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close this bookManual on the Prevention of Post-harvest Grain Losses (GTZ)
close this folder8. Pest control using insecticides
View the document(introduction...)
View the document8.1 Insecticides
View the document8.2 Application techniques
View the document8.3 Calculating the dosage of insecticides in stored product pest control
View the document8.4 Precautionary measures
View the document8.5 Equipment
View the document8.6 Further literature

8.1 Insecticides

General Principles

· Insecticides are always used as a supplement to hygiene measures and can never substitute them.
· A high level of infestation makes control with insecticides more difficult. Care should therefore be taken to perform control of any pest infestation in time.
· The choice of the correct products is made by taking into account the following aspects:

· insect species present (sensitivity, resistance)
· Method of storage (bags, bulk)
· Climatic conditions (decomposition of the products by moisture and high temperatures)
· Stored commodity
· Legal restrictions (Use only approved products. If the country has no legislation, refer to the FAO/WHO code of conduct)
· Availability and price.

· Insecticides must not be stored for longer than the indicated shelf life. Buy only quantities which will certainly be used within one storage season (also see section 8.4) in order to avoid overstorage and decomposition of the product.
· Prevent the development of resistance by changing the active ingredient (if possible) annually.
· The safety regulations for handling of insecticides must always be followed. Attention must be paid to any warning signs on packages which draw the user's attention to particular dangers in the use of the insecticide concerned (see also section 8.4.2).

8.1.2 Scope of Application in Central Storage

Good store management and high standards of hygiene are the basic requirements for successful pest control.

In central storage, i.e. in warehouses, the following scopes of application for insecticides apply:

Surface treatment of empty warehouses

This is an efficient curative control method to clean up warehouses before intake of new stocks. Attention must be paid to the use of suitable insecticide formulations. On surfaces the activity of EC formulations is generally very poor. WP formulations tend to perform better. All insecticides are more persistent on smooth, concrete surfaces than on rough and alkaline (white washed) ones. In extensive laboratory tests conducted in Germany the active ingredients tetrachlorvinphos, deltamethrin and phoxim showed the best overall performance.

During the last years it has been proven in several tests that residual spraying of surfaces like store walls and floors is not economic because the residual activity of most insecticides is very short under these circumstances. On absorbent surfaces like limewash, whitewash, cement, and bricks the activity of insecticide residues tends to be very short, irrespective of the active ingredient or formulation. Pyrethroids form an exception, being considerably more persistent. For residual surface protection inert dusts seem to offer considerable potential (cf. section 10.2).

It goes without saying that insecticides are much more effective on clean surfaces than on dirty ones. The cleaning of empty stores before any surface spraying should be an indispensable standard practice.

Surface treatment of bag stacks

For a long time it has been common practice to protect bag stacks from becoming infested by means of treatment with long-term contact insecticides. However, this method is unsatisfactory as the pesticides decompose too rapidly. On non-absorbent bag surfaces like polypropylene insecticide activity is better than on the highly absorbent jute bags.

As a conclusion it can be recommended to practice surface spraying of bag stacks only as non-residual application at the time of stack fumigation. Continued surface spraying could multiply resistance because of the repeated exposure of stored product insect populations to sublethal dosages.

It would be more effective to treat the individual layers of bags during stacking. Anyway, this is regarded as being less practicable.

An alternative to surface treatment consists in covering the stack of bags with light cotton or nylon sheets which provide a barrier to crawling or flying insects. Impregnation of the sheets with repellent substances (e.g. neem oil, see section 4.3.2.2) increases the effect of keeping the stored commodity uninfested. However, regular observation of the stack becomes more difficult and the costs must be considered.

Space treatment of warehouses

Space treatment is best done by fogging. It requires a tightly sealable store. This method is particularly suited for curative control of flying pests.

Evaporation strips hanging up in a well closed store serve for preventive moth control.

All kind of treatments in stores must be followed up in order to check the success.

8.1.3 Formulations

The insecticides sold on the market by manufacturers are referred to as commercial products. They contain one or more active ingredients as well as carriers and special additives. The latter improve the adhesion of the active ingredient on the surface treated and the stability, act as synergists or simply colour the insecticide as a warning agent.

Depending on the formulation, the commercial products either have to be mixed with a liquid, which is generally water, to form a spraying mixture or are sold ready for use.

The most common formulations are listed below. Abbreviations are in accordance with the FAO specifications.

- Dust formulations (DP) to be mixed with the stored produce or for surface treatment. They contain between 0.1 and 5% active ingredient and are ready-for-use. They are mainly applied in small farm storage.
- Emulsifiable concentrates (EC) for admixture to the produce or for surface treatment. They contain between I and 100% active ingredient and are mixed with water, giving a stable emulsion, and primarily used in warehouses,
- Wettable powders (WP) for surface treatment. They contain between 10 and 50% active ingredient. They are mixed with water, giving instable suspensions, which have to be constantly stirred as the powder will otherwise settle.
- Flowable concentrates (SC) for surface treatment. These are liquid concentrates which are relatively stable and similar to the EC formulations. They are not yet commonly used in storage pest control.
- Hot fogging concentrates (HN). The so called 'FOG'-formulations contain up to 100% active ingredient. They are either ready-for-use or must be diluted with diesel or kerosene. Some heat resistant EC formulations can also be used for fogging.
- Aerosols as evaporation strips (VP) or in the form of smoke tins (FD) or smoke cartridges (FP) for use against moths. They are ready-for use. Smoke cartridges are also used to a certain extent in small farm and village storage They have a good knock-down effect on adult insects.
- ULV formulations (UL) for surface treatment. They are ready-for-use and applied with special ULV applicators. Their utilisation in storage is very limited.

8.1.4 Requirements for insecticides in Storage

While there is a great number of products against agricultural pests there are only few products available which meet the special requirements of pest control in storage.

Insecticides for stored product protection should meet the following requirements. None of the existing products, however, will entirely fulfil all of them:

· Good effect against most storage pests (broadspectrum effect)
· Long persistence
· Stable under various climatic conditions
· Low toxicity to warm-blooded animals
· Low tendency to create insect resistance
· No harmful residue left in stored produce
· No influence on the smell or taste of the stored produce
· No chemical reaction with the ingredients of the stored produce (proteins, fats, etc.)
· Simple to use
· Low price

It is pan of the user's responsibility to select the correct insecticide meeting most of his specific requirements. The following information is intended as an aid for the right choice.

8.1.5 Groups of Active ingredients in Storage Pest Control

There are two main groups of active ingredients used in stored product protection, organophosphorous compounds and pyrethroids:

Organophosphorous compounds

They are effective against most storage pests, although less against the Bostrichidae (Rhyzopertha dominica, Prostephanus truncates, Dinoderus spp.). Some of these compounds are sensitive to hot and moist conditions. The following products are commonly used:

Active Ingredient

Brand names

Pirimiphos-methyl

Actellic

Fenitrothion

Folithion, Sumithion

Chlorpyrifos-methyl

Reldan

Methacrifos

Damfin

Dichlorvos (DDVP)

Nuvan, Vapona

lodofenphos

Nuvanol

Tetrachlorvinphos

Gardona

Phoxim

Baythion

Malathion

Malathion, Malagrain etc.

Pyrethroids

They are very effective against Bostrichidae, though less against other species of beetles. They also provide a good moth control. The most common are:

Active ingredient

Brand names

Deltamethrin

K-Othrin

Permethrin

Permethrin

Fenvalerate

Sumicidin

Cyfluthrin

Baythroid

Combined products

Combined products, also known as "cocktails", containing an organophosphorous compound and a pyrethroid have been used as broadspectrum insecticides for some years and have performed very well in cases of mixed infestation. The following preparations are commonly found on the market:

Active ingredients

Brand names

Pirimiphos-methyl + Permethrin

Actellic Super

Pirimiphos-methyl + Deltamethrin

K-Othrine Combi

Fenitrothion + Cyfluthrin

Baythroid Combi

Fenitrothion + Fenvalerate

Sumicombi

Other groups of active ingredients

Chlorinated hydrocarbons which were used for a long time in storage pest control are today no longer admissible due to their high persistence and health hazard.

In the group of carbamates, carbaryl ("Sevin") is used to a limited degree in storage pest control. It is quite effective against Rhizopertha dominica.

8.1.6 Choice of insecticide with Respect to the Species and the Properties of the Surfaces to be Treated

· In the case of infestation with beetles other than Bostrichidae organophosphorous compounds should be applied.
· Where Bostrichidae predominate (Rhyzopertha dominica, Prostephanus truncatus Dinoderus spp.) pyrethroids are recommended.
· In the case of mixed infestation with Bostrichidae and other species of beetles, a combined insecticide may be used provided this is permitted by local regulations. Otherwise an organophosphorus insecticide and a pyrethroid may be applied alternately.
· Against moths, the organophosphate dichlorvos (DDVP) gives good control. The most effective application is by fogging, otherwise by surface treatment or by using evaporation strips.
· For the treatment of whitewashed walls (rapid decomposition of most insecticides under alkaline conditions), iodofenphos and tetrachlorvinphos are the most favourable.

The following applies in general for insecticides sprayed on surfaces:

- Their effect is better

- on clean than on dirty surfaces
- on smooth than on rough surfaces

- The persistence is better

- on woods and metals than on concrete or alkaline paint
- with WP formulations than with EC formulations

In the following two tables results of laboratory tests on the persistence of stored product insecticides under tropical climate conditions are presented. These tables provide information for the choice of appropriate insecticides according to climatic conditions. For each pest insect species the maximum period with at least 90% control effect is indicated. Under practical conditions we have to suppose a shorter residual effect.

Effect Or Stored Product insecticides under Arid Conditions (Temperature: 36°C; relative humidity: 50%)


Figure 104

Effect of Stored Product insecticides under Humid Conditions (Temperature 28°C: relative humidity: 75%)


Figure 105


Figure 106

* per 100 kg of grain (according to manufacturers' instructions)

The following pages provide a summary of the most commonly used insecticides in storage pest control and their properties, as well as notes on their application. For each active ingredient, the following data are provided (for definitions refer to sections 8.1.7 and 8.1.8):

· Toxicity, indicated as oral LD50 in mg active ingredient kg of body weight
· Maximum residue limit (MRL) for cereal grains in ppm as recommended by the FAO/WHO Joint Codex Committee on Pesticide Residues
· Recommended dosage rate for admixture to cereals in ppm
· Recommended concentration of spray mixture for surface treatments in %
· Recommendations for space treatments in stores
· General remarks

- Organophosphorous compounds:

Chlorpyrifos-methyl
LD50: 1630 - 2140 mg/kg
MRL: 10 ppm
Dosage rate (admixture to cereals): 10 ppm
Dosage rate (surface treatment): 0.5 - 1%

Remarks:
Insecticide with contact, stomach and vapour action it has a moderate persistence and controls a wide spectrum of stored product pests (except
Rhyzopertha dominica). Resistances have been repotted repeatedly for several insect species.

Dichlorvos (DDVP)
LD50: 56 - 108 mg / kg
MRL: 2 ppm
Dosage rate (admixture to cereals): 2 ppm
Dosage rate (surface treatment): 0.250/D
Space treatments: dilute with diesel to up to 1% (I - 2//1 000 m³); preventive: 1 strip/30m³

Remarks:
Insecticide with a high vapour pressure and strong "knock down" effect. It is efficient against most stored product pests; especially against larval stages within the grain (high penetrating effect) and also against moths. Short residual stability. Recent tests proved DDVP to be potentially carcinogenous, so that restricted use or ban can be expected for the future.

Fenitrothion
LD50: 800 mg / kg
MRL: 10 ppm
Dosage rate (admixture to cereals): 10 ppm
Dosage rate (surface treatment): 0.5%

Remarks:
Fenitrothion has a broad-spectrum effect against all species, though it is not fully effective against Rhyzopertha dominica Good stability for more than 12 months. Suitable for use under traditional storage conditions as dustable powder.

lodofenphos
LD50: 2 100 mg/kg
MRL: no recommendation
Dosage rate (surface treatment): 1 - 2%

Remarks:
lodofenphos is only used for surface treatments. It has a wide range effect against stored product pests, though less effective against Rhyzopertha dominica and Trogoderma granarium. It is often used for pest control on cement surfaces in warehouses as it shows relative good stability under alkaline conditions. The persistence is lower than that of fenitrothion.

Malathion
LD50: 2 800 mg/kg
MRL: 8 ppm
Dosage rate (admixture to cereals): 8 ppm
Dosage rate (surface treatment): 2%

Remarks:
Malathion has been widely used for over 20 years what has led to marked resistances of stored product pests world-wide. In the United States the production has been stopped since 1991, partly in consequence of studies proving that intolerably high residues are very common in foodstuff in countries, where this cheap and generally efficient insecticide has not been intensively applicated in the past and resistances are not usual, it can still be used in small farmers' storage as dustable powder. Malathion has a weaker effect than most other organophosphorous insecticides and degrades comparatively quickly under hot and humid and alkaline conditions.

Methacrifos
LD50: 678 mg/kg
MRL: 10 ppm
Dosage rate (admixture to cereals): 10 ppm
Dosage rate (surface treatment): 0.5%
Space treatments: 5% as fog-solution (1l/1 000 m³)

Remarks:
Methacrifos acts as contact, vapour and stomach poison against all important storage pests and their larval stages within the grain. It is effective against many malathion-resistant insects. It has a pronounced "knock-down" effect and controls also Rhyzopertha dominica. It degrades significantly at high temperature and humidity.

Phoxim
LD50: 1975 mg / kg
MRL: no recommendation
Dosage rate (surface treatment): 0.2% in empty stores
Space treatments: 5% as fog-solution (I - 2//1 000 m³)

Remarks:
Phoxim is mainly used for surface and space treatments in empty warehouses and concrete silos. It has a broad spectrum of activity with stomach and contact action. Phoxim is a short-term insecticide with a "knock-down" effect. It shows cross resistance to malathion-resistant insects.

Pirimiphos-methyl

LD50: 2050 mg/kg
MRL: 10 ppm
Dosage rate (admixture to cereals): 10 ppm
Dosage rate (surface treatment): 0.5%
Space treatments: dilute with diesel to up to 5% (1 - 2 l/1 000 m³)

Remarks:
Pirimiphos-methyl is a fast acting wide range insecticide with contact and vapour action. It has long lasting effect to a wide range of stored product pests, but it is not sufficiently effective against Rhyzopertha dominica. The effect is comparable to that of fenitrothion and chlorpyrifos-methyl, but pirimiphos-methyl appears more potent to malathion-resistant strains. In the last years pirimiphos-methyl-resistances have already been occasionally reported.

Tetrachlorvinphos
LD50: 4 000 tug kg
MRL: no recommendation
Dosage rate (admixture to cereals): 15 ppm
Dosage rate (surface treatment): 1 - 2%

Remarks:
This insecticide has shown to be effective against many species stored product pests. It has a good persistence on alkaline surfaces and is therefore used for structural treatments of warehouses and concrete silos.

- Pyrethroids:

Cyfluthrin
LD50: 500 mg / kg
MRL: 2 ppm (in Australia)
Dosage rate (admixture to cereals): 1 - 2 ppm
Dosage rate (surface treatment): 0.4 - 0.8%

Remarks:
Cyfluthrin provides reliable protection of stored products against crawling and flying insects. It controls insect strains resistant to organophosphorous compounds. It has a long residual activity also on alkaline surfaces.

Deltamethrin
LD50: 135 - 5 000 mg / kg
MRL: 1 ppm
Dosage rate (admixture to cereals): 1 ppm
Dosage rate (surface treatment): 0.1 - 0.1 5%
Space treatments: dilute with diesel to up to 1% (1 l/1 000 m³)

Remarks:
Deltamethrin is one of the most potent active ingredients of the synthetic pyrethroids. It is effective against most storage pests (exception: Sitophilus spp.), in particular against all species of the family of Bostrichidae such as Rhyzopertha dominica and Prostephanus truncates. It shows a delayed but long lasting action.

Fenvalerate
LD50: 451 mg/kg
MRL: 2 ppm
Dosage rate (admixture to cereals): 2 ppm
Dosage rate (surface treatment): 0.5%

Remarks:
Fenvalerate has shown to be effective against Rhyzopertha dominica It acts as a contact and stomach poison against most insect species and has an adequate stability.

Permethrin
LD50: 430 - 4 000 mg/kg
MRL: 2 ppm
Dosage rate (admixture to cereals): 2 ppm
Dosage rate (surface treatment): 0.25%

Remarks:
Permethrin is effective against a wide range of stored product pests, particularly against Rhyzopertha dominica and Prostephanus truncates, but it has only weak action on species of Tribolium. It is most valuable when used in combination with organophosphorous insecticides and has a long stability.

- Combined products:

The following dosages apply for the admixture of combined products formulated as dustable powders to stored cereals:

Fenitrothion + Cyfluthrin:

8 + 0.2 ppm

Fenitrothion + Fenvalerate:

5 + 1 ppm

Pirimiphos-methyl + Deltamethrin:

5 + 0.5 ppm

Pirimiphos-methyl + Permethrin:

8 + 1.5 ppm

For combined products MRL are not yet defined by FAO/WHO, as it is difficult to assess synergistic effects which may occur.

8.1.7 Toxicity of Insecticides

Insecticides are not only poisonous to the target organisms but also in varying degrees to humans, animals and the environment.

The "LD50" of an insecticide is used to assess its potential danger. LD stands for Lethal Dose. The LD50 is stated in milligrams (mg) of the relevant insecticide per kilogram body weight of test animals, usually rats. The LD50 is the amount of an active ingredient which will lead to the death of 50% of a group of test animals after a single application. As the toxicity of an insecticide also varies according to the kind of contact with the body, a distinction is made between LD50 (oral) and LD50 (dermal).

Insecticides are classified according to their toxicity as follows:

Classification

LD50 for rats (mg/kg body weight)


oral

dermal

Extremely hazardous

under 25

under 50

Highly hazardous

25 - 200

50 - 400

Moderately hazardous

200 - 2 000

400 - 2 000

Slightly hazardous

over 2 000

over 2 000

The LD50 applies to the pure active ingredient of an insecticide although the concentration as well as the type of formulation and application also play a role. Insecticides with a high LD50 have a relatively low acute toxicity. This does not affect, however, possible long-term (chronic) dangers to health.

In order to estimate the long-term effects of an insecticide, the "no effect dose" is used. This dose is generally referred to as NOAEL (no observed adverse effect level). This refers to the highest concentration of an active ingredient in mg kg body weight of test animals administered daily in long-term tests without causing any symptoms of poisoning.

This value is divided by a safety factor, usually of 100. The result is the maximum amount of an active ingredient (in mg/kg body weight) which a person can consume daily over the complete lifespan without any damage to her or his health according to the present state of knowledge. This value is called the ADI (acceptable daily intake). Regrettably, adequate data are still lacking today in many respects, particularly as far as the combined effect of a number of chemicals is concerned.

8.1.8 Residues

Contamination with insecticide residues takes place first and foremost by eating contaminated produce. Insecticides and their decomposition products can still be found as residues in the produce treated a fairly long period ago. If the insecticide concerned has a high acute toxicity, immediate illness may result (see section 8.4.3).

Active ingredients which are chemically very stable and thus decompose slowly (i.e. which have a high persistence) may have a long term effect. Even with lower acute toxicity they may lead to chronic poisoning as a result of their accumulation, particularly in fatty tissue. This is the case, for example, with DDT and other chlorinated hydrocarbons. Regrettably, these are still used in stored product pest control, even though they are no longer officially permitted in most countries.

"Maximum residue limits" (MRL) have been laid down for all insecticides to protect consumers. MRL refer to the relevant foodstuff As a large amount of the insecticide is decomposed in processing, higher residue limits are permitted in primary products (e.g. raw grain) than in processed products (e.g. flour). it is assumed that people eating foods containing insecticide residues not exceeding the MRL will not reach the amount stated in the ADI value (see section 8. 1.7).

As it may occur that produce treated, e.g. grain, will be eaten soon after treatment, the amount of insecticide applied must not exceed the admissible maximum residue limits. Attention is paid to this requirement in the recommendations of the FAO/WHO Joint Codex Committee and in national legislations.

In the last years preoccupation with human health has brought forth new approaches to the toxicological evaluation of insecticides. Therefore the TMDI (theoretical maximum daily intake) has been introduced, which is based on the multiplication of the MRL of staple items of national diets by their estimated daily consumption.

As it is practically impossible to predict the TMDI it has been proposed by FAO/WHO to use the estimated maximum daily intake (EMDI) for a more realistic assessment of health hazards for consumers. A further refinement is the estimated daily intake (EDI). Calculation of these factors is, however, only possible for countries with reliable databases, so that their practical value is rather limited.

In order to keep the intake of insecticides as low as possible, the following measures should be taken:

- Only use active ingredients and formulations which are officially approved for use in stored product pest control!
- Exactly observe the recommended application rates!
- Treat stored product only once and avoid local overdosages!
- Avoid any unnecessary insecticide treatment!
- Do not treat the produce shortly before it is sold or eaten!

8.1.9 Resistance

Resistance means that the target pests are no longer controlled by the originally recommended application rate of an insecticide. Resistance develops as a result of a selection process. In a pest population there are always individuals which react less sensitively than the majority to any insecticide treatment. They have a chance of surviving and of reproducing. If they succeed they pass on their insensibility to the next generation. Thus, over a period of time, a process of selection of resistant insects takes place.

Insect species with a high rate of reproduction (short generation periods, large number of offspring) build up resistance more rapidly than others. The climate in the tropics and the resulting short generation periods are particularly favourable to the development of resistance. The process of resistance development is speeded up, if

· the same active ingredient is used over a long period
· the insecticide is partly degraded due to overstorage
· the insecticide is underdosed
· the insecticide is partly degraded due to overstorage
· the active ingredient is unevenly distributed
· insecticide applications are performed frequently
· bad hygiene conditions exist.

An insect population may become resistant to two different insecticides, even if they have only been treated with one of them. I his phenomenon is called cross resistance and may even occur if the two insecticides belong to two different chemical groups.

When insects show resistance against different active ingredients as well as against different groups of insecticides multi-resistance exists.

A distinction is made between various forms of resistance:

- Physiological resistance: the insects have the ability to neutralize the active ingredient in their metabolism before it can take toxic effect.
- Morphological resistance: the insects have adapted their physical structure, e.g. wax layer or hairs to avoid penetration of the insecticide into the insect's body.
- Behavioural resistance: the insects actively avoid coming into contact with the insecticide.

The following measures should be taken to prevent resistance:

- Change the active ingredient regularly (if possible once a year)!
- Use insecticides only under perfect hygiene conditions!
- Ensure that dosage and application are correct!
- Do not use insecticides unless necessary!

Increasing the amount of insecticide is no solution as it promotes further resistance. I his approach is also uneconomical and not permitted because of let al stipulations of maximum residue limits.

Note: As a result of the intensive application of malathion in recent years, resistance to this insecticide has developed world-wide. It is therefore no longer possible to generally recommend it for pest control in storage. Malathion-resistant insects frequently exhibit cross resistances to newer organophosphorous compounds.

Attention should be paid to the fact, that resistances against chorpyrifos-methyl are already rather widespread and first resistances against pirimiphos-methyl have already appeared in countries, where it is applied frequently. Resistant insect strains may be world-wide distributed by the trade.