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Soybean Aphid Update

July 24, 2003

Since the most recent issue (no. 17, July 18, 2003) of the Bulletin was printed and mailed, there has been considerable telephone and e-mail traffic regarding soybean aphids in northern Illinois. We have received numerous reports regarding insecticide applications to control the aphids in soybean fields. Although some fields still do not have densities of aphids large enough to warrant control, the densities in a fairly large number of fields have exceeded economic thresholds. In addition, some people are finding very few natural enemies in some fields, so aphid population densities have increased rapidly.

Drs. David Voegtlin (Illinois Natural History Survey) and David Onstad (Department of Natural Resources and Environmental Sciences) recently visited several soybean fields in Kendall County. They have been studying soybean aphid populations there since 2001. In fact, one of the first fields of soybeans Dave Voegtlin visited in 2000 was in Kendall County. They have provided an overview of the soybean aphid situation and have reiterated our soybean aphid management guidelines. Following is an article they wrote upon their return from northern Illinois last week.--Kevin Steffey

Fourteen fields in Kendall Township, Kendall County were sampled for soy-bean aphids on Wednesday, July 16 by David Onstad, David Voegtlin, Ramie Wells, Doris Lagos, Mike White, and Mike Just. Each field was sampled by counting all the aphids on each of 50 plants, noting the presence of winged aphids as well as predators and parasite mummies. This is the third year of the intensive sampling program in the same township.

The numbers of aphids seen were dramatically larger than in previous years for the same time period. The mean number of aphids per plant was 276, compared to 16 in 2001 and 5 in 2002, already exceeding the mean peaks observed in the last 2 years in August. All plants sampled were infested, and the mean number of aphids per plant ranged from 110 to 636. Aphids were observed throughout the plants: on stems and lower trifoliates as well as the growing tips. Ignoring lower populations in severely hail-damaged fields, the mean density in Kendall Township rose 100 times since the first observations 3 weeks ago. (Note that in a similar effort in Champaign County, the team found virtually no aphids in 14 fields.)

One of the surprising observations was the presence of many winged individuals (alates). It was uncommon to find a plant without the presence of alates. We believe that the alates that develop on any plant will leave that plant and fly before settling on another soybean plant for feeding and reproduction. It was common to see winged aphids feeding on the plants with a few first-instar nymphs around them. It seems that even plants that already have quite large populations have winged individuals landing on them and producing offspring. In addition, teneral (newly molted) alates were seen on most of the plants, with populations in the hundreds, and on many of these plants large numbers of alatoid nymphs were observed. These will molt and disperse from the field, moving in whatever direction the wind will take them.

We have also found winged aphids for the first time this year in three of the nine suction traps around the state (Freeport, DeKalb, and Joliet). This is another indication that aphid flight is much greater and earlier this year than in the past. Much of central Illinois could be infested if we have a moderate north wind carrying the large number of winged aphids being produced in the northern part of the state.

(Fun with numbers: Let's assume that in an 80-acre field, 100,000 plants per acre, with 100% of the plants infested at an average of 500 aphids per plant, 10% of the population will become winged and emigrate each day. So, 80 x 100,000 x 500 = 4 billion aphids per field. Ten percent would mean that 400 million alate aphids emigrate from one 80-acre field daily.)

The most abundant predator group noted was syrphid adults. These were abundant in every field, often landing on the counter's hand or on the plant where the aphids were being counted. Syrphid larvae were the most abundant predators on the plants. Relatively few Harmonia axyridis were observed; probably not even half of the plants examined had either adults or larval coccinellids on them. Eggs of chrysopids were common. Parasitized aphids were seen on maybe 20% of the plants (Aphidiidae, Braconidae) in fields with the greatest aphid densities. We don't believe there were enough predators or parasites present in any of the fields to have any significant impact on the aphid numbers.

What do we recommend? First, we suggest rereading the soybean aphid fact sheet prepared in 2002 (online; portions are reprinted at the end of this article). We plan to send a team back to Kendall County on Monday, July 21 to measure changes in aphid populations and in particular to document the percentage of the population that is alatoid nymphs. With the moderate densities of natural enemies and potential emigration of newly formed winged aphids, it is possible that the aphid population in a field may decrease.

For fields with an average of over 625 aphids per plant or of 25 aphids per leaflet on 25 randomly sampled leaflets (fact-sheet sampling protocol), an insecticide may be worthwhile. However, we strongly recommend sampling the field twice over 4 to 5 days to note increases or decreases. Don't spray if the population is declining by the second sampling. The problem with using an insecticide is the potential for the aphid population to rebound after all the natural enemies have been killed in a field. Remember, other fields in the area are sending out winged aphids that may reinfest your field. The best insecticide application is not likely to kill more than 90% of the aphids, while at the same time it will kill most of the natural enemies. With dozens of aphids surviving per plant and more flying in, a rebounding population is likely several weeks following an insecticide application.

The decision is not an easy one. According to past research, pod set is the critical stage that should be protected. Researchers at the University of Illinois, Illinois Natural History Survey, and USDA are trying to develop better guidelines and predictive tools. For research and extension purposes, you may want to set aside a strip without insecticide for later harvesting and comparison to treated areas.

From the Soybean Aphid Fact Sheet

The need to treat soybean aphids with an insecticide should be based on information gathered from regular and thorough scouting procedures. The combination of predators, weather, and aphid biology often causes soybean aphid populations to "crash" within given fields. Therefore, we strongly encourage appropriate assessment of the situation in each field before making a control decision. Consider the following as guidelines for a soybean aphid management program.

Natural control. Intense rainfall may kill many aphids by knocking them off the soybean plants. In addition, higher humidity increases the potential for fungal disease organisms to infect soybean aphids.

Naturally occurring predators, parasitoids, and pathogens can suppress soy-bean aphid populations. Lacewings, predatory bugs, and the multicolored Asian lady beetle are particularly im-portant predators of soybean aphids. For example, each lady beetle can consume several hundred aphids during its lifetime. Thus, if you see one lady beetle on every plant, then biological control may effectively suppress soybean aphid populations.

Soybean aphid populations also may decline rapidly because stresses on the crop and crowding by the aphids cause a generation of winged aphids to form. Subsequently, almost all aphids fly away from the field. This dispersal from a field can be predicted by monitoring for the percentage of alatoid nymphs in the population. A high percentage of alatoid nymphs (nymphs with "shoulder pads") within a field indicates the forthcoming occurrence of winged adults that will leave in search of other fields.

Insecticide application. Before considering use of an insecticide, determine whether natural enemies or the presence of alatoid nymphs will cause a population "crash" within the week. Also, note that because an aphid population can increase 10-fold in one week, especially without predators to control them, the pest may rebound after an insecticide application to levels similar to those before the application.

To protect pod set, sample the aphid population at least twice during late V and early R stages. An application of an approved insecticide may be justified if these conditions prevail:

  • Density of aphids is above 25 per leaflet on average.
  • Density of aphids increases from first to second sample.
  • Percentage of alatoid nymphs is less than 50%.
  • Soybean stage is R1 or R2.

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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