Section 14 – PESTICIDE CALCULATIONS

Pesticide Calculations

Most insecticides are marketed in highly concentrated forms and these are not generally intended for direct application (Calderon 1970). The chemical formulation is further diluted with a suitable carrier (generally water) to a much lower concentration to meet a recommended dosage. If proper rates of dilutions are not used, the insecticide may be too weak (and therefore ineffective for insect control) or too strong (as to cause damage to the crop or injury to animals).

To the herbicide user, accuracy in dilution is critical in achieving the desired dosage of the weed killer. In conjunction with using the proper equipment for applying the prepared spray mixture, using the correct concentration will ensure effective weed control. Improper dilution may damage the crop along with the weeds.

Formulations

Most common pesticides (insecticides, herbicides, and fungicides) are available commercially in one or more of the following formulations: wettable powder (WP), dusts, granules (G), or emulsifiable concentrate (EC). In its pure form, the toxicant may be a solid or a liquid. Because the insecticide is highly toxic, the manufacturer has to dilute it before making it available to farmers (Calderon 1970, Moody and De Datta 1977).

Solid formulations

In commercial solid formulations (dust, WP, and G), a certain weight of the toxicant is mixed or impregnated with a certain weight of inert powder or granules. The concentration of the active ingredient (ai) is thus expressed as a percentage of the weight of ai to total weight of the commercial solid formulation. Thus,

                                                       Weight of ai

ai       =          ————————————- x 100

                                     Total weight of dust, WP, or G

Examples:

Sevin 85 WP means that there is 85 g of 1-naphthyl-N-methylcarbamate for every 100 g of commercial 85% wettable powder.

Basudin 10% granules mean that, for every 100 g of the commercial granules, there is 10 g of pure diazinon or diethyl phosphorothioate.

Liquid formulations

In commercial liquid formulations (EC), a certain weight of the toxicant is dissolved in a certain volume of solvent with an emulsifying agent (69). The concentration of the ai is thus expressed in two ways:

Percentage

                                                             Weight of ai

                        % ai in EC = ————————– x 100

                                                            Volume of EC

Pounds per gallon or grams per liter

Example: Endrin 20% EC means that, for every 100 cm3of commercial Endrin 20% EC, there is 20 g of pure endrin (200 g/li).

Calculations

Pesticides are generally recommended in two ways. For spray applications, the percent toxicant contained in the mixed spray solution is indicated and applied per unit area, whereas granular applications are normally recommended on the basis of the weight of the toxicant to be applied per unit area. The following procedures show how correct amounts of different insecticidal formulations and recommendations are computed, along with some examples (Xuan and Ross 1979).

1.     For EC expressed in % ai or kg ai/li which calls for a recommendation in liters of the toxicant (EC) required.

Formula 1 

                                                                        ai desired x specified spray

                                                                          volume (liter)

EC required (li)                    = —————————————-

                          % ai in commercial EC

Example:

500 gallons of 0.04% Folidol spray is needed to control insects. What is the volume of commercial Folidol 19.5%? How much EC? is required?

Data given:

a.       Concentration of commercial Folidol 19.5% EC

b.       Specified spray volume                 = 500 gal x 3.5 li/gal

                                                                                                            = 1900 li

c.       Recommended concentration         = 0.04% ai

Calculation:

                                                                        0.04 x 1900

Folidol required (li)  = ———————          = 3.89

                               19.5

2.     For EC expressed in g ai/li which calls for a recommendation in ai of the toxicant (EC) required.

Formula 2 

% ai desired x specified spray 

                                      volume (li)

EC required (li)                    = ——————————————–

                          100 x kg ai in commercial EC

Example:

300 liters of 0.05% Folidol spray is to be prepared to control insects. If Folidol 500-E, an EC containing 500 g ai/li, is used, calculate the required volume of toxicant (EC).

Data given:

a.  Concentration of Folidol 500-E      = 500 g/li

   or    = 0.5 kg/li

b.  Specified spray volume                 = 300 li

c.  Recommended concentration         = 0.05% ai

Calculation:

                                                                                                 0.05 x 300

Folidol 500-E required (li)     =   —————- = 0.30

                                                                                                 100 x 0.5

3.     For EC expressed in % ai which calls for a recommendation in kg ai/ha of the toxicant (EC) required.

Formula 3

Recommended rate (kg ai/ha) 

   x area (ha)

             EC required =     ———————————————- x 100

   % ai in commercial EC

Example:

When the recommendation calls for 2 kg ai/ha of gamma-BHC to control rice stem borers and an EC containing 20% gamma-BHC is available, how many liters of this formulation are needed to treat 8,000 m2

Data given:

a.   Area to be treated            = 8,000 m2

b. Concentration of gamma-BHC EC: 20%

c.   Recommended rate          = 2 kg ai/ha

Calculation:

                                                                                                2 x 8000

gamma-BHC required (li)          = ——————- = 8

                                                                                                 20 x 100

4.     For EC expressed in lb (US) gal which calls for a recommendation in kg ai/ha of the toxicant (EC) required.

Formula 4

Recommended rate (kg/ha) 

  x area (ha)                             

EC required (li)                    = ——————————————

                               kg ai/li in commercial EC

or use the formula below when square meters are used.

                                                                         Recommended rate (kg/ha) x area (m2

            EC required (li)   = ————————————————–    

                          kg ai/li in commercial EC x 10,000

Example:

When the recommendation calls for Folidol EC at the recommended rate of 1.5. kg ai/ha and if Folidol EC containing 6 lb (US)/gal is available (79), how many liters of Folidol are needed to treat 3.5 ha?

Data given:

a.        Area to be treated            =          3.5 ha

b.       Recommended rate          =          1.5 kg ai/ha

c.        Concentration of Folidol EC

                                                                                    =         6 lb (US)/gal x 0.120

                                                                                     =          0.720 kg/li

Calculation:

                                      1.5 x 3.5

Folidol required (li)              = —————– = 7.29

                                         0.72

5.     For WP expressed in % ai which calls for a recommendation in % ai of the WP.

Formula 5

                                                                        % ai desired x specified spray (li)

WP required (kg) = ———————————————

                             % ai in WP

Example:

2,000 liters of 0.09% carbaryl is to be prepared. The WP Sevin to be used contains 85% carbaryl. What is the required weight of Sevin?

Data given:

a.        Concentration of commercial Sevin             =          85% ai WP

b.       Specified spray volume                             =         2,000 li

c.        Recommended concentration                     =          0.09% ai

Calculation:

                                                                         0.09 x 2,000 

Sevin required (kg)   = ———————— = 2.11

                                                                                     85

6.     For WP, dust, and G expressed in % ai which calls for a recommendation in kg a.i./ha of the solid formulation.

Formula 6

                                                                                                 Rate (kg/ha) x area (ha)

Weight of WP, dust, or G           = ——————————— x 100

                                                                                                 % ai in WP, dust or G

or

                                                                                                 Rate (kg/ha) x area (m2)

Weight of WP, dust, or G            = ———————————– x 100

                                                   % ai in WP, dust, or G                                                                              

Example:

Basudin 10G granules containing 10% ai are to be used to control pink stem borers at a rate of 2 kg ai/ha. How many kilograms of Basudin 10G are needed for a 2,500-m2plot?

Data given:

a.        Concentration of Basudin 10G                   =          10% ai

b.       Recommended rate                                              =         2 kg ai/ha

c.        Area to be treated                                                =         2,500 m2

Calculation: 

2 x 2,500

Weight of Basudin 10G required         = ——————

                                                                                                            10 x 100

                                                                      = 5 kg