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The Soybean Aphid Enigma—Will They Or Won’t They?
| Kevin Steffey Professor and Extension Entomology Specialist Department of Crop Sciences (217) 333-6652; ksteffey@illinois.edu |
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After the “invasion” of soybean aphids in 2000, the situation settled down a bit in Illinois in 2001. Although some areas of Michigan, Minnesota, and Wisconsin were heavily infested with soybean aphids (Aphis glycines) in 2001, heavy infestations in Illinois were elusive. In many fields in northern Illinois, populations of soybean aphids increased to large densities and then “crashed,” seemingly overnight. The populations crashed from a combination of activity of natural enemies, weather, and population aging. The latter was an important finding in 2001. We learned that when a large percentage of colonies is comprised of alatoid nymphs (nymphs with wing pads, which then become winged adults), most of the aphids disappear from the field within a week. We incorporated this information into our management recommendations for the 2002 growing season (see below).
Figure 1. Soybean aphids on a soybean leaf.
Although field surveys and determination of distribution of the soybean aphid in Illinois will continue, we are focusing more on the impact of soybean aphids on soybean yield in 2002. Research entomologists in the northern states reported a wide range of yield responses to infestations of soybean aphids in 2001. Yield reductions in excess of 16 bushels per acre were measured in non-replicated strip trials in Minnesota in 2001, with an average loss of more than 6.2 bushels per acre (13.9 percent yield loss). In one replicated insecticide efficacy trial in Michigan, the average density was almost 7,000 aphids per plant. Researchers recorded a 46 percent difference in yield between the best insecticide treatment and the untreated check.
Soybean Aphid in Illinois, 2001 |  |
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Figure 2. Distribution of soybean aphids in Illinois in 2001.
However, the potential for yield loss caused by soybean aphids is affected by many factors that have not been investigated thoroughly—planting time, soybean cultivar, time of initial infestation of aphids, time of peak infestation of aphids, and effects of weather and natural enemies. Preliminary studies indicate planting date plays a role in aphid population build-up and subsequent feeding injury; late-planted soybeans seem to be more susceptible to large aphid population build-up and injury. This hypothesis will be tested in many fields in Illinois in 2002 because of the lateness of soybean planting this year.
We will continue to try to correlate densities of soybean aphids with soybean yield. In addition, plant pathologists and entomologists will investigate the potential for soybean aphids to transmit viruses such as soybean dwarf virus and soybean mosaic virus. In the meantime, we offer the following management guidelines.
To protect pod set, we recommend that you sample the aphid population at least twice during the late V and early R stages. An application of an approved insecticide may be justified if the following conditions prevail:
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Average density is 25 or more aphids per leaflet.
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Density of aphids increases from first to second sample.
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Alatoid nymphs make up less than 50 percent of the population.
- Soybean stage is R1 or R2.
Increased application pressure and gallons per acre ensure good coverage and improve efficacy of an insecticide for control of soybean aphids. Also remember that spraying blossoming soybeans can be extremely hazardous to bees. Coordinate with local beekeepers before applying sprays.
As more information about the population dynamics of soybean aphids becomes available, we will adjust our management recommendations accordingly.
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