Insects Active in Soybeans and Corn
Posted: July 12, 2012
Insect and mite pest populations continue to build across Pennsylvania, but of course they are not everywhere. I scouted several fields yesterday and found minimal pest populations; therefore, scout your fields to assess the situation locally and only treat when necessary. Use of preventative sprays often does more harm than good as applications are not timed well enough to be effective or alternatively kill natural enemies that can help suppress pest species. We recognize that there is an increasing tendency to tank mix insecticides when other pesticide applications are to be made. Keep in mind that these applications can flare secondary pests like aphids and spider mites, and if products are oil-based applying them in the hot sun can have phototoxic effects.
Potato leafhopper populations remain high across Pennsylvania. As I mentioned last week, potato leafhoppers are occasionally a problem in soybeans, and in the last week I have seen plenty of soybean fields with hopperburn, so keep an eye on your fields for this type of damage. Symptoms of damage in soybeans look very similar to hopperburn commonly seen in alfalfa with yellowing developing at the leaf tip and then spreading in a V-shape down the midrib. Appropriate thresholds are five leafhoppers per plant for V4 stage soybeans, or nine hoppers per plant for R1-2 stage beans.
Other pest species to keep in mind are grasshoppers and two-spotted spider mites. These two species thrive in hot, dry conditions and are becoming increasingly common in many soybean and corn 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 than 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 U.S. 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. 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.
Two-spotted spider mites produce a distinctive stipling on leaves, and fields will slowly bronze as more and more leaves suffer from increasing damage. This bronzing will often start on a field edge, and this tendency of spider mite damage to accumulate on edges can be exploited for management purposes. If the interiors of fields are not infested, treatments can be applied to only the field perimeters.
If mites are present on the edge of fields, assess the rest of the field by moving at least 100’ into the field and scouting again. In this hot, dry weather, growers might want to develop an aggressive scouting schedule (about every other day). If you see leaf discoloration, can identify mites present in the field, and hot, dry weather continues to be forecast, a treatment may be necessary. Rains can help keep populations in check by reducing plant stress and allowing key beneficial fungi to be more effective. If you have a large field and see damage only along the field borders, a spray directed on the affected area and into the field about 100 feet farther may contain the problem. If you have a small field, plan on treating the entire field. Products labeled against spider mites do not kill eggs and have a short residual, so hatching spider mites can begin rebuilding the population in a few short days. For more details on spider mites, see our fact sheet.
One further note on chemical control of spider mites. Dimethoate is preferred by many for managing spider mites but it is good to realize that spider mite populations vary in their susceptibility to the toxin. Some populations are easily killed by the product whereas others are more difficult to manage. This variation has been recognized for some parts of the country for a while, but may also apply to Pennsylvania. Last season we heard of dimethoate applications that seemed to have no effect on two-spotted spider mites. We would be curious to hear from readers of other instances where they feel that this product was not effective, so please drop us an email. If you encounter a population that does not respond to dimethoate, you will need to consider other products labeled against two-spotted spider mite, such as products containing the active ingredients bifenthrin or chlorpyrifos. See the Penn State Agronomy Guide for details.