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What's New with the Soybean Aphid?

November 3, 2000
Despite the onset of autumn, considerable activity regarding the soybean aphid, Aphis glycines, has continued since we wrote our last article about it in the Bulletin (issue no. 22, September 8, 2000). Entomologists and plant pathologists from many states in the Midwest and elsewhere have been delving into the literature to learn more, meeting to discuss what we know and don't know, and preparing grant proposals to obtain funding to mount a robust research program. We want to be able to offer some reasonable management suggestions if the soybean aphid resurges in 2001 and colonizes soybean fields. People will want answers, and we still have a great deal to learn.

Regarding the soybean aphid's distribution, we have learned that this invasive pest moved far and wide in a relatively short time in 2000. In issue no. 22 (September 8, 2000) of the Bulletin, we listed counties in which we had found the aphid in Illinois during our surveys in late August and early September. Well, the list of counties has become almost meaningless; we have found the aphid in every county in Illinois that we have surveyed. Although we did not examine soybean fields in all counties, the counties in which we found the soybean aphid include counties along Illinois's entire perimeter. The heaviest infestations in soybeans occurred in counties north of I-80, but we were able to find small numbers (a female and a small number of young [nymphs] on some leaflets) in most of the fields we sampled elsewhere. During August and September 2000, entomologists and plant pathologists found the soybean aphid in Illinois as far north as Winnebago County, as far south as Pope County, as far west as Adams County, and as far east as Vermilion County. That's it, folksthe aphid spread throughout our state in 2000.

You probably recall that other states that had heavy populations of soybean aphids were Wisconsin, Michigan, and Minnesota. During September, we learned that the soybean aphid also was found in Indiana, Iowa, Missouri, and Ohio. Entomologists from Kentucky confirm one finding of the aphid in their state. And someone found the aphid in West Virginia. That's some serious traveling in a year.

So, what should we expect in 2001? The wide distribution of the soybean aphid in 2000 may not mean a lot if it doesn't survive the winter well. And overwintering survival is not dependent solely on weather. In late summer and fall, winged soybean aphids fly to their primary overwintering hosts, species of Rhamnus, also known as buckthorn, a woody perennial. The aphids reproduce sexually on Rhamnus, and females lay eggs that overwinter. In the spring, the aphids reproduce asexually and complete three generations on Rhamnus. Adults of the third generation develop wings and fly from Rhamnus in search of soybeans. So it's obvious that the availability of Rhamnus will play a pivotal role in the soybean aphids' survival and distribution.

Three species of Rhamnus are native to Illinois. Rhamnus alnifolia is very rare in the northern part of Illinois and is listed as state endangered. Rhamnus lanceolata is rare to occasional in the northern two-thirds of Illinois. Rhamnus caroliniana is not very common in the southern third of the state. Much more common in Illinois are the introduced species: R. cathartica (common buckthorn), R. frangula, and R. davurica. The acceptability of the abundant exotics and less abundant native species of Rhamnus will have a significant role in the potential of the soybean aphid to become a serious pest throughout the soybean-growing regions in the United States.

What effect will large numbers of soybean aphids in 2001 have on soybean yields? In the short term, this is one of the most important questions we must answer. Results from a very preliminary study conducted by entomologists and plant pathologists in Wisconsin suggested that 80 to 100 soybean aphids per leaflet might cause an 8-bushel-per-acre yield loss. Results from a study in China indicated that soybean aphid injury could cause as much as 28% yield loss. However, that study focused on aphids attacking young soybean plants. The effects of soybean aphids feeding on reproductive-stage soybeans have not been studied thoroughly. Therefore, some of our research planned for 2001 is focused specifically on studying the relationship among numbers of aphids, time of attack, and subsequent effects on plant growth and yield.

How many soybean aphids are "too many," and what insecticides will control the aphids if applications seem justified? Because we know so very little about the potential impact of the aphids' feeding on soybean yield, let alone the potential for the aphids to transmit viruses, treatment guidelines do not exist for these aphids in North America. In addition, no insecticide has been registered specifically for control of soybean aphids in North America. Therefore, we will be flying by the seat of our pants in 2001 until we have more definitive information.

In an attempt to determine what insecticides might control soybean aphids, John Shaw (research scientist in the Center for Economic Entomology in the Illinois Natural History Survey) established an insecticide efficacy trial in Carroll County on August 25, 2000. The trial was a randomized complete block with four replications, with two untreated check plots per replication. Eighteen insecticide treatments were included in the trial, including different application rates for some treatments. Nine of the insecticides were labeled for use on soybeans, six were not labeled for use on soybeans. Only the data for labeled insecticides are presented.

John sampled the aphids in each plot before he applied insecticides. The average number of aphids in the untreated check plots was 53.9 aphids per leaflet. Three days after the treatments were applied, aphid densities in the untreated check plots had increased to an average of 144.9 aphids per leaflet. However, aphid densities "crashed" (effects of natural enemies and dispersal) to an average of 8.6 aphids per leaflet in the untreated check plots 10 days after the treatments. Therefore, measurement of the residual efficacy of different insecticides for 10 days was tenuous. Nevertheless, several insecticides provided good control of soybean aphids for the short duration of the study.

Results from the insecticide efficacy study in Illinois in 2000 are presented in Table 2 and Table 3. The data in Table 2 are the percentage reductions in numbers of aphids from pretreatment levels to levels on both 3 and 10 days after treatments were applied. These reductions are based on average numbers in the plots before treatments were applied and average numbers in the same plots 3 and 10 days after treatments were applied. No comparisons with average numbers of aphids in the untreated check plots are included. The data in Table 3 reflect percentage control of soybean aphids 3 and 10 days after treatments were applied. Percentage control is based on the average number of aphids in plots treated with a given insecticide compared with the average number of aphids in the untreated check plots on that same day. However, keep in mind that densities of aphids had "crashed" in the entire plot area 10 days after the plots were established.

At least five insecticides reduced numbers of aphids by 95% or more by 3 days after treatment (DAT) (Table 2). By 10 DAT, numbers of aphids had been reduced by at least 90% in seven plots treated with insecticides. All but one registered insecticide, Pounce 3.2EC, reduced numbers of aphids by at least 75% by 10 DAT. Control of aphids by all but one registered insecticide, Safer Soap, exceeded 80% by 3 DAT. By 10 DAT, control of aphids by Pounce 3.2EC, Warrior T at 0.02 lb a.i./A, Asana XL, and Safer Soap was less than 50%. Dimethoate, Lannate SP, Lorsban 4E, Penncap-M, and Warrior T at both 0.025 and 0.015 lb a.i./A seemed to provide the best control of soybean aphids for the duration of the study.

So, where do we go from here? A large team of entomologists and plant pathologists from the Department of Crop Sciences, Department of Natural Resources and Environmental Sciences, Illinois Natural History Survey, and USDA­ARS have assembled to address many aspects of research related to the soybean aphid. Our team in Illinois will interact regularly with similar teams being assembled in other states. At the very least, we will monitor their distribution and dispersal in 2001 and 2002 and study their biology in greenhouses and laboratories. We also will determine the aphids' ability (or lack thereof) to transmit viruses. If infestations of soybeans develop in 2001, we will mount a significant research program to examine population dynamics; effects of feeding injury on soybean plant growth, development, and yield; impact of natural enemies on aphid populations; and efficacy of insecticides. In addition, we will attempt to develop treatment guidelines or economic thresholds. We have our work cut out for us, but we believe it is important to address soybean producers' concerns very quickly if the soybean aphid poses a significant threat to soybean production in the United States.--Kevin Steffey and Mike Gray

Author: Kevin Steffey Mike Gray

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

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