Vector Control - Methods for Use by Individuals and Communities. © 1997, WHO.

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Insecticides for residual spraying


Characteristics of good residual insecticides
Resistance
Formulations
Dosages and cycles
Type of sprayed surface
Commonly used insecticides
Preparation of insecticide suspension


Insecticides can be directed against the larval stages of mosquitos, which live in water, or against free-living adult stages. In the latter case they can be applied in two ways:

· Release into the air in the form of a vapour or aerosol, by means, for example, of mosquito coils and aerosol spray cans and by space-spraying. This method knocks down or kills flying and resting insects immediately after they absorb the particles by inhalation or contact, but offers only brief protection.

· Application to a surface as a spray or deposit or impregnation for prolonged action. Residual insecticides kill insects that land on or crawl over a treated surface. The duration of action depends on many factors, such as the nature of the surface, the insecticide, its formulation and the dosage. Examples are insecticidal dusts used against lice and fleas, impregnated mosquito nets and residual spraying of the walls in a house.

Different insecticides are suitable for different application methods. For example insecticides that evaporate quickly at ambient temperature are not suitable for residual application on walls; they may, however, be suitable for use in vaporizers or space sprays.

Characteristics of good residual insecticides

A residual insecticide should be:

· Highly toxic to target insects. Insecticides may lose their effectiveness if the target insects develop resistance. From time to time, samples of the target insect should be collected and checked for the development of resistance. If resistance is observed another insecticide without cross-resistance has to be used.

· Long-lasting on a given surface. The toxicity should remain high over a sufficiently long period to prevent the need for frequent reapplication, which is costly and time-consuming.

· Not repellent or irritant to target insects to ensure that the insects pick up a lethal dose.

· Safe to humans and domestic animals. There should be no danger to spray workers, inhabitants or animals accidentally contaminated with the insecticide during or after spraying.

· Acceptable to house owners. Some insecticide formulations are less acceptable because of their smell or because they leave unattractive deposits on walls.

· Stable during storage and transportation; mix well with water; harmless to spraying equipment.

· Cost-effective. Calculation of the cost should be based on how the insecticide is applied, at what dosage and how many times a year.

Resistance

Resistance is a common result of insecticide use and selection pressure on the insect population. When resistance does emerge, the choice of a replacement insecticide will depend on the mechanism of resistance, known susceptibility, cost-effectiveness and availability. Ideally, the available insecticides should be used as part of an overall strategy to maximize the useful life of each product.

DDT used to be the most commonly applied insecticide and can still be employed where mosquito vectors are susceptible.

Where insects are resistant to DDT, the next insecticide of choice is usually one of the organophosphorus compounds, especially malathion. If the target insects have developed resistance to malathion, fenitrothion, which is more expensive and more hazardous or pirimiphos methyl, which is also more expensive, can be used. The carbamates are also more expensive alternatives. The pyrethroids are normally used when resistance occurs to all other types of insecticide; they are among the safest such products currently available when applied at the recommended dosages. Vector control experts in the local health services or research institutes may be able to advise on locally effective pesticides.

Formulations

Insecticides are rarely applied in their pure form. They are available as special formulations, which are adapted to the requirements of the various application methods.

Residual insecticides for spray application are generally formulated as water-dispersible powders, emulsifiable concentrates or suspension concentrates.

Water-dispersible powder

This is a dry powder of insecticide mixed with a surface-active agent that allows the insecticide to dissolve in water. The insecticide remains in suspension in the water with occasional stirring.

The products are usually packaged as powders containing 5 - 80% active ingredient. One kilogram of a 75% powder formulation would consist of 250g of inert material and 750g of the pure insecticide. Such products are ready for mixing with water to form a spray suspension, normally containing 1 - 5% of active ingredient.

Emulsifiable concentrate

An emulsifiable concentrate consists of a solvent and an emulsifying agent in which the insecticide is dissolved. When mixed with water it forms a milky, white emulsion composed of finely suspended oil droplets. It remains in suspension with a minimum of agitation.

Suspension (or flowable) concentrate

A suspension concentrate consists of particles of the insecticide with a wetting agent and some water, which can be used to make a water-based suspension. A distinct advantage is that the ingredients are not flammable. The insecticide particles are larger and remain available on wall surfaces longer than those of emulsifiable concentrates. However, the particles are smaller than those of water-dispersible powders, and are therefore less effective on porous surfaces. The residues left on the wall are aesthetically more acceptable than those of water-dispersible powders. This type of formulation is available for several insecticides listed in Table 9.1.

Water-dispersible powder, emulsifiable concentrate or suspension concentrate?

For indoor spraying purposes, the water-dispersible powder is the most effective formulation in most countries. This is because it is most suited for porous surfaces such as brick and mud walls. The insecticide particles are comparatively large and absorption is comparatively slight. More active ingredient therefore remains available on walls to be picked up by resting mosquitos and crawling insects and the residual effect lasts longer.

The water-dispersible powder is also lighter and easier to transport than the emulsifiable concentrate. It can be prepacked for use in the field and is less toxic to humans.

The suspension concentrate is also suitable for rough surfaces, but special care is needed during the formulation process in order to avoid caking of solid materials at the bottoms of containers and, as it is a liquid, it requires relatively expensive containers and careful handling to avoid spillage.

The emulsifiable concentrate is more expensive and used for spraying impervious surfaces and walls with fine coverings because it does not cause spots and stains.

The residual effect of emulsifiable concentrates depends on the absorption capacity of the wall and on the physical properties of the insecticide. Usually, water-dispersible powders and suspension concentrates have a longer residual effect, except on non-absorbent surfaces where the effectiveness and persistence of the three kinds of formulation are equivalent.


Table 9.1 Insecticides used for residual wall-spraying

Insecticide

Dosage (g/m2)

Duration of effectiveness (months)

Insecticidal action

Safety class of active ingredienta

Organochlorines

DDT

1 - 2

6 or more

contact

MH

lindane

0.2 - 0.5

3 or more

contact

MH

Organophosphorus compounds

malathion

1 - 2

1 - 3

contact

SH

fenitrothion

1 - 2

1 - 3 or more

contact, airborne

MH

pirimiphos methyl

1 - 2

2 - 3 or more

contact, airborne

SH

Carbamates

bendiocarb

0.2 - 0.4

2 - 3

contact, airborne

MH

propoxur

1 - 2

2 - 3

contact, airborne

MH

Pyrethroids

alphacypermethrin

0.03

2 - 3

contact

MH

cyfluthrin

0.025

3 - 5

contact

MH

cypermethrin

0.5

4 or more

contact

MH

deltamethrin

0.05

2 - 3 or more

contact

MH

lambdacyhalothrin

0.025 - 0.05

2 - 3

contact

MH

permethrin

0.5

2 - 3

contact, airborne

MH

a MH = moderately hazardous; SH = slightly hazardous.

Dosages and cycles

The dosage rate is the amount of insecticide applied to a unit of surface area. It is usually given in grams of insecticide per square metre (g/m2). The optimal dosage rate may vary with place and season, with species of mosquito or other vector and with the material of the sprayed surface. Table 9.1 gives dosage rates which usually provide satisfactory results. Local vector control experts should advise on the most appropriate dosage rates.

The spray cycle is the time between consecutive insecticide spray rounds. In small communities where spraying can be done quickly, houses should be sprayed in the weeks preceding the onset of the transmission season. If this season lasts only three months an insecticide that persists for three months or more needs to be sprayed only once a year.

In areas where transmission occurs throughout the year, several spray cycles may be needed to cover the whole period. Residual effectiveness is normally extended when a relatively high dosage rate is used. A lower dosage can be applied when the transmission season is short.

Type of sprayed surface

The persistence of an insecticide sprayed on a surface varies not only with the type of insecticide and its formulation but also with the nature of the surface. Most insecticides last longer on wood and thatch than on mud. Mud surfaces absorb some insecticide, and certain types of mud may also break it down chemically. For example, malathion sprayed on wood may last three months or more, whereas on some mud surfaces it may last only three weeks.

If local data are not available, it is suggested that for spraying on wood or for short transmission periods the lower dosage rates given in Table 9.1 be chosen. The higher dosage rates may be used for applications on mud surfaces and where long persistence is needed. As discussed above, the persistence of an insecticide is also influenced by its formulation.

Commonly used insecticides

Organochlorines

Of this group of insecticides only DDT is discussed here in detail. Dieldrin was commonly used but, as it is highly toxic to humans and domestic animals, it is no longer available. Lindane has been used in areas where DDT resistance occurs. It is more toxic than DDT but can be applied safely when suitable precautions are taken. It is more expensive than DDT and less persistent, and consequently lindane spraying is rather costly. Because of resistance it is now of limited importance.

DDT

This was one of the first and most commonly used insecticides for residual spraying. Because of its low cost, high effectiveness, persistence and relative safety to humans it is still used for indoor wall spraying. However, the development of resistance and restrictions imposed in a number of countries have led to its replacement by other insecticides that are more expensive. A WHO Study Group met in November 1993 to consider the use of DDT for controlling vector-borne diseases. It concluded that it may be used for vector control, provided that certain conditions are met (1).

Commonly available formulations: 75% water-dispersible powder (the most commonly used) and 50% water-dispersible powder; 25% emulsion concentrate.

Dosage: 1 - 2g/m2 depending on the surface (more on mud-bricks, less on timber) and the length of the transmission period (the higher dosage lasts longer).

Storage: it is stable and can be stored in tropical countries without deterioration if heat, bright sunlight and high humidity are avoided.

Residual effectiveness: six months or more.

The effectiveness and importance of DDT

The discovery of DDT in the 1940s led to a breakthrough in the control of malaria. The insecticide is highly effective in killing indoor-resting mosquitos when sprayed on house walls. It is cheap and remains effective over a period of many months. In many countries, malaria control programmes achieved substantial success because of the spraying of houses once or twice a year with DDT. However, in many areas the spraying could not be maintained because of the high cost of the operations and declining cooperation of the population. In addition, in many areas malaria mosquitos developed resistance to DDT, necessitating increased costs for more expensive replacement insecticides. Nevertheless, DDT is still an effective insecticide in a number of countries.

However, the use of DDT is increasingly being opposed by environmentalists who correctly point out that it is harmful when used for agricultural purposes. DDT does not break down quickly when sprayed on crops. It remains in the soil for a long time and can enter rivers and water supplies. Animals that eat insects poisoned with DDT or predators further up the food chain slowly become poisoned themselves. Humans eating contaminated vegetables and other products may also accumulate DDT in various tissues. In most countries this has resulted in a ban on the use of DDT.

This situation affects the availability and use of DDT for malaria control purposes. DDT is still one of the cheapest insecticides available (Table 9.2) and, if used for wall spraying, is relatively safe for humans and the environment. In spite of its widespread use in malaria control, there have been no reports of intoxication of humans as a result of wall spraying.


Organophosphorus compounds

This group of insecticides was developed after the organochlorines. Following the development of resistance to DDT the organophosphorus compounds became important as alternative residual insecticides. The most commonly used are malathion and fenitrothion. They are more costly than DDT and have a shorter residual effectiveness (Table 9.2).

Table 9.2 Cost comparison of insecticides as applied in residual spraying, excluding operational costs

Insecticide

Dosage
(g/m2)
(technical grade)

Approximate duration of residual effect on mud
(months)

Number of applications
(6-month period)

Total dosage per 6-month period
(g/m2)

Formulationa

Total amount of formulation per m2 per 6-month period

Approximate cost/tonneb
(US$)

Cost/m2
(US cents per 6-month period)

Cost ratio
(DDT= 1)

DDT

2

6

1

2

75% WDP

2.67

3000

0.8

1

malathion

2

3

2

4

50% WDP

8

2100

1.68

2.1

fenitrothion

2

3

2

4

50% EC

8

7500

6

7.5

propoxur

2

3

2

4

20% EC

20

9300

18.6

23.25

deltamethrin

0.025

6

1

0.025

2.5% WDP

1

25000-28000

2.5

3.125

permethrin

0.125

3

2

0.250

25% WDP

1

30000

3

3.75

a WDP: water-dispersible powder; EC: emulsifiable concentrate.
b Excluding freight costs.
Source: 2.

Malathion

This has become one of the most commonly used residual insecticides, following the development of resistance to DDT in many countries. It is classified as slightly hazardous. The absorption of particles by spray workers through inhalation, ingestion or contact with the skin reduces the activity of the enzyme cholinesterase in the nervous tissue. Signs of severe poisoning are muscle twitching and weakness followed by fits and convulsions. Spray personnel should not work with malathion for more than five hours a day, nor for more than five days a week. If the insecticide is stored for long periods in hot areas, impurities may develop which make the product more toxic to humans. Malathion is the least expensive organophosphorus insecticide and the safest when manufactured according to WHO specifications. It is commonly used as a residual spray in the control of malaria and Chagas disease. Acceptability to house owners is sometimes a problem because of its unpleasant smell.

Commonly available formulations: 50% water-dispersible powder and 50% emul-sifiable concentrate.

Dosage: 1 or 2g/m2.

Residual effectiveness: at the higher dose it may last up to six months on thatch or wood but only 1 - 3 months on mud and plaster surfaces. Mud surfaces with a high alkali content (minerals) tend to break down the malathion most rapidly.

Fenitrothion

Fenitrothion is classified as moderately hazardous and is more toxic than malathion to humans. Spray personnel and workers handling the insecticide must observe strict precautionary measures. As with malathion, repeated exposure may lead to a reduction of cholinesterase in the nervous tissue. Spray personnel should be monitored regularly for blood cholinesterase activity; if the level is low they should stop spraying until it has returned to normal. Fenitrothion is a contact poison but it also has an airborne toxic effect on insects which may last up to two months after spraying. The airborne effect may be useful where target mosquitos bite but do not rest in houses. It is often effective against pests that have developed resistance to malathion.

Commonly available formulations: 40% and 50% water-dispersible powder; 5% emulsifiable concentrate.

Dosage: 1 or 2g/m2.

Residual effectiveness: on wood surfaces, 1g/m2 may remain effective for up to 2.5 months; on mud surfaces it lasts 1 - 2 months.

Carbamates

Propoxur

This product is classified as moderately hazardous. If absorbed it reduces cho-linesterase activity, which, however, returns quickly to normal once exposure ceases. It is fairly toxic to fish, birds, bees, livestock and wild animals. Propoxur has an airborne effect inside and near houses for up to two months after spraying. It is used in areas where resistance occurs to organochlorine and organophospho-rus insecticides.

Commonly available formulations: 50% water-dispersible powder and 20% emul-sifiable concentrate.

Dosage: 1 or 2g/m2.

Residual effectiveness: at 2g/m2 it may last 2 - 3 months.

Bendiocarb

Bendiocarb is classified as moderately hazardous. It is rapidly metabolized after absorption, and metabolites are totally excreted from the body within 24 hours. It inhibits cholinesterase, but recovery is very rapid once exposure ceases. When used with appropriate safety precautions, it is safe for operators, householders and livestock, but ducks are particularly susceptible.

Commonly available formulation: 80% water-dispersible powder in preweighed sachets, one sachet to be used per spray charge.

Dosage: 0.2 - 0.4g/m2.

Residual effectiveness: remains effective for 2 - 3 months.

Synthetic pyrethroids

This group includes the most recently developed residual insecticides. The compounds that have been tested for wall-spraying are permethrin, deltamethrin, lambdacyhalothrin, cypermethrin and cyfluthrin. They are used where resistance occurs against the previous groups of insecticides. The pyrethroids are moderately hazardous and under normal conditions of use they are safe for spray personnel and house owners.

Deltamethrin: available as 2.5% and 5.0% water-dispersible powder and as 2.5% and 5.0% emulsifiable concentrate. At a dosage of 0.05g/m2 it usually remains effective for 2 - 3 months on mud and thatch surfaces, but nine months has been reported for other surfaces.

Permethrin is available as 25% water-dispersible powder. At a dosage of 0.5g/m2 it remains effective for 2 - 3 months.

Lambdacyhalothrin is available as 2.5% emulsifiable concentrate and as 10% wettable powder in preweighed sachets. At a dosage of 0.025 - 0.05g/m2 it may remain effective for 2 - 3 months.

Cypermethrin is available as 5% and 25% emulsifiable concentrate. At a dosage of 0.5g/m2 it may remain effective for four months or longer.

Preparation of insecticide suspension

If the standard spraying procedure (see p. 376) is adopted the spray liquid will be applied at a rate of 40ml per m2 or one litre per 25m2. This amount of suspension normally stays on the surface without run-off.

Water-dispersible powder

One litre of spray suspension can be prepared using the following formula:

where:

X = weight of water-dispersible powder required (g)
Y = recommended application rate (g/m2)
C = concentration of active ingredient in formulation (%).

Example

DDT (75% water-dispersible powder) is to be sprayed at a dosage of 2g/m2.

For an eight-litre tank, the amount of water-dispersible powder needed is:

8 × 66.6 = 533.3g

The insecticide should be packed in small bags containing 533.3g each. In the field, put water in a mixing bucket until the eight-litre mark is reached. Mix the contents of one bag with the water, using a wooden paddle. Pour the solution into the sprayer through a funnel with a screen, close the tank and shake it.


Emulsifiable concentrate

To prepare an insecticide suspension from an emulsifiable concentrate, use the same formula as for the water-dispersible powder, with:

X = amount of emulsifiable concentrate needed (ml)
Y = recommended application rate (g/m2)
C = concentration of active ingredient in formulation (%).

To prepare one litre of suspension, add X ml of emulsifiable concentrate to (1000 - X) ml of water.

Example

DDT (25% emulsifiable concentrate) is to be sprayed at a dosage of 1g/m2.

To prepare one litre of suspension, add 100ml of emulsifiable concentrate to 900ml of water. For an eight-litre tank, add 800ml of emulsifiable concentrate to 7200ml of water.


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