Most pesticide application in Sierra Leone is done with backpack, pump-type sprayers. The main function of a sprayer is to break the liquid pesticide solution into droplets of effective size and distribute them uniformly over the surface or space to be protected. Another function is to regulate the amount of pesticide to avoid excessive application (which might prove harmful, wasteful, and costly) or insufficient application (which would fail to give effective control). This chapter describes the common formulations of locally available pesticides and explains step by step how to dilute them to obtain the desired concentration for field application.
I. Pesticide Formulations
Most common pesticides are available commercially in one or more of the following forms: dust, wettable powder, emulsifiable concentrate, or granules.
Dusts are widely used in agriculture, especially where it is difficult to obtain large volumes c' water for diluting sprays. The toxicant mixed or dilated with a carrier such as, talc, and mechanically blended by the manufacturer. A low concentration of the toxicant is helpful in making an even application.
Dusts are applied dry with hand dusters, ground dusting equioment, or aerial equipment and are not intended to be mixed with water or oil. In order to avoid drift it is best to use dusts when there is little or no wind, and the crops are wet from dew or rain. Because they contain a minimum of solvents and no emulsifiers, dusts are least likely to harm delicate plants.
2. Emulsifiable Concentrates
An emulsifiable concentrate is an oil-based liquid compound containing a high concentration of the toxicant. These concentrates contain emulsifiers (agents which facilitate mixing with water), as well as wetting and sticking agents to make them readily emulsifiable in water and liable to adhere to plants. Before ´:se, they must be diluted in water. They can be applied as ground sprays, or with aerial spray equipment.
3. Wettable Powders
Wettable powders, although similar in appearance to dusts, contain a wetting agent which permits them to become dispersed and suspended when mixed with water and applied as a liquid spray. Since powders do not dissolve but rather stay in suspension, spray tanks must be equipped with agitators to keep the particles in suspension. Wettable powders contain no oil or solvent, and they are consequently safer than some emulsifiable concentrates for use on plants with delicate foliage.
Granular pesticide formulations consist of free-flowing grains or inert materials either mixed or impregnated with a toxicant. Granules offer these advantages they do not need any further mixing or dilating they present no drift problem during applications: and they can be broadcast directly by hand without expensive equipment (since almost invariably they are made up of compounds showing low toxicity to humans).
All of these different forms of pesticides have their place in insect control because of differences in weathering properties, uptake characteristics, life cycles of insect pests, and/or growth characteristics of crops.
II. Pesticides Available in Sierra Leone
MALATHION - ID50 Values Oral: 1375 Dermal:
Organophosphate (contact insecticide)
Formulation: emulsifiable concentrate, wettable powder Active Ingredient: 25% or 50%
Use: Insect control in rice, vegetables
GAMA-BHC - ID50 Values Oral: 600
Chlorinated hydrocarbon (systemic insecticide)
Formulation: wettable powder Active Ingredient: 25% uSE: Insect control in rice, vegetables
PHOSDRIN - ID50 Values Oral: 6 Dermal:
Organophosphate (contact, systemic insecticide)
Formulation: emulsifiable concentrate Active Ingredient: 24%
Use: Insect control in rice, vegetables EXTREMELY Dangerous - NOT Recommended
BIDRIN ID50 Values Orals 21 Dermal: 43
Organophosphate (contact, systemic insecticide) Formulation: emulsifiable concentrate Active Ingredient: 24%
Use: Insect control in rice, vegetables EXTREMELY DANGEROUS - NOT RECOMMENDED
KOCIDE 101 ID50 Values Orals Dermal:
Active Ingredient copper (contact fungicide) Formulation: wettable powder Active Ingredient: 54%
Use Control of black pod disease in cacao
III. Sprayer Calibration
If and when it becomes necessary to apply pesticides it is likely that application will be made with a pump-type backpack sprayer. These sprayers are relatively inexpensive, simple, and easy to operate. It is important that the extension agent be well versed in the use and calibration (calculation of the rate of spraying) of the backpack sprayer. Even if the extension agent does not actively encourage farmers to apply pesticides, it is quite possible that s/he will be approached by farmers owning their own equipment for instruction in the use of the sprayer. One of the best ways to help decrease pesticide abuse is to provide accurate information about the proper calibration and safe use of spraying equipment.
Before each application of pesticide, it is necessary to calibrate the sprayer to determine the exact rate of spraying. The rate of spraying is the amount of liquid emerging from the sprayer (in normal use) to cover a given area. This differs significantly from the rate of Pesticide application, which is the amount of pesticide applied per unit area. The rate of spraying varies according to the physical characteristics of the sprayer and has nothing to do with the properties of the spray itself, but the rate of pesticide application varies according to the rate of spraying and the concentration of the mixture in the tank. (This is an important distinction and should be understood clearly.) Only once the sprayer has been calibrated (to determine the rate of spraying) will it be possible to determine how much pesticide to mix with water in the tank to achieve the desired rate of pesticide application.
The rate of spraying depends on four major factors'
1) Pressure in the spray tank
Keep the pressure in the tank as constant as possible. A pressure regulator or pressure gauge is ideal, but usually these are lacking. Smooth, even pumping will help keep the pressure relatively constant.
2) Size of nozzle orifice
The size of the nozzle orifice regulates the amount of spray passing through the nozzle. The nozzle orifice wears with use and therefore must be recalibrated regularly.
3) Spray swath
The spray swath, or "path" covered by the droplets, varies according to the height at which the tip of the nozzle is held. Keep the swath as constant as possible.
4) Walking speed of Svrayperson
Walking speed directly affects the area covered per unit time, which in turn determines the amount of spray applied per unit area. Maintain an even walking speed when spraying, to the extent possible.
When calibrating a sprayer, it is important to keep in mind that the calibration will be valid only for that Particular sprayer in that Particular swamp with that particular spray-Person, since a change in any one of these variables will afect a change in the rate of spraying.
To calibrate a sprayer, follow this procedures
1. Prepare sprayer
a) Rinse supply tank and fill with clean water.
b) Remove nozzle; check and clean if necessary.
c) Flush pump, hose, and lance with clean water.
d) Apply pressure (i.e. pump) and check sprayer for leaks.
2. Determine walking speed of spravperson
a) Fill tank with clean water.
b) In an actual paddy, mark starting point with a stake.
c) Using your wristwatch, begin 1 minute trial. Walk at a constant and normal speed, carrying the filled sprayer on your back. Pump the sprayer handle with one hand to maintain pressure and direct the nozzle with the other hand to obtain a spray swath of approximately 1m width.
d) Stop walking at the end of exactly 1 minute and mark the stopping point with a second stake.
e) Measure the distance between the starting and stopping points. Record the distance in meters. Walking speed can be expressed in terms of m/minute.
f) Repeat trial at least three times to obtain an average walking speed.
3. Calculate area sprayed in one minute
If the spray swath was kept at (approximately) 1m, the area sprayed in one minute can be calculated easily using the known walking speed:
Area sprayed in one minute = spray swath (1m) x walking speed (m/minute). The answer is expressed in terms of m2/minute.
4. Determine nozzle discharge in one minute
a) Fill sprayer with clean water and pump sprayer handle to build up pressure.
b) Dip end of nozzle into a graduated cylinder.
c) Using your wristwatch, begin 1 minute trial. Open the cut-off valve and spray into the graduated cylinder. Make sure none of the spray escapes.
d) Cut off the discharge at the end of exactly 1 minute.
e) Note the volume (in liters of liquid collected. This is the nozzle discharge, expressed in terms of liters/minute (1/min).
f) Repeat trial at least three times to obtain an average nozzle discharge.
Now you can easily compute the rate of spraying'
Rate of spraying = (nozzle discharge (1/min)) / (area sprayed
Since most pesticide application rates are given in terms of 1/ha, the rate of spraying should be converted to the same units of measurement'
Rate of spraying = 1/m2 x 10000m2/1 ha = 1/1 ha ha
i.e., simply multiply the rate calibrated for 1/m2 x 10000.
IV. Pesticide Dilution
Once the sprayer has been calibrated, it is very easy to calculate the amount of pesticide which must be diluted with water to achieve the desired mixture. Since the rate of spraying has been calibrated in terms of liters (spray)/ha, and since pesticide application rates are given in terms of liters (e.c. of pesticide), the desired ratio of emulsifiable concentrate to water can be determined by simple division'
Mixture = desired rate of pesticide application (liters e.c./ha) / calibrated rate of spraying (liters water/ha)
= liters emulsifiable concentrate / liters water