for plant disease control on grape in France.

· 1860s: Paris green, an arsenical, used as an insecticide for control of Colorado potato beetle.

· 1873: DDT first made in the laboratory.

· 1882: Bordeaux mixture discovered in France for control of plant diseases.

· 1883: John Bean invents pressure sprayer for pesticide application leading to efficient applications to crops.

· 1886: Hydrogen cyanide fumigant use in California citrus.

· 1892: Lead arsenate discovered for gypsy moth control in Massachusetts.

· 1894–1900: Steam-, mechanical-, and horse-driven pesticide spray equipment developed.

· 1907–1911: Industry begins production of lead arsenate.

· 1910: Passage of Federal Insecticide Act (precursor to today's Federal Insecticide, Fungicide, and Rodenticide Act).

· 1921: First use of airplane to apply a pesticide.

· 1927: Tolerance established for arsenic on apples by U.S. Food and Drug Administration.

· 1932: Methyl bromide first used as a fumigant in France.

· 1932–1939: Insecticidal properties of DDT studied and described in Switzerland.

· 1936: Pentachlorophenol introduced as a wood preservative.

· 1942: DDT made available for U.S. military use (civilian use available in 1945).

· 1942: Herbicidal properties of phenoxy acetic acids described, including 2,4-D.

· 1944: Introduction of warfarin for rodent control

· 1946: Organophosphates insecticides, developed in Germany, made available in United States.

· 1950s–1960s: Massive industrial research, development, and commercialization of multiple classes and families of pesticides.

· 1961: Bacillus thuringiensis first registered.

· 1962: Publication of Silent Spring by Dr. Rachel Carson.

· 1965: Atrazine registered as an herbicide.

· 1970: Formation of the U.S. Environmental Protection Agency (responsible for pesticide registration).

· 1971: Herbicidal properties of glyphosate described.

· 1972: DDT uses cancelled by the EPA.

· 1973: Development of first photo-stable synthetic pyrethroid insecticide, permethrin.

· 1978: EPA releases first list of restricted-use pesticides.

· 1980s: EPA cancels many uses of chlorinated hydrocarbon pesticides.

· 1996: Monsanto introduces Roundup Ready® soybeans, the first transgenic crop with major market prospects.

· 1996: Food Quality Protection Act becomes law.

· 1990s and 2000s: Mergers and buyouts in the pesticide industry. 

2. Types of Pesticides

Pesticide active ingredients are described by the types of pests they control or how they work. People often use the term "pesticide" to refer only to insecticides, but it actually applies to all the substances used to control pests.

The most well-known pesticides include insecticides, herbicides, fungicides, plant growth regulators, and rodenticides, etc.

The following is a more complete list of pesticides, which will help you understand the wide range of types of pesticides:

Table 1  Classification of insecticides

Pesticide

Target Pest / Function

Acaricide

Mites, ticks

Algaecide

Algae

Anticoagulant

Rodents

Attractant

Attracts insects or birds

Avicide

Birds

Bactericide

Bacteria

Defoliant

Plant leaves

Desiccant

Disrupts water balance in arthropods

Fungicide

Fungi

Growth regulator

Regulates insect and plant growth

Herbicide

Weeds

Insecticide

Insects

Miticide

Mites

Molluscicide

Snails, slugs

Nematicide

Nematodes

Piscicide

Fish

Predacide

Vertebrate predators

Repellent

Repels vertebrates or arthropods

Rodenticide

Rodents

Silvicide

Woody vegetation

3. Insecticides/acaricides

The most important chemical groups of insecticides and their mode of action are summarized in appendix 1.

So far, the most commonly used insecticides are the chemical groups of organophosphate, carbamate, and pyrethroid.

Organophosphates Although a few organophosphate (OP) formulations remain available for vector control, their use has dramatically decreased because of resistance to OPs, the potential for non-target effects, and the development of alternative products. Members of this group contain phosphorous in their molecules. Products currently labeled for vector control include naled, malathion, and some formulations of dursban. Organophosphates are considered by most to pose a greater human health risk for pesticide applicators than other families of pesticides.

Carbamates are chemically similar in structure to organophosphates, but whereas OPs are derivatives of phosphoric acid, carbamates are derivatives of carbamic acid. Pesticides in this group used for vector control in California include carbaryl (Sevin®) for dusting rodent burrows to control fleas, propoxur (Baygon®) for use against insect pests, and certain brands of bee and wasp control sprays. Carbamates also pose a relatively high risk for human poisoning. Some carbamates are herbicides. 

Pyrethroids are synthetically produced molecules that are chemically similar to pyrethrins. Pyrethroids are not persistent. At rates applied for vector control, they break down quickly in sunlight, and are rarely present after just a few days. The mode of action of pyrethroids is the same as that of pyrethrins. Most pyrethroids are also synergized with PBO. Several generations of pyrethroids have been produced, with the latest formulations being effective at extremely small doses. Some of these new compounds may not break down as readily in sunlight as do pyrethrins, and in some cases pyrethroid synergists may not markedly improve their effectiveness.

For better understanding insecticides, we also need to know how insecticides enter the target organism or where they act. Insecticides can enter the body of insects through one or two or more of the following ways (parts of insect body):

(1) Stomach (stomach toxicants)  In some cases, an insect will feed on a treated leaf surface. The insect ingests the insecticide and absorbs it through the stomach lining. In this case, the insecticide is able to attack the site of action more quickly than when the insect simply walks across the treated surface. Ingestion usually is more toxic to the insect than direct contact, so an ingested insecticide will induce a more severe response than the same amount of material an insect encounters through direct contact, but there are exceptions.  For example, trichlorfon can kill insects because it is degraded into more toxic DDVP (dichlorvos) in the gut of insects. Dimethoate is oxidized into more toxic omethoate. Insecticidal crystal proteins of Btk or Bti is hydrolyzed into smaller molecules which are called endo-toxins and toxic to insects. Ants, cockroaches, and other pest insects with chewing mouthparts can be controlled by incorporation of insecticides into baits of various types.

(2) Body surface (contact toxicants)  How insects encounter insecticides Insects may encounter insecticides in several ways. Perhaps the most common way is by direct contact. In this case, insecticide residues remain on the surface of the plant you have treated. The insect comes in contact with the material as it walks across the treated surface. The insecticide enters the insect through its feet and then makes its way to the site of action (for example, nerve cells or hormone sites). If the insect is present at the time you apply the insecticide, the spray also may cover the insect and penetrate its body directly. Most adult mosquito control products are contact toxicants.

Often, an insect will experience both contact and ingestion, thereby getting a double exposure to the insecticide. For example, sod webworms or cutworms usually come in contact with insecticides as they move from the thatch to the surface to feed and also consume some of the treated turfgrass. The combined effect of contact and ingestion proves difficult for the insect to overcome.

Target insects walk across or feed on the plant material to which you have applied the insecticide. Insecticides that work in this manner are contact insecticides. They remain where you applied them and do not move on or inside the plant. Most traditional insecticides are primarily contact materials,for example the pyrethroid insecticides, some organophosphate and carbamate insecticides have both contact and stomach toxicity to insects.

3Through breathing apparatus ( fumigants)

Some insecticides change to a vapor quite readily. These materials, fumigants, enter the insect's breathing apparatus. These kinds of products are useful in enclosed areas where the vapors can remain concentrated, such as greenhouses or storage bins, but usually do not work well in open landscapes. However, some insecticides may create a bit of fumigant activity at the time an insect is moving across the treated surface.

(4) Systemic Toxicants

They are absorbed by plants, pets, or livestock and are disseminated throughout the organism via the vascular system. When an insect pest feeds on the plant, they ingest the toxicant. Some toxicants are quickly lethal to the pest; others work to prevent the pest from maturing.

A few insecticides have systemic qualities. This means the plant absorbs the material, which then translocates (moves via the vascular system) to other parts of the plant.

Some products translocate upward. In this case, material taken up by the roots can move up into above-ground parts. Other materials translocate downward; pesticide entering the leaves moves to lower regions of the plant.

Most of the organophoshates, carbamates, nereistoxin analogue insecticides (bensulatap, cartap, thiocyclam, and thiosultap, etc.), and neonicotinoid insecticides (clothianidin, dinotefuran, imidacloprid, imidaclothiz, thiamethoxam, nitenpyram, nithiazine, acetamiprid, imidacloprid, nitenpyram, paichongding, and thiacloprid, etc.) are systemic insecticides.

For contact insecticides, they must be sprayed onto crop leaves or surfaces uniformly and cover all parts of the crop plants or surfaces. Systemic insecticides do not need to do so.

When use insecticides you need to bear in mind the following tips:

· Using more than one insecticide product in the same location can increase or decrease each one's effectiveness. It may also result in a greater risk to healthand/or the environment.

· Broad-spectrum insecticides are effective against all insects, even the good ones. Other insecticides target certain insects. Using a targeted insecticide minimizes the risk to beneficial or non-target insects.

· Some insecticides work immediately to kill insects while others may need some time to take effect.

· Insect growth regulators like pyriproxyfen and methoprene do not kill insects; they make it impossible for exposed insects to molt (grow) or lay eggs properly.

· Insecticidal baits can be used instead of spraying large areas, especially for social insects like ants. This can decrease the risk of exposure, but remember to place baits where children and pets won't have access.

4. Fungicides

Definition of Fungicide: a chemical or physical agent that kills or inhibits the growth of fungi. Fungicides can be classified a number of different ways, including (1) mobility in the plant, (2) role in protection of plants, (3) breadth of activity, (4) mode of action, and (5) chemical group.

4.1 Classified by Mobility in the Plant

(1) Protectant (or Contact) Fungicides

Protectant fungicides are active on plant surfaces where they form a chemical barrier between the plant and fungus. There is no movement of the fungicide into the plant. Protectant fungicides must be applied prior to infection and re-applied to new growth if conditions remain favorable for disease development.

Many protectants are potentially phytotoxic (toxic to plants) and can damage the plant if absorbed. Repeated applications are needed to protect new growth of the plant and to replace material that has been washed off by rain or irrigation, or degraded by environmental factors such as sunlight.

Protectant (contact) fungicides, such as the inorganics (copper) and sulfur, the dithiocarbamates (maneb, mancozeb, thiram, ziram) and chloronitriles (chlorothalonil), have low chance for fungicide resistance to develop.

In all cases, a protectant fungicide’s chemistry disrupts fungal growth and development, either non-specifically or in multiple manners. Because of this, there is a much lower chance for fungi to develop resistance to them. Protectant fungicides are contact fungicides, meaning they must be present on the leaf surface prior to the arrival of the fungus and must then come into direct contact with the fungus.

(2) Systemic Fungicides

Systemic fungicides are absorbed and translocated in the plant. They serve to prevent the development of disease at the site of uptake as well as in other plant regions. Translocation is simply a term used to describe the movement of any compound within the plant from the site of application to distant tissues.

Local penetrant (also known as local systemic) fungicides are absorbed into the immediate area of application but are not translocated far from the site of uptake. They serve to prevent the development of disease at (and in a small zone surrounding) the site of uptake. The term local systemic is often used but is not the best description of these fungicides.

You may also see the terms curative and eradicant used to further describe certain systemic and local penetrant fungicides. Curative or eradicative fungicides have the unique ability to stop the progress of infections that may have occurred a few hours or days before the application. Table 2 gives examples of systemic fungicides. Table 3 is comparison of protectants and systemic fungicides.

Table 2  Examples of Systemic Fungicides

Group Name

Common Name

Mode of Action

Phenylamide

Metalaxyl,

Mefenoxam

Nucleic acid synthesis

Strobilurin

(Quinone outside Inhibitor (QoI))

Azoxystrobin

Kresoxim-methyl Pyraclostrobin   Trifloxystrobin   

Respiration

Antibiotic

Streptomycin Oxytetracycline

Amino acids and proteins

Quinoline

Quinoxyfen

Signaling

Demethylation

Inhibitor (DMI)

Fenarimol  Myclobutanil  Fenbuconazole Tebuconazole, Triflumizole

Sterol synthesis

Phosphonate

Fosetyl-AL, phosphorous acid

Unknown

 

 

 

Table 3  Comparison of Protectant and Systemic Fungicides.

Contact Fungicide

Systemic Fungicide

Protective

•Not translocated

New growth is not protected

Typically for foliar diseases only

•Broad spectrum

•Little possibility of resistance developing

Typically not used for root pathogens

•Protective & curative

Translocated

New growth is protected

For foliar and root diseases

•Specific mode of action

•More possibility of resistance developing

Effective on root pathogens

4.2 Fungicides classified by chemical groups

Roughly, fungicides can be divided into inorganic and organic groups based on their chemical nature. Of which, the organic fungicides are the synthesized modern fungicides which are summarized in appendix 2 with examples and information on mode of action of each group of fungicides. Inorganic fungicides together organic fungicides and their characteristics (uses) are summarized in appendix 3. For readers’ convenience, appendix 3 also summarizes the characteristics of major groups of fungicideswhich help understand the features of each groups of fungicides. Some fungicides are mainly protectants, some are both protectant and systemic, and some may be only systemic.

4.4 Fungicides classified by the nature of their use

(1) Seed protectants: Captan, thiram, organomercuries, carbendazim, carboxin etc. 

(2) Soil fungicides (preplant): Bordeaux mixture, copper oxy chloride, Chloropicrin, Formaldehyde Vapam, etc.

(3) Soil fungicides: Bordeaux mixture, copper oxychloride, Capton, PCNB, thiram etc.

(4) Foliage and blossom: Capton, ferbam, zineb, mancozeb, chlorothalonil etc.

(5) Fruit protectants: Captan, maneb, carbendazim, mancozeb, etc.

(6) Eradicants: Organomercurials, lime sulphur, etc.

(7) Tree wound dressers: Boreaux paste, chaubattia paste, etc.

 

5. Herbicides

5.1 Herbicide classification

In terms of chemical nature, herbicides can be classified into many types as shown in appendix 4.

Herbicides are also classified as to their selectivity. Some herbicides are designed to control a broad range of weeds, while others are designed to control only selected types of weeds.

1Selective Herbicides

Selective herbicides can be used to control certain plant species without injuring others. This characteristic can be used to control weeds while avoiding harm to desirable plants. Other selective herbicides may affect foliage of plants while leaving plant roots unaffected. Still other examples of selective products are herbicides that can be applied to soil or water before (pre-emergence) or after (post-emergence) the active growing season of plants. Some of these products can control growth of all plants, others affect only certain species. Perhaps the most common selective herbicides are those that affect broad-leaf plants, but not grasses. This selectivity is based almost entirely on the shape and size of the target foliage. In these cases, some herbicide formulations will wet broad-leaved plants but run off on grasses.

The application of selective herbicides may kill only the parts of the plant actually sprayed. In this case they are considered contact herbicides. Complete weed kill using contact herbicides requires well-directed and properly applied sprays. Complete coverage of the weed is a must.

Some herbicides are applied to the leaves of plants and absorbed into their stems and roots (translocated) causing the death of the entire plant. Because species of plants vary in their susceptibility to these systemic herbicides, these herbicides are selective to some degree. Examples of selective herbicides are 2,4-D, dicamba, and picloram. Pre-emergent herbicides with selective properties are atrazine, trifluralin, and oryzalin.

2Non-selective Herbicides

Some herbicides are non-selective and must be used with extreme caution. They are used primarily in situations where complete removal of vegetation is desired, such as on transportation rights-of-ways. Some commonly used nonselective herbicides include glyphosate, imazapyr, bromacil and paraquat.

Non-selective herbicides can be applied to foliage as contact herbicides or as translocated herbicides. They may also be applied to soils where they kill nearly all plants growing there. Some soil-applied herbicides are available as fumigants.

Even selective herbicides can damage desirable plants if not used in strict compliance with their labels. In this regard, accurate dilution and calibration of equipment is critical. Since the distinction between weeds and non-weeds is so subjective, designing sound weed control strategies requires considerable knowledge and planning.

Extra care must be taken in applying non-selective pesticides. Do not apply them in sloping areas or where soil may be taken for use in a different location.

When crops are treated with herbicides, some herbicides can be absorbed by crops and translocated into other parts of crops, some are not be translocated. Therefore the latter are called contact herbicides. The herbiicides that can be translocated by crops are called systemic herbicides.

Table 4 as below summarizes the two commonly used terms with regard to selectivity and gives an example of each category.

Table 4  Selectivity of Herbicides

Term

Definition

Example

Selective

Herbicide formulated to control specific weeds or weed categories. A material that is toxic to some plant species but less toxic to others.

2,4-D

(Selectively toxic to broadleaf weeds)

Nonselective

(a.k.a. Broad spectrum)

Herbicide formulated to control both broadleaf and grass weeds.

Paraquat, Glyphosate

According to the ability of herbicide absorption and /or transportation in crop plants, herbicides can also be classified into contact and systemic.

1Contact Herbicides

Contact herbicides are applied directly to the plant, and may affect only the part of the plant contacted. This type herbicide can be used for preventing growth of brush and tree limbs into pathways. Bromoxynil, paraquat, and diquat are examples of contact herbicides.

2Systemic Herbicides

Herbicides that move from one part of the plant to another such as from the leaf to the roots are called systemic. These formulations are particularly useful for control of deep rooted perennial vegetation.

Systemic herbicides may enter the plant through the roots or the leaves then move via the plant's vascular system to affect the entire plant. Commonly used systemic herbicides applied to plant foliage include MSMA, glyphosate, dichloprop, 2,4-D, dicamba, picloram, and chlorsulfuron.

Simazine, diuron, pronamide, and EPTC are examples of soil applied systemic herbicides. Some herbicides including triazines and thiocarbamates will translocate through both processes; however, they primarily work through root uptake which is the recommended method of application.

5.2 Herbicide application method

There are several techniques that can be used to apply herbicides. Some of the most common are outlined below.

(1) Foliar spraying

Here, the herbicide is diluted with water or another diluent as specified on the product label, and sprayed over the foliage to point of runoff (until every leaf is wetted, but not dripping).

The method is most suited to shrubs, grasses and dense vines less than 6 m tall so that complete coverage is achieved. Advantages include speed and economy. Disadvantages include the potential for spray drift and off-target damage.

Foliar spraying can be done in a number of ways, depending on the size of the weed plant or the infestation. Blanket spraying, using a boom spray from a tractor or aircraft, can be used to treat areas completely infested with weeds, especially with selective herbicides.

For large infestations that need targeted applications of herbicide, a hose and handgun can be used to spray solution from a herbicide tank and pump carried by a tractor or vehicle. Smaller infestations can be sprayed using a backpack/knapsack spray unit. Spot spraying is used to treat individual weed plants or areas that only have small clumps of weed infestations.

(2) Basal bark spraying

This method involves mixing an oil soluble herbicide with a diluent recommended by the herbicide manufacturer and spraying the full circumference of the trunk or stem of the plant. Basal bark spraying is suitable for thin-barked woody weeds and undesirable trees.

Basal bark spraying is also an effective way to treat saplings, regrowth and multi-stemmed shrubs and trees. This method works by allowing the herbicide to enter underground storage organs and slowly kill the targeted weed.

The whole circumference of the stem or trunk should be sprayed or painted with herbicide solution from ground level to a height of 30 cm. It is important to saturate the full circumference of the trunk, and to treat every stem or trunk arising from the ground.

Basal bark spraying is a very effective control method and is a good way to tackle inaccessible areas such as steep banks. This method will usually kill difficult-to-kill weeds at any time of the year, as long as the bark is not wet or too thick for the solution to penetrate. The work is often best performed by contractors.

(3) Stem injection

Stem injection involves drilling or cutting through the bark into the sapwood tissue in the trunks of woody weeds and trees. Herbicide is immediately placed into the hole or cut. The aim is to reach the sapwood layer just under the bark (the cambium growth layer), which will transport the chemical throughout the plant.

It is essential to apply the herbicide immediately (within 15 seconds of drilling the hole or cutting the trunk), as stem injection relies on the active uptake and growth of the plant to move the chemical through its tissue.

Stem injection methods kill the tree or shrub where it stands, and only trees and shrubs that can be safely left to die and rot should be treated this way. If the tree or shrub is to be felled, allow it to die completely before felling. The use of chainsaws, particularly in the felling of trees, is a dangerous activity that should only be undertaken by an appropriately trained person.

One method is the 'drill and fill method' also referred to as tree injection, and is used for trees and woody weeds with stems or trunks greater than 5 cm in circumference. A battery-powered drill is used to drill downward-angled holes into the sapwood about 5 cm apart. The placement of herbicide into the hole is usually made using a backpack reservoir and syringe that can deliver measured doses of herbicide solution.

Another method is the 'axe cut method' which involves cutting through the bark into the sapwood tissue in the trunk, and immediately placing herbicide into the cut. This method can be used for trees and woody weeds with stems or trunks greater than 5 cm in circumference. Using an axe or tomahawk, cuts are made into the sapwood around the circumference of the trunk at waist height. While still in the cut, the axe or tomahawk is leaned out to make a downward angled pocket which will allow herbicide to pool. The herbicide is then immediately injected into the pocket. Cuts should be made no further than 3 cm apart. This method of using an axe to make the cut is often referred to as frilling or chipping. A hammer and chisel can be used to make the pocket cuts, or a spear to make cuts in the trunk closer to ground. It is important not to entirely ringbark the trunk, as this will decrease the uptake of the herbicide into the plant.

(4) Cut stump application

Here, the plant is cut off completely at its base (no higher than 15 cm from the ground) using a chainsaw, axe, brush cutter or machete (depending on the thickness of the stem/trunk). A herbicide solution is then sprayed or painted onto the exposed surface of the cut stump emerging from the ground, with the objective of killing the stump and the root system.

It is imperative that the herbicide solutions are applied as soon as the trunk or stem is cut. Refer to the product label instructions for information on timing, as delayed application will give poor results.

Two operators working as a team can use this method effectively. The herbicide can be applied from a knapsack, or with a paint brush, drench gun or a hand-spray bottle. It is a good idea to use a brightly coloured dye in the solution to mark the stumps that have been treated.

For trees with large circumferences, it is only necessary to place the solution around the edge of the stump (as the objective is again to target the cambium layer inside the bark). The stump circumference should be bruised with the back of an axe and each successive blow treated with herbicide.

This method has the appeal of removing the weed immediately, and is used mainly for trees and woody weeds. This method is also referred to as cut and spray or cut and paint.

(5) Cut and swab

This method is similar to the cut stump method, but is suited to vines and multi-stemmed shrubs. Here, the plant stems are cut through completely, close to the ground. Herbicide is then applied immediately to the cut surface emerging from the ground, via spray or brush application.

In the case of Madeira Vine and some other vines with aerial tubers, both ends of the cut stems must be treated with herbicide. An effective way of doing this is to hold both 'bunches' of cut stems in a container of herbicide for 15 seconds after cutting, so that maximum translocation occurs to both ground and aerial tubers. Extra care should be taken when doing this to ensure spillages do not occur.

(6) Stem scrape

Stem scraping is used for plants and vines with aerial tubers. A sharp knife is used to scrape a very thin layer of bark from a 10 cm section of stem. Herbicide is then immediately applied to the exposed soft underlying green tissue.

This method is also called bark stripping or stem painting. Some woody weeds can have their bark surface peeled away and the exposed wood painted or sprayed with herbicide.

(7) Wick application

This method of applying herbicide consists of a wick or rope soaked in herbicide from a reservoir attached to a handle or assisted with 12 volt pump equipment. The wetted wick is used to wipe or brush herbicide over the weed.

For obtaining ideal efficacy, the following factors must be considered when apply herbicides.

5.3 Factors affecting herbicide application

(1) Factors affecting foliar applied herbicides

There are many factors that affect the results of foliar herbicide applications. Some of these are:

l The age of the plants treated.

l The season of the year of applications.

l The life cycle stage of the plants (budding, flowering, overwintering, etc.).

l The type of life cycle of the plant (annual, biannual, or perennial).

l The degree of maturity of the plant.

l The time of day of the application.

l Weather conditions at the time of application.

l The life form of the plant treated (woody, succulent, broad-leaved, grassy, etc.).

l The morphology of the plant treated (cuticle thickness, presence of leaf hairs, etc.).

(2) Factors affecting soil applied herbicides

Soil characteristics

The physical and chemical characteristics of the soil as well as the climatic conditions will determine the effectiveness of a soil applied herbicide, the persistence of the herbicide in the soil, and the potential movement of the herbicide through the soil (leachability).

Both soils and herbicides vary in their polarity of their constituent particles. Both can be negatively or positively charged or have a neutral charge. This will affect the movement of herbicides thorough soil and also the persistence of the herbicides applied.

Soil texture also will influence movement and persistence of herbicides. Light soil types (sands and sandy loams) tend to have large pore openings between the particles that allow water to move down through the soil profile rapidly. This will promote the more rapid movement of herbicides through these soils, but more rapid leaching, and thus lower persistence.

Herbicides applied to heavy soils (clay loams and clays) behave in the opposite manner. They move slowly through these soils and tend to remain longer. Medium texture soils (loams and silt loams) respond in an intermediate fashion to herbicides applied.

Before application of an herbicide to a soil a pesticide technician should know the characteristics of the soil to be treated. This can be determined by a soil test. A local county extension office or a Natural Resources Conservation Service (NRCS) Office can furnish information on collection of soil samples for testing. Herbicide labels have recommended rates of application based on the soil texture. The texture of soil basically is determined by the percentage of sand, silt, clay, and organic material in it. Generally, heavier soils require higher amounts of herbicide for plant control than lighter soils.

Other factors that can affect herbicide applications to soils include that amount of organic matter in the soil, the degree of compaction of the soil, the moisture content of the soil, and whether an underlying hard-pan is present below the soil surface.

Herbicide persistence

Other factors beyond those already discussed that affect herbicide persistence include the rate of application, soil temperature, exposure to sunlight, microbial and chemical decomposition, solubility of an herbicide, and precipitation. These factors also affect how fast an herbicide will be degraded, and how deep it will leach through the soil.

(3) Factors to consider in planning herbicide applications

When choosing an herbicide to use for weed control, consider the vegetation that is close to the application site. Take precautions to avoid movement of herbicides into surrounding areas, especially if valuable vegetation is nearby.

Herbicide applications should be avoided when it is raining, or in areas where overland water flow is likely to occur. Applications should likewise be avoided in heavy winds. The danger of drift in high wind conditions is especially high in open areas with little protection from wind.

Volatile herbicides such as the 2,4-D ester formulation and dicamba will vaporize during hot summer days, and danger of herbicide drift will be greater under these circumstances. Danger from volatilization will be included on the pesticide label.

(4) Application timing

Different herbicides have different mode of action, therefore the application timing is different for obtaining effective control of weeds, causing no phytotoxicity to crops. Some herbicides must be applied before planting crops, some after planting but before emergence, some post emergence, etc. Table 5 gives examples of different herbicides which are applied at different timing.

Table 5  Herbicide Application Timing

Term

Definition

Example

Preplant

Herbicides that are applied before planting the crop - typically from several days to just before planting

EPTC

Glyphosate

Preemergence

Herbicides applied anytime before the weed seedlings emerge through the soil surface

Simazine

Postemergence

Herbicides applied after the crop seedlings (or weed seedlings) have emerged through the soil surface

2,4-DB

Bromoxynil

Established stands

Herbicides applied after the roots systems have developed sufficiently enough to allow selective use.

Diuron

Terbacil

hexazino

 

6. Rodenticides

Nearly half of all species of mammals are rodents, but only a few are of public health importance. Domestic rats and mice are the primary targets of pesticide applicators in urban and suburban situations, but rodents also are associated with rural diseases such as plague and hantavirus diseases. Control of these kinds of diseases by use of rodenticides is impractical except in unusual circumstances. Other rodents that may present problems for public health agencies are squirrels, gophers, hares, and rabbits. These problems may involve the roles of the vertebrates as disease reservoirs, or may involve activities of rodents such as ground squirrels or gophers in damaging water impoundment dikes used for mosquito control.

Rodenticides may be typically classified as first or second generation. There are a few instances where the product may not fall under either heading. A first generation rodenticide requires higher concentrations (usually between 0.005 and 0.1%) and consecutive intake over multiple days so a lethal dose may bio-accumulate. There are considered less toxic than second generation agents. Second generation Rodenticides are applied in lower concentrations in baits (usually in order 0.001-0.005%) and are lethal after a single ingestion of bait. Second generation rodenticides are also effective against rodents that are resistant to first generation anticoagulants. The second generation anticoagulants are sometimes referred to as "superwarfarins".

The most widely used group of rodenticides is the coumarins. The best-known member of this group is Warfarin, which derives its name from the Wisconsin Alumni Research Foundation, where it was originally developed. Coumarins affect all mammals, including humans, by serving as blood anticoagulants. Coumarins kill rodents over time by two related effects. They inhibit prothrombin formation, thus disrupting clotting, and they also damage capillaries, resulting in internal bleeding.

Warfarin was very successful as a rodenticide when it was first introduced because rodents did not exhibit bait shyness because of the extended period of action of the coumarins. However, physiological resistance to coumarins has been reported in rats in some areas. Some newer coumarins have been developed (e.g., brodifacoum, bromadiolone) that will kill rodents in 4–7 days after a single feeding. These materials can be used where rodents are encountered that are resistant to conventional anticoagulants.

Indandiones is another group of rodenticides. Although indandiones belong to a different chemical class than the coumarins, they also are anticoagulants. Diphacinone, pindone, and chlorophacione belong to this group. Pindone was the first anticoagulant developed, and requires daily feeding to cause rodent death. Diphacinone will cause death after a single feeding. Both of these chemicals may induce bait shyness in rodents. Chlorophacione will also result in rodent death after a single feeding, and unlike most of the anticoagulants, does not cause bait shyness.

Benzenamines are chemicals that are not anticoagulants. The only rodenticide in this group is bromethelin (Vengeance®, Fastrac®, Gladiator®). These materials are particularly effective against Norway rats, roof rats, and house mice. When used with baits, rodents stop all feeding after a single dose, and death occurs shortly thereafter.

Cholecalciferol (Vitamin D3) is the active ingredient in the rodenticides Quintox®, Rampage®, and Muritan®. These materials cause calcification of soft tissues, which can be fatal to rats after extended feeding. Cholcalciferol is used in baits and is tasteless. It is less toxic to humans than most rodenticides, but may poison small pets.

Some rodenticides are extremely dangerous to all mammals, including humans and their pets, and must be used with extreme care by applicators. Compound 1080, sodium fluoroacetate, is one of the most poisonous pesticides known. This material has gained considerable notoriety in connection with coyote control programs. Government predator control programs are now the only permitted use of Compound 1080.

Strychnine is a botanical rodenticide. It is highly toxic to all warm-blooded animals. It is somewhat commonly used for gopher and other underground pest control when pets and people are not present. Elsewhere it is rarely used because of its high toxicity and its relative poor performance as a rodenticide in comparison with anticoagulants.

Because people, rodents, and many domestic animals and pets are closely related genetically, rodenticides have a high potential for accidental poisoning. This danger can be minimized by use of protected bait boxes, and as always, usage in strict compliance with the pesticide label.

7. Plant growth regulators

A Plant Growth Regulator is a compound or a number of compounds used to affect the rate of growth, whether in the root zone, stem region or any other portion of the plant. This is not done with the use of micro or macro nutrients, but with plant hormones which are naturally produced molecules within the plant. These hormones are responsible for the plant’s formation, rate of growth, size of fruits and flowers, etc. The major plant growth regulators are listed in table 6.

 

 

 Table 6  Major plant growth regulators

Type

Explanation

Example

Auxins

The first plant hormone discovered in the 1880s. Auxin is derived from the Greek word “Auxein” which means “to grow”. Charles Dawrin studied them in the late 1800′s. Some of the effects can be: cell elongation and promotion of roots on cuttings.

(1) 1-naphthalenacetic acid (NAA)

(2) 2,4-D

(3) 3-indoleacetaldehyde acid (IAld).

(4) 3-indoleacetic acid (IAA).

(5) 3-indolepyruvic acid (IPA).

(6) indolebutanoic acid (IBA).

Gibberellins (GA)

Also known as GAs. There are 134 known GA’s to date. They are derived from plants – Ascophylluuum.

(1) GA4, GA7(2) GA3.

Cytokinins

Produces a process of cytokinesis or cell division.
Founded in 1913. It is considered a synthetic hormone although it is naturally derived most commonly from corn. Some of the effects could be: 

Larger Leafs due to Cell ElongationOpening of the StomatalPromotion of Rooting of CuttingsEncourages Cell Division.

(1) Forchlorfenuron (CPPU);    (2) kinetin.

Ethylene/Ethylene releasers

Has been in use since the Egyptians used it to ripen figs. It is a gas hormone that helps in the ripening stage when sprayed onto the plant. It is developed from methionine, a natural compound found in all plants and oxygen. Mostly used commercially on tomatoes and peppers. These are some the effects that would be noticed:

Promotes Stem and Root Growth; Helps in Fruit Color; Promotion of Ripening; Opening of Flowers.

(1) ethephon; (2) ethylene.

Inhibitors/Retardants

(1) abscisic acid (ABA; (2) ancymidol; (3) carbaryl; (4) chlormequat;   (5) chloro IPC; (6) daminozide; (7) flurprimidol; (8) hydrogen cyanamide (H2CN2);    (9) mefluidide; (10) mepiquat chloride; (11) paclobutrozol; (12) prohexadione calcium; (13) succinic acid (SADH) .

The major role of plant growth regulators may play in the following aspects: a. abscission; b. dormancy; c. fruit abscission; d. fruit ripening; e. fruit set; f. leaf expansion [ethylene]; g. plant senescence; h. root initiation; i. seed germination;     j. stem elongation. 

8. Biopesticides (biocontrol agents)

However, USEPA classifies biopesticides into the following three categories.

(1) Microbial pesticides consist of a microorganism (e.g., a bacterium, fungus, virus or protozoan) as the active ingredient. Microbial pesticides can control many different kinds of pests, although each separate active ingredient is relatively specific for its target pest[s]. For example, there are fungi that control certain weeds, and other fungi that kill specific insects.

(2) Plant-Incorporated-Protectants (PIPs) are pesticidal substances that plants produce from genetic material that has been added to the plant. For example, scientists can take the gene for the Bt pesticidal protein, and introduce the gene into the plant's own genetic material. Then the plant, instead of the Bt bacterium, manufactures the substance that destroys the pest.

(3) Biochemical pesticides are naturally occurring substances that control pests by non-toxic mechanisms. Conventional pesticides, by contrast, are generally synthetic materials that directly kill or inactivate the pest. Biochemical pesticides include substances, such as insect sex pheromones, which interfere with mating, as well as various scented plant extracts that attract insect pests to traps. Because it is sometimes difficult to determine whether a substance meets the criteria for classification as a biochemical pesticide, EPA has established a special committee to make such decisions.

Biopesticides play an important role in providing pest management tools in areas where pesticide resistance, niche markets and environmental concerns limit the use of conventional chemical pesticide products.

According to BPIA (Biopesticide Industry Alliance), USA, biopesticides can be classified into two main categories based on their sources or natures: Microbial Biopesticides (bacteria, fungi, protozoa, viruses, yeast) and Biochemical Biopesticides Introduction (plant growth regulators, insect growth regulators, organic acids, plant extracts, pheromones, minerals/other).

Plant-Incorporated-Protectants (PIPs) are not included in BPIA’s categories of biopesticides may be because that are only controlled by a few multi-national companies, e.g. Syngenta, Monsanto, Bayer, etc.

Biopesticides can also be classified into insecticides, herbicides, fungicides, and nematicides, etc., based on their uses or functions.

(1) Insecticides

The common used bio-insecticides are produced based different types of microbials which are shown in table 7. It should be pointed out that plant extracts that directly kill insects are not considered as bio-insecticides by EPA. Also in Australia, the products based on plant extracts which have known chemical identity are treated as chemical pesticides when apply registration.

Table 7  Microbial insecticides

Type of microbial

Examples of microbial used as bio-insecticides

Bacteria

Bacillus thuringiensis, B. sphaericus, Paenibacillus popilliae, Serratia entomophila

Viruses

Nuclear polyhedrosis viruses, granulosis viruses, non-occluded baculoviruses

Fungi

Beauveria spp, Metarhizium, Entomophaga, Zoopthora, Paecilomyces fumosoroseus, Normuraea, Lecanicillium lecanii

Protozoa

Nosema, Thelohania, Vairimorpha

Entomopathogenic nematodes

Steinernema spp, Heterorhabditis spp

Others

Pheromones, parasitoids, predators, microbial by-products

(2) Herbicides

The following fungi and bacteria are used as herbicides.

Fungi- Colletotrichum gloeosporioides, Chondrostereum purpureum, Cylindrobasidium laeve

Bacteria - Xanthomonas campestris pv. Poannua

(3) Fungicides

The following are examples of fungicides based on microbials.

Fungi - Ampelomyces quisqualis, Candida spp., Clonostachys rosea f. catenulate, Coniothyrium minitans, Pseudozyma flocculosa, Trichoderma spp

Competitive and Soil Inoculants - Bacillus pumilus, B. subtilis, Pseudomonas spp, Streptomyces griseoviridis, Burkholderia cepacia

(4) Nematicides etc.

Nematode Trapping Fungi - Myrothecium verrucaria, Paecilomyces lilacinus.

Bacteria - Bacillus firmus, Pasteuria penetrans.

Mollusc parasitic nematode - Phasmarhabditis hermaphrodita.

Currently the most widely used biopesticide is Bacillus thuringiensis (Bt) which is an insecticide with unusual properties that make it useful for pest control in certain situations. Bt is a naturally occurring bacterium common in soils throughout the world. Several strains can infect and kill insects. Because of this property, Bt has been developed for insect control. The target insect species are determined by whether the particular Bt produces a protein that can bind to a larval gut receptor, thereby causing the insect larvae to starve. The insecticidal activity of Bt was first discovered in 1911. However, it was not commercially available until the 1950s. In recent years, there has been tremendous renewed interest in Bt. Several new products have been developed, largely because of the safety associated with Bt-based insecticides.