Issue No. 18, Article 2/July 23, 2004
Insects Infesting Soybean? Or Not?
Few of us will forget the outbreak of soybean aphids and the associated insect-control activities in 2003, in contrast with the relative lack of insect infestations in soybean in 2004. Thus far, both bean leaf beetles and soybean aphids have failed to make their presence known. However, some defoliation by Japanese beetle adults and other leaf chewers has been reported recently, and scouting for soybean aphids continues. So there is still plenty to discuss regarding management of insects in soybean.
Let's deal with soybean aphids first. Almost every report about soybean aphids we have received this year conveys the same message: There are very low numbers of soybean aphids, or none, in Illinois soybean. The average densities of aphids in soybean fields in Champaign, Kendall, Tazewell, and Woodford counties, reported by Dr. David Onstad (Department of Natural Resources and Environmental Sciences) and his crew, ranged from 0.005 to 0.43 per plant. These densities are far below the densities that have been found in Kendall County during this same period in previous years. Kendall County has been a hotbed of soybean aphid activity in Illinois since the pest's discovery in 2000.
Dr. David Voegtlin, Center for Ecological Entomology in the Illinois Natural History Survey, continues to obtain data from the suction traps located at nine sites throughout the state--(from north to south) Freeport, DeKalb, Joliet, Monmouth, Eureka, Champaign-Urbana, Perry, Brownstown, and Dixon Springs. Thus far in 2004, no soybean aphids have been collected from any of the suction traps. For comparison, during the week ending July 18, 2003, 176 and 131 soybean aphids had been captured in suction traps in Freeport and DeKalb, respectively. During the week ending July 25, 2003, the following captures of soybean aphids were recorded for the suction traps located at Freeport, DeKalb, Monmouth, and Eureka, respectively: 112, 940, 59, and 142. During the week ending August 1, 2003, 6,755 soybean aphids were captured in the suction trap in DeKalb. You can review the captures of soybean aphids in suction traps for 2001 (the year the traps were erected), 2002, and 2003 on our IPM Web site. The chart for 2004, with lots of zeroes, will be added soon.
It seems unlikely that soybean aphids will build up to economically threatening levels in Illinois this year. However, we must emphasize that we are still learning about this invasive pest, and the occurrence of soybean aphids is still unpredictable. During their journeys in the state, both David Onstad and David Voegtlin have noticed late-planted soybeans that may be attractive targets for buildup of soybean aphid populations. These fields also will serve as a "bridge" for aphids late this summer just before the aphids return to buckthorn to mate, lay eggs, and overwinter. We also are aware that some people have encountered "hotspots" of aphids in isolated fields. These "hotspots" contain very large numbers of aphids, but the areas infested, at least right now, are quite small. So continue to monitor for soybean aphids well into August. We may not have to deal with soybean aphids this summer, but we certainly do not want to be surprised by a sudden upsurge in their numbers.
On the other hand, Japanese beetles are causing some concern in some areas, and some fields of soybean have been treated with insecticides to prevent further defoliation. Japanese beetles have been pests of soybean in Illinois since the 1950s, at least in east-central counties. As you know, however, the distribution of Japanese beetles in Illinois has expanded to encompass most of the state, so excessive defoliation by this insect can cause yield loss in soybean almost anywhere in Illinois. Ron Hines, senior research specialist at the University of Illinois Dixon Springs Agricultural Center, has recorded some extraordinarily high numbers of Japanese beetles captured in traps in southern Illinois (e.g., 33,578 captured at the St. Clair County site during the week ending July 13).
Japanese beetles feeding on soybean leaves, July 9, 2003. (Photo courtesy of Angie Rusk.)
Fortunately, we have some very reliable thresholds associated with insect defoliation in soybean, thresholds that are based on percentage of defoliation rather than on counts of insects. Treatment to control insects defoliating soybean is warranted if defoliation reaches or exceeds 30% before bloom or 20% between bloom and pod fill. These thresholds can be adjusted slightly to accommodate higher or lower prices for soybean (e.g., thresholds are lowered slightly if the price of soybean increases). Determining which insecticide to apply if treatment is warranted requires that you know which insect is feeding on soybean leaves--bean leaf beetle, grasshopper, green cloverworm, Japanese beetle (suggested insecticides are listed in Table 1), thistle caterpillar, or woollybear caterpillar. However, the defoliation thresholds for all of these insects are the same.
We conclude this article with a strong expression of concern regarding some insecticide applications that have been made thus far in 2004, and some of which continue. We are aware that some insecticide sales people recommended inclusion of an insecticide with Roundup when the herbicide was applied last month. These preventive insecticide applications made no sense economically (insect pests were not present at economically threatening levels), and they violate one of the tenets of IPM--apply an insecticide for insect control only when insects reach or exceed published economic thresholds. Recommending insecticides to prevent insect infestations is foolish and irresponsible. People who applied insecticides to prevent infestations of soybean aphids in June wasted their money.
Furthermore, we have become aware of promotion of application of a fungicide (Quadris) and an insecticide (Warrior) to soybean at stage R3, with a guarantee of a yield benefit. We will not address whether the fungicide is necessary or not; we leave that to our colleagues in plant pathology. However, recommendation of the inclusion of an insecticide, regardless of the product and without justification for insect control when densities reach or exceed published economic thresholds, is irresponsible. We are beginning to wonder whether the principles of IPM are being considered at all when such recommendations are made. Economics is important in IPM, but so is the environment. Continued abuses of IPM principles with these types of recommendations will draw negative attention to agriculture. Let's not undo what has taken us decades to accomplish with IPM. Please reconsider these recommendations and stop application of insecticides for reasons that are tenuous, at best.--Kevin Steffey and Mike Gray