Conservation of Beneficial Insects while Controlling Cropland Grasshoppers
Contributors: David K. Weaver, Tyler J. Lane, Samuel S. Anderson, Tim Seipel, and Hayes Goosey
If you are planning on controlling grasshoppers in cereal or other crops this year, you should consider impacts on non-target organisms, such as honeybees, native pollinators, specialist parasitoids, and predators that kill pests. All these organisms are vulnerable to the modes-of-action of most insecticides used to control grasshoppers. All these beneficial insects help with crop yields, by either pollinating or by killing pests that will impact yield at harvest, such as wheat stem sawfly.
Two native parasitoids, Bracon cephi and B. lissogaster are known to survive only by using wheat stem sawfly larvae as their primary host. They kill larval wheat stem sawflies by consuming them inside the stems, where the larvae are feeding. Growers who have had to treat grasshoppers in their cereal crops, especially for multiple applications over several years, have frequently reported increased levels of wheat stem sawfly damage. This is directly related to killing wheat stem sawfly parasitoid adults, which help prevent yield loss and reduce wheat stem sawfly populations.
Pollinator and parasitoid mortality occurs when insecticides are used to control grasshoppers. The insecticides in question are those that are less expensive options for grasshoppers and can kill both immatures and adults. Such insecticides are contact insecticides and are strongly recommended to be applied in the morning to control grasshoppers - which are day-active. Thus, even the guidelines for spraying at dusk or at night to protect pollinators are not considered when managing grasshoppers.
Diflubenzuron, or Dimilin, is more expensive and requires scouting to detect bands of immature cropland grasshopper nymphs earlier in season. It can be applied without impact on parasitoids or pollinators because it kills immature insects that must go through a molt or molts before finishing metamorphosis. Diflubenzuron will not kill adults; thus, it is safe for use with both early and late-season adult parasitoids - or honeybees or native pollinators.
It's important to note: 1) Diflubenzuron will also kill cutworms in crop when targeting grasshopper nymphs, and 2) USDA-APHIS has a large spray program in MT rangeland that targets grasshoppers in areas that are 10,000 contiguous acres. This helps reduce grasshopper populations on the landscape that might migrate into crops. APHIS only applies diflubenzuron and stops spraying when grasshopper populations are adults, because of these same concerns about beneficial insects.
For more information on grasshopper monitoring, see Grasshopper Management and Control in Montana - https://apps.msuextension.org/magazine/articles/5527
Contact: Hayes Goosey, Ph.D.
Assistant Professor and Extension Forage Specialist, Montana State University
Dept. of Animal and Range Sciences, Office: 406.994.5688, [email protected]