Insects Active in Crop Fields
Posted: July 3, 2012
Scout fields and apply insecticides as necessary to prevent this damage from developing because typically once you see the damage economic loss has already occurred. We have received at least one report of aphid populations, likely the cowpea aphid, developing in an alfalfa field following a pyrethroid application to control leafhoppers. Quite likely this is a case of secondary pest outbreak caused by the insecticides knocking back populations of natural enemies of the aphids. This phenomenon is somewhat common with pyrethroid applications; therefore, treated fields should always be scouted in the weeks following applications to determine if the treatment was effective against the target pest species and to see if other pest populations have developed. If you have populations of cowpea aphids, an appropriate threshold would be 10–12 per stem on new regrowth just after cutting, or more than 60 per stem when the hay is 12 inches or taller. This threshold is from the University of California, which deals with cowpea aphid far more routinely than those of us in Pennsylvania.
Potato leafhoppers are occasionally a problem in soybeans, and with populations as we currently see soybean fields should be watched. Symptoms of damage in soybeans look very similar to the 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.
Another pest species that bears mentioning is soybean aphid. Since 2001, soybean aphids had been on a two-year cycle with economically damaging populations developing in scattered fields in odd-numbered years, and only minor populations developing in even-number years. This pattern broke down in 2011 when aphid populations failed to materialize across Pennsylvania. Now in 2012, we are not exactly sure what to expect, but have had three reports from Berks, Dauphin, and Lancaster Counties of fields with smaller numbers of aphids in soybean fields. Growers should keep this in mind and scout your fields for the small pale-green aphids on the newest growth and underside of leaves. Do not confuse with slow-moving aphids with the fast-moving and brighter green potato leafhoppers, which are common in soybeans this year and also found on the underside of leaves. The economic threshold for soybean aphid is 250 aphids per plant, and growers should keep in mind that treating prior to populations reaching this threshold is likely to be uneconomical and perhaps even disruptive (i.e., secondary pest outbreaks) if natural enemy populations are killed with a premature insecticide application. Of all crop species, soybean harbors the best natural enemy populations that contribute natural pest control against soft-bodies insects like aphids.
- PSU Entomology Specialist