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close this bookVector Control - Methods for Use by Individuals and Communities (WHO, 1997, 425 p.)
close this folderChapter 9 - House-spraying with residual insecticides
close this folderInsecticides for residual spraying
View the document(introduction...)
View the documentCharacteristics of good residual insecticides
View the documentResistance
View the documentFormulations
View the documentDosages and cycles
View the documentType of sprayed surface
View the documentCommonly used insecticides
View the documentPreparation of insecticide suspension

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.