|Manual on the Prevention of Post-harvest Grain Losses (GTZ)|
|8. Pest control using insecticides|
Great care should be paid in calculating the amount of insecticide to be used, because:
- if the dosage it too low, it
· lack of efficient control
· waste of money
· promotion of resistance
- if the dosage is too high, it
· danger to users and consumers
· uneconomic usage
Only the correct dosage guarantees optimum pest control whilst also keeping all risks to a minimum (see sections 8.1.5 and 8.1.6). The details as to the recommended application rates and to the active ingredient content are to be found on the label of the container in which the insecticide is sold.
Stored produce which is to be treated directly must be weighed.
Surface areas to be treated must be measured and calculated.
For organizations with a number of stores, it is advisable to introduce standardized stack sizes and issue technical instructions for treatments. These must include the following details on making up spray mixtures:
· Amount of water per standardized stack (in
· Specification and amount of insecticide per standardized stack (in ml for EC formulations and g for WP formulations)
This simplifies carrying out treatments and avoids wrong dosages being made.
8.3.1 Calculating the Dosage for Surface Treatment Using Dust Formulations
Recommended application rates given in g/m²
(= g commercial product/m² surface area)
Details required for calculation:
- surface area to be treated (in m²)
- recommended application rate of insecticide (in g/m²)
The calculated surface area to be treated (in m²) is multiplied by the recommended application rate of the insecticide.
Example: A stack of bags has a surface area of 120 m². It is to be treated with a 5% dustable powder formulation. The recommended application rate is 10 g / m².
10 g/m² × l20m² = 1200g
of the 5% dust formulation is thus required to treat the stack.
8.3.2 Calculating the Dosage for Surface Treatment Using EC and WP Formulations
Two questions have to be answered in order to calculate the dosage for EC and WP formulations:
1. How much spray mixture is required to treat the surface area?
2. How much insecticide is required for the correct amount of spray mixture?
188.8.131.52 Amount of Spray Mixture for Surface Treatments
Details for calculation:
· surface area to be treated (in m²)
· recommended application rate of spray mixture (in l/100 m²).
The following basic principle applies: The smoother the surface, the less spray mixture will be required.
The following amounts are recommended for surface treatments of:
smooth walls: 3 - 51/100 m²
rough walls: 6 - 8//100 m²
jute bags: 8 - 101/100 m²
plastic bags: 3 - 5//100 m².
To calculate the required amount of spray mixture, the recommended application rate is multiplied by the actual surface area to be treated.
Example: A stack of jute bags has a surface area of 160 m². The recommended application rate is 8l/100 m².
8l/ 100 m² × 160 m ²= 12.8 l
of spray mixture is thus required to treat the stack.
184.108.40.206 Calculating the Amount of insecticide Required for the Spray Mixture
- Recommended application rate given in ml/l (EC) or g/l
(= ml or g of commercial product/l of spray mixture)
Details for calculation:
· amount of spray mixture (in l)
· recommended application rate of insecticide (in ml/l for EC or g/l for WP formulations)
The recommended application rate of the insecticide is multiplied by the amount of spray mixture calculated.
Example: 12.8 l of spray mixture is required to treat a stack of bags
a) it is to be treated with an EC formulation with 50% active ingredient. The recommended application rate is 20 ml/l.
20 ml/l x 12.8 l = 256 ml
of the EC formulation is thus required.
b) it is to be treated with a WP formulation with 40% active ingredient. The recommended application rate is 30 g/l.
30 g/l x 1 2.8 = 384 g
of the WP formulation is thus required.
- Recommended application rate of insecticide given in %
(= % of active ingredient in spray mixture)
Details for calculation: · Amount of spray mixture required (in 1) · Active ingredient content of the insecticide (in %) · Recommended application rate of the insecticide (in %)
In this case, the amount of insecticide required can be seen from the following per cent table by means of the concentration of active ingredient in the commercial product and the recommended rate of the active ingredient in the spray mixture. The amounts of insecticide listed are in ml/l for EC and g/l of spray mixture for WP formulations. The dosage can be calculated in four steps with the aid of the % table:
1. The top row shows various application rates in %. Find the correct column for the amount stated!
2. The left column shows various concentrations of active ingredients in insecticides in %.
Find the correct row for the concentration of the active ingredient stated on the label of the insecticide used!
3. Find the intersection of the chosen row and column! The figure listed there is the amount of EC or WP formulation in ml or g required for 1 litre of spray mixture.
4. Calculate the amount of insecticide to prepare the actual amount of spray mixture required!
Example: 12.8 litres of spray mixture are required. A SO EC insecticide is to be used (concentration of active ingredient = 50%). The recommended application rate is 0.25% (concentration of active ingredient in the spraying mixture).
The point where the 50% row crosses the 0.25% column shows the amount required for 1 litre of spray mixture: 5 ml of the EC50 formulation.
5 ml/l × 12.8 l = 64 ml
of the insecticide are thus required for 12.8 litres of spray mixture.
The increase in volume of the spray mixture resulting from adding the insecticide to the water can be disregarded. Calculations of WP formulations are made as for EC formulations.
Table for the calculation of the amount of insecticide needed for 1 litre of spray mixture
8.3.3 Calculating the Dosage for Fogging
The dosage of a togging concentrate depends on the volume of the free space in the store. It is thus necessary to first determine the total volume of the store and deduct the volume of the stacks from this figure.
Recommended application rates of ready-to-use fog formulations are generally stated in ml/100 m³ volume.
A store which is 40 m long, 15 m wide and 8 m high contains 10 stacks of bags of the same size, all measuring 5 m x 5 m x 4 m. Infestation with moths is to be dealt with using Dichlorvos, a ready-to-use commercial fog formulation. The recommended application rate is 100 ml/100 m³
Calculation of the volume of empty space in the store:
Store: 40 m x 15 m x 8 m = 4 800 m³
Stacks: 5 m x 5 m x 4 m = 100 m³ x 10 = 1 000 m³
Free space: 4 800 m³ - 1000 m³ = 3 800 m³
100 ml/100 m³ x 3 800 m³ = 3 800 ml = 3.8 l
of the fog formulation are thus required.