Issue No. 18, Article 2/July 22, 2005
Overview of Insects and Mites in Illinois Soybean Fields
The two primary pest concerns in soybeans in Illinois right now are twospotted spider mites and soybean aphids. The mites have been building in drought-stricken areas for weeks, and economic infestations are becoming more widespread. Both central and northern Illinois counties are affected. In northern Illinois, after a couple of weeks of slow development in June, soybean aphid population densities have increased noticeably during the first half of July. Other insects also are showing up in soybean fields, so it seems appropriate to provide an overview of all this activity.
Soybean aphid. If you are reading other states' weekly newsletters and pest reports, you already know that population densities of soybean aphids have been increasing significantly over the past couple of weeks. Michigan was the first state to experience widespread economic infestations of soybean aphids, and economic infestations are now appearing in Minnesota and Wisconsin. We became aware of above-threshold numbers of soybean aphids in northern Illinois during the week of July 11, and average densities and percentages of plants infested continue to increase in the counties we have surveyed. Increasing numbers of soybean aphids also are being observed in both Indiana and Iowa.
Our weekly surveys continue, and although average densities of aphids per plant did not increase dramatically from the July 11-12 to the July 18-19 sampling dates, a slow, steady increase was obvious. Compare Table 1 in this article with Table 1 in last week's issue (no. 17, July 15, 2005) of the Bulletin. The number of fields with 100% of plants infested also increased. The counts of soybean aphids in 2 (Kendall 1 and Kendall 2) of the 14 fields sampled in Boone, Kane, Kendall, Lake, and McHenry counties suggested treatment within 7 days, according to the "speed scouting" protocol developed at the University of Minnesota.
Reports from other individuals in Illinois indicate that although the increase in soybean aphid densities is not a widespread explosion (yet), several hand grenades have detonated. Jim Donnelly with Ag View FS reported that "aphids appear to be exploding in parts of our area, with southern Lee County and northern Bureau County in particular." Jim observed an estimated 1,500 aphids per plant in one field in Lee County. Several other fields in the area had densities of aphids quickly approaching 250 aphids per plant (125 to 175 aphids per plant).
Dave Feltes, Extension educator in IPM in the Quad Cities, has been assessing soybean aphid densities in northwestern Illinois for a couple of weeks. He easily found nymphs and winged adults in fields in Carroll, JoDaviess, and Stephenson counties on July 14 and 18, after not finding many aphids during the week of July 4. On July 14, Dave estimated 90% of the plants being infested in Carroll County, 80% (three to five aphids per leaflet) in JoDaviess County, and 100% (one to five aphids per leaflet) in Stephenson County. From July 14 to July 18, the number of aphids increased dramatically (although still below threshold) in one location in JoDaviess County.
Gary Bretthauer, Kendall County educator in crop systems, has also reported a noticeable increase in soybean aphid activity. He reported scouting for aphids in four fields, and one of them was "absolutely loaded." This field was scheduled to be treated with an insecticide. A small plot near the Kendall County Extension office also was heavily infested.
Soybean aphids on a leaflet in a small plot of soybeans in Kendall County (photo courtesy of Gary Bretthauer).
Soybean aphids on soybean petioles in a small plot of soybeans in Kendall County (photo courtesy of Gary Bretthauer).
Many people are now seeing aphids more fully distributed within the canopy, a typical finding when densities of aphids increase. More and more aphids are distributing themselves on the stems and petioles deeper in the canopy. Also, quite a few people are finding fair numbers of multicolored Asian lady beetles and minute pirate bugs (Orius insidious), both of which prey on and potentially suppress soybean aphid populations. The predators are not keeping up with soybean aphid populations in the "explosive" fields, but they still may hold the line where densities are lower. Ants that tend the aphids also are being found in many fields, and these ants often will defend the aphids against the predators. There are some very interesting insect dynamics taking place in soybean fields right now.
Soybean aphids and tending ants (photo courtesy of Kevin Black, Growmark, Inc.).
It is important to note that most of the fields being sampled still have densities of soybean aphids well below the threshold of 250 aphids per plant with 80% of plants infested. So scout now and frequently, keeping an eye on population trends (increasing or decreasing densities). Aphid densities seem to be increasing in many areas, despite the recent hotter temperatures. A very good article written by Eileen Cullen, extension entomologist at the University of Wisconsin, in Wisconsin Crop Manager summarizes the relationship between temperature and soybean aphids. The research was conducted by entomologists at the University of Minnesota.
Insecticides for control of soybean aphids in soybeans were listed in Table 1 of issue number 16 (July 8, 2005) of the Bulletin. As the number of soybean fields that will be treated with insecticides to control soybean aphids (and other pests) increases, applicators need to be very cognizant of the potential negative impact of insecticides on honey bees visiting flowering soybeans.
Twospotted spider mite. We are receiving increasing reports of economic infestations of twospotted spider mites in areas where drought conditions persist. The most recent (July 12) U.S. Drought Monitor map indicates severe drought in an area extending from central Illinois into north-central and northeastern counties. Reports of infestations of twospotted spider mites are most common from that area.
We also have heard from a handful of people that in soybean fields sprayed with miticides to control spider mites in June, densities of spider mites are increasing again. This is typical when hot, dry conditions prevail. Based on preliminary information from a miticides efficacy trial in Champaign County, we believe it is advisable that if a field must be treated a second time, the miticide applied the second time should be different from the miticide applied the first time. We will follow up with this recommendation after we obtain results from the trial late in the week of July 18.
Bean leaf beetle, western corn rootworm. Both bean leaf beetles and western corn rootworms are being observed in soybean fields throughout Illinois. The numbers of bean leaf beetles (the adults of the first 2005 generation) are low, but the numbers of western corn rootworm adults are increasing. An increase in the numbers of western corn rootworms being found in soybean fields suggests that the variant is beginning to fly from corn to soybeans and back. Egg laying will commence in soybean fields, so there will be a threat from corn rootworm larvae in these same fields when corn is planted in 2006. We still strongly discourage spraying insecticides in an attempt to prevent western corn rootworm adults from laying eggs in soybeans, but insecticide applications to control other arthropods (soybean aphids, spider mites) in soybeans undoubtedly will kill visiting western corn rootworms, too. Unfortunately, there is no guarantee that the timing of such an application will have a significant influence on egg laying by rootworm adults. Some followup treated and untreated strips in cornfields next year would help us address this issue a little better. For now, this approach for rootworm management is not recommended for corn to be planted after soybeans.
Multiple pests, drought stress, and thresholds. Many "What should we do?" questions have been associated with infestations of more than one pest (e.g., spider mites and soybean aphids) and the double-whammy of both insect- (and mite-) and weather-related stresses. Should economic thresholds be lowered under these conditions? If the answer is yes, what should the adjusted thresholds be? The answers to both questions are simple, although possibly not completely satisfyingÑ"Yes" and "I don't know," respectively. Entomologists have long said that economic thresholds should be adjusted when more than one pest infestation exists and crop plants are suffering from one or more additional stresses. In other words, drought-stressed plants will experience yield loss at lower densities of insect pests than healthy plants. Unfortunately, the research to provide quantitative adjusted thresholds is extremely complex and labor-intensive, so very little work to investigate these interactions has been conducted. So although all of us intuitively know that stressed plants will suffer more greatly from insect (or mite) attack than healthy plants, we do not have the information necessary to provide more specific guidelines. So as I have said in the past, use your experience and best judgment to make the call.--Kevin Steffey