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Insect Issues: Growing Insect Populations

Posted: June 26, 2012

As the worst of the armyworm outbreaks seem to be in the past, we are able to turn our attention to other pests of concerns.

Potato leafhoppers are at the top of the list because their populations are quite high in some areas and alfalfa fields are succumbing to hopper burn. Scout those fields to prevent this damage from developing because typically once you see the damage economic loss has already occurred.

Other pests that are starting to become noticeable in fields are grasshoppers and Japanese beetles. Grasshopper populations did very well in 2011 when it was hot and dry, so plenty of eggs were likely laid last year. This year we are starting to see the result of those many eggs with populations of grasshopper nymphs developing locally in small grain, corn and soybean fields. Grasshopper populations tend to develop first along field edges, often after adjacent alfalfa or small grains have been harvested, so treating border rows can be a good approach. Controlling adults is more challenging that controlling nymphs, so keep an eye on fields so you are not caught off-guard. Economic thresholds for grasshoppers in corn and soybeans are poorly developed for the eastern US. Growers can use the informal range of 20-40 grasshoppers per square yard to help guide their treatment decisions. Organophosphates tend to provide less residual control against grasshoppers than pyrethroids. If our weather continues to be hot and dry, insecticide use against grasshoppers has potential to flare spider mite populations, so if you decide to treat for grasshoppers return to those fields regularly to watch for development of other pest populations. See the Penn State Agronomy Guide for insecticide options.

Japanese beetle populations also appear to be growing. This pest species goes in cycles with low years followed by high years. For the past few years populations have been low, but this year populations appear to be growing again and growers will need to watch their soybean fields for developing populations. Control of Japanese beetle (and other defoliators) should be driven by threshold-based decisions. Soybean thresholds for leaf-feeding pests are: 30% defoliation during vegetative stages; 15% defoliation for bloom to pod-fill; and 25% pod-fill to maturity. Assess defoliation on the canopy as a whole. One sampling tactic is to randomly sample 10 plants and pick one trifoliate leaf from the top one-third of the canopy, one from the middle third, and one from the bottom third. From each trifoliate, discard the least and most heavily damaged leaflets, then with the remaining 30 leaflets, estimate the percent defoliation.

Contact Information

John Tooker
  • Penn State Entomology Specialist