Grape Colaspis In Corn and Aphids in Soybean: Will There Be a Repeat Performance in 2001?

Kevin Steffey

Professor and Extension Specialist in Entomology

Office: S-322 Turner Hall
Phone: (217) 333-6652


The year 2000 will be remembered by many in Illinois as a year of unusual occurrences in the insect world. For the third consecutive year, secondary insect pests of corn caused problems in various regions of the state. The grape colaspis was particularly problematic throughout the central third of Illinois, frustrating both growers and their advisers. The damage was severe enough in some fields to warrant one or two replantings. Our inability to predict their occurrence and the lack of reliable control measures continue to hamper our ability to manage this pest.

The appearance of an insect pest of soybean formerly not known to occur in North America posed another challenge for producers this year. The soybean aphid, Aphis glycines, infested many soybean fields in the upper Midwest, including northern Illinois, late in the 2000 growing season, generating a great deal of activity and media coverage. By late August, densities of soybean aphids began to decline dramatically, leaving us with many unanswered questions. The stage is set for some meaningful research in 2001.

Grape Colaspis

Situation in 2000

Grape colaspis was the focus of many growers' concerns by early May and became the most notorious insect pest of corn in the central third of Illinois for the next six weeks. Seedling corn plants infested with grape colaspis larvae showed classic symptoms of injury-stunted, wilted plants with purple stems and "burned" leaf edges. Most of the fields infested in 2000 had been planted to soybean in 1999. In the past, I have written, "The grape colaspis is a sporadic pest most often found in corn planted after red clover or mammoth clover, and occasionally in corn planted after sweet clover, alfalfa, or soybeans." Information to support that quotation originated from old scientific literature. Apparently, we need to reconsider the statement about "occasionally in corn planted after ... soybeans." Very few acres of clover currently are grown in Illinois, so that preferred host is no longer readily available. The grape colaspis may have adapted to modern corn-soybean rotation to survive.

Insecticide Efficacy Trial, 2000

We conducted an insecticide efficacy trial to compare products for control of grape colaspis in Menard County in 2000. For each replicate by treatment combination, we dug a trench 1 meter (3 feet) long and 6 inches deep and recorded the numbers of living corn seedlings. We examined all of the soil from the trench for grape colaspis larvae and recorded the numbers found. The plants in many of the plots were stunted and purple, symptoms of injury attributable to both grape colaspis and white grub larvae, which also occurred in the plot. We did not attempt to distinguish which insects caused the injury.

We observed considerable variation in numbers of live plants and grape colaspis among the plots. Only plots treated with Regent 4SC had significantly more live plants (eight plants per meter) than in the untreated control (about four plants per meter). The numbers of grape colaspis larvae per meter of row in insecticide-treated plots were not significantly different from the number of grape colaspis larvae in the untreated control. This observation compares well with past testimonials regarding lack of performance of soil insecticides against grape colaspis.

One final note about this trial is in order. No soil insecticides currently are registered for control of grape colaspis larvae. Results from this experiment should not be used to justify the application of a product for grape colaspis control.

Soybean Aphid

Situation in 2000

In late July, entomologists and plant pathologists at the University of Wisconsin observed heavy infestations of aphids in soybean fields in southern Wisconsin. They tentatively identified the aphid as the cotton aphid, Aphis gossypii, and alerted us to their presence. We verified the presence of aphids in soybean fields in northern Illinois in early August. Some infestations were heavy, especially in fields of late-planted soybean, and applications of insecticides were common in some areas. After receiving some specimens in early August, David Voegtlin, an aphid specialist at the Illinois Natural History Survey, identified the aphids as Aphis glycines, a species heretofore known to occur only in Asia, Australia (recent occurrence), and some Pacific Islands. The soybean aphid is 2-4 mm long and pale green with black cornicles ("tailpipes") on a pale rear end.

By the end of September, the soybean aphid had moved far and wide. Although we did not examine soybean fields in all counties, the counties in which we found soybean aphids include counties along Illinois' entire perimeter. The heaviest infestations occurred north of I-80, but we found small numbers (a female and a small number of young on some leaflets) in most of the fields we sampled elsewhere. During August and September, we 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. Heavy populations of soybean aphids also occurred in Wisconsin, Michigan, and Minnesota. By September, soybean aphids also had been found in Indiana, Iowa, Kentucky, Missouri, Ohio, and West Virginia.

By late August, densities of aphids began to decline dramatically. Heavy rains and the activity of some natural enemies, especially the multicolored Asian lady beetle and a fungal organism, reduced the numbers of aphids. Within about six weeks, we experienced a significant rise and subsequent fall of aphid populations, leaving us little time to initiate much research.

Life Cycle and Injury Caused

The life cycle of the soybean aphid can be summarized as follows. Female soybean aphids reproduce in soybean throughout the summer, giving birth to living young and producing as many as 18 generations. In late summer and fall, winged females leave soybean fields and fly to their overwintering host, Rhamnus species, a woody perennial commonly called buckthorn. On buckthorn, males are produced, and males and females mate. Females lay eggs that overwinter on buckthorn. In the spring, nymphs hatch, then become adults, and adult females give birth to living young on buckthorn, completing two to three generations on this host. In late spring, winged aphids leave buckthorn to colonize soybean fields.

Soybean aphids suck fluids from soybean plants, and injured leaves turn yellow and crinkled. Sooty mold may grow on the "honeydew" that is excreted by aphids. The Asian literature indicates that the aphids can cause as much as 28 percent yield loss if they infest seedling soybeans. However, little information about infestations later in the summer exists. Preliminary information from the University of Wisconsin suggests that 80-100 aphids per leaflet may cause an 8-bushel-per-acre yield loss. The soybean aphid is known to transmit viruses in Asia; its ability to transmit viruses in North American is not known.

Insecticide Efficacy Trial, 2000

John Shaw, Research Scientist at the Illinois Natural History Survey, conducted an insecticide efficacy trial in Carroll County to determine what insecticides might control soybean aphids. Nine of the insecticides were labeled for use on soybean; six were not labeled.

John sampled the aphid populations in each plot before applying insecticides. The average number of aphids in the untreated check plots was 53.9 aphids per leaflet. Three days after the treatments (DAT) 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 DAT. Therefore, measurement of the residual efficacy of insecticides for 10 days was tenuous. Nevertheless, several insecticides provided good control of soybean aphids for the short duration of the study.
shows the percentage reductions in numbers of aphids from pretreatment levels to levels on both three and 10 DAT, based upon average numbers in the plots before treatments were applied and average numbers in the same plots three and 10 DAT. Shows percentage control of soybean aphids three and 10 DAT, based upon 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.

At least five insecticides reduced numbers of aphids by 95 percent or more by three DAT. By 10 DAT, numbers of aphids had been reduced by at least 90 percent in seven plots treated with insecticides. All but one registered chemical insecticide, Pounce 3.2EC, reduced numbers of aphids by at least 75 percent by 10 DAT. By 10 DAT, control of aphids by Pounce 3.2EC and Asana XL was less than 50 percent. Dimethoate, Lannate, Lorsban, Penncap-M, and Warrior seemed to provide the best control of soybean aphids.

Management of Grape Colaspis and Soybean Aphid in 2001

We do not know whether these two pests will occur in significant numbers in 2001. Winter weather conditions may dictate their population levels in 2001. We also do not know how successfully (or unsuccessfully) soybean aphids found buckthorn plants on which to overwinter. Consequently, we enter the 2001 growing season with considerable uncertainty about both pests. Unfortunately, we still have very few options for controlling grape colaspis; no soil insecticides or seed treatments are registered for control of this pest.

We will monitor first occurrences of the soybean aphid in 2001 and keep track of their population dynamics. If we detect escalating densities and widespread occurrence of soybean aphid colonies, we will sound the alert to prepare people for their onset. We also will conduct insecticide efficacy studies and establish research trials to measure the impact of soybean aphids on soybean yield and detect their potential for transmitting viruses. Although we have much to learn, we will share the results of our efforts as quickly as possible

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