No. 21 Article 2/August 17, 2007

Soybean Aphids Continue to Cause Concern, Although Most Soybeans Are in "Safe" Stage of Development

We continued to receive reports of heavy infestations of soybean aphids in soybean fields during the week of August 13, with most of the most recent reports from central and northeastern counties. In many instances, aphid densities increased from relatively low to threatening in a relatively brief period, despite the high temperatures that have prevailed. In other fields, aphid population development slowed or even stopped. We observed both population trends among the infestations of soybean aphids in the 26 commercial soybean fields we have been surveying regularly since mid-June. During August 6 through 8, average densities of soybean aphids in our surveyed fields ranged from 21.5 aphids per plant (Bureau County) to 916 aphids per plant (one field in Stephenson County). Many of the 26 fields in our survey were sprayed for control of soybean aphids, so we discontinued surveys in these fields. However, some cooperators agreed to leave untreated check strips in the fields, so we have continued our surveys in the more confined areas.

Soybean leaflet treated with an insecticide (top leaflet) and not treated (bottom leaflet) , JoDaviess County, Illinois, August 9, 2007 (University of Illinois).

We have learned that the multicolored Asian lady beetle is beginning to show up in soybean fields heavily infested with soybean aphids. The late arrival of these predators should have a suppressive effect on soybean aphid populations before winged aphids depart soybean fields to seek buckthorn for overwintering. On August 9, I visited a soybean field in JoDaviess County that has been monitored all summer by David Feltes, University of Illinois Extension IPM educator in the Quad Cities. Multicolored Asian lady beetle larvae were feasting on the large numbers of soybean aphids present.

Multicolored Asian lady beetles feeding on soybean aphids, JoDaviess County, Illinois, August 9, 2007 (University of Illinois).

We received several telephone calls from individuals who had observed near-threshold numbers of soybean aphids on stage 5 or stage 6 soybeans and were wondering what to do. Our advice was to not treat. As we have noted before, although the economic threshold is 250 aphids per plant, economic yield loss does not occur until soybean aphid densities exceed 600 aphids per plant. Most data to date suggest that an insecticide application to R6 soybeans infested with soybean aphids will not pay for itself in yield benefit. Consequently, a density of 200 or 300 aphids per plant on late-stage 5 or on stage 6 soybeans will not threaten yield.

As I stated in last week's issue (No. 20, August 10, 2007) of the Bulletin, we invite others who have untreated areas in otherwise treated fields to provide us with yield data when they obtain them, including stage of soybean development and average density of soybean aphids at the time of treatment. We are particularly interested in obtaining data for fields of R5 or R6 soybeans that were (or will be, although not recommended) treated.--Kevin Steffey

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