Counteracting adverse weather effects on deposition and degradation of insecticides used in berries

Michigan is in the middle of a period of extreme heat and drought that affects not only the crop physiology, but also the way insect pests respond to pesticides used.

Currently, with the harvest of blueberries and raspberries in progress, there is a need to prevent insects and diseases to become a post-harvest problem. At this time, spotted wing Drosophila and Japanese beetles are the main concern for fresh and processed blueberries and raspberries. Spraying pesticides under the prevailing weather conditions presents some challenges that growers need to consider before applying pesticides to their crops.

Insecticides, as any other pest control tool require specific conditions to obtain the maximum benefit or to be effective in preventing pest damage. To be effective, the spray carrying pesticide needs to reach the target (the crop or the pest) to put the pest in contact with the pesticide. Once the product is deposited in the target area, its activity will depend on its duration as residue on the target and on the probability of the pest getting in contact with the residue. Mobile pests like blueberry maggots, Japanese beetles and spotted wing Drosophila have a greater opportunity to enter in contact with the pesticide residue than aphids or sessile pests that do not move a lot.

Most pesticides are affected by the environmental conditions in which they are released. Temperature and relative humidity are two major factors responsible for the permanence of the pesticide in the target area and for the metabolization of the pesticide by the insect pest.

High temperature and low relative humidity will cause pesticide droplets to evaporate quickly, reducing drastically their volume before reaching the target. This may reduce the amount of product deposited on the insect habitat. This, in turn, will reduce the amount of pesticide deposition needed for an effective pest control and may create pesticide drift that will be lost in the environment.

The other important impact of high environmental temperatures is in the performance of the insecticide. Temperature is one of the major factors for environmental pesticide degradation. It also influences the methabolic activity of the insect that may lead to an increased insecticide detoxification or toleration. According to Dr. Janet Knodel from North Dakota State University Extension, some organophosphate (OP) insecticides increase their toxicity to insects with increased environmental temperatures. However, pyrethroids insecticides such as Asana XL (esfenvalerate), Baythroid XL (beta-cyfluthrin), Delta Gold (deltamethrin), Mustang Max (zeta-cypermethrin), Ambush or Arctic (permethrin), or Warrior II (lambda-cyhalothrin), decreases as temperature increases. (For more information, see Hot Temperatures will Impact Soybean Aphids and Insecticide Control by Dr. Knodel.)

Our current recommendation for controlling spotted wing Drosophila and Japanese beetles includes the use of OP and pyrethroid insecticides. Both insecticides are affected by high environmental temperatures. For example, Malathion sprayed on strawberry flowers decreased to 2.70 percent of the initial concentration within two days of application, 0.93 percent after three days and 0.50 percent within seven days (according to the National Pesticide Information Center’s Malathion Technical Fact Sheet). This decrease on the amount of insecticide residue is temperature-dependent; as temperature increases, Malathion residues decrease.

To prevent the loss of activity or effectiveness of insecticides applied under high temperatures and drought conditions against pests affecting both blueberries and raspberries, we recommend:

  1. Spray at day hours when temperatures are below 90 degrees Fahrenheit (32 degrees Celsius). At high temperatures, Malathion residues decline rapidly due to volatilization.
  2. Adjust the sprayer to deliver large droplets: more than 300 microns in diameter.
  3. Avoid concentrated sprays; use the maximum recommended volume of water.
  4. Finally, calibrate the spray equipment to match the conditions of the target crops, i.e., type of canopy and height of the target crop, to deliver the right volume of water and maximize pesticide deposition on the target.

If you have questions or require assistance with your pesticide application program, please contact MSU Extension small fruit educator .(JavaScript must be enabled to view this email address) at 616-260-0671.

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