Crop Scene Investigation - 53 Solved: Sucking the life out of Sebastian's soybeans

Tuesday, January 6, 2015

Sebastian's insecticide failed to control the spider mites in his soybean field because the water he used in the sprayer was very alkaline.

"If you want an insecticide to give you top performance, it's important to mix it in water that has the correct pH level," explains Willy Gubbels, UAP Canada's eastern sales manager. "The vast majority of pesticides require water within a specific pH range. However, they're all different. That's why it's important to know your pesticide and the pH range."

The optimal pH range for dimethoate, the active ingredient in Sebastian's insecticide, is 5.0 to 7.0. However, tests showed the water the farmer used had pH ranging from 8.5 to 9.2. Without the proper water pH, Gubbels explains that the active chemical quickly decomposes and the insecticide loses its effectiveness.

"If you use water with the optimal pH, an active like dimethoate should have a half life of 12 hours. That means the insecticide will be effective if applied within 12 hours. Because Sebastian used the wrong pH, his half life was only 48 minutes."

When Sebastian stopped for supper and didn't start spraying until two hours after mixing the insecticide, he had already exhausted the half-life and was effectively spraying at half the recommended rate. That's why he got such poor spider mite control.

Gubbels says it's important for farmers to be aware of pH when spraying. One way for farmers to tackle the issue is by using an acidifier/surfactant like LI 700 to ensure a neutral pH.  

"You also need to follow the proper procedure when filling the sprayer," he says. "In the case of dimethoate, for example, you need to put the water in the spray tank, add LI 700 as per label instructions with the agitator going to ensure it's mixed, and then add the insecticide."

Congratulations to Steve Smith, Smiths' Apples and Farm Market, for his correct answer. BF