No. 14 Article 3/June 24, 2005

Don't Wait to Begin Examining Soybean Fields for Spider Mites

The situation with twospotted spider mites infesting soybeans in areas of Illinois that are dry has worsened. We have received a fair number of reports of fields of soybeans in central Illinois (Champaign, Christian, Logan, Menard, Morgan, Moultrie, Sangamon, and Scott counties, to name a few) that have been treated or will be treated with miticides to prevent further injury and stop the spread of the infestations. The primary problem this year is that twospotted spider mites became established on relatively small soybean plants and infestations did not necessarily begin at field edges, although infestations along field edges are common. We also have received reports that fields of no-till soybeans have worse infestations than fields of soybeans that were tilled this spring.

At an invitation from Dan Schaefer, Illini FS, I visited a field near Tolono (Champaign County) on June 21, and I believe we figured out the "epidemiology" of the infestation, at least in that field. It is likely that the explanation in other infested fields is similar. This field is about 40 acres, and it had been planted no-till into corn stubble. The distribution of the spider mite infestation was notable in that there were infested patches throughout the field rather than a concentrated infestation along the field edges. In the infested patches, the soybean plants were severely stunted, with stippling caused by spider mite feeding evident. The leaves on many plants appeared to be dying. All stages of twospotted spider mites, including eggs, were numerous on infested plants. The areas in the field that appeared to be "healthy" were infested with mites, too, although the symptoms were not as dramatic.


Soybean plants infested with twospotted spider mites, Champaign County (University of Illinois).


Soybean plants infested with twospotted spider mites, Champaign County (University of Illinois).

One marked difference in development of spider mite infestations in 1988 (the benchmark year for spider mites in recent history) and 2005 is the early development of infestations in spots throughout the field this year. In 1988, most spider infestations developed along field edges in June, and infestations throughout fields occurred later as spider mite populations increased and spread. Consequently, the plants infested were larger and more capable of tolerating spider mite injury, up to a point. This year, however, several people have reported that the small soybean plants that have been infested may not survive. The continuing dry conditions are not helping.

It was apparent in the field I visited that the patches of spider mite infestations were associated with patches of henbit that had been killed with Roundup early this spring. In relatively "healthy" areas of the field, we observed very few dead henbit plants. However, in heavily infested areas, dead henbit plants were common. No fall burndown herbicides were applied to the field in 2004, so some late-season weeds (e.g., henbit) were present. In the spring of 2005, Roundup was applied to the Roundup Ready soybeans just as the seedlings were emerging. Henbit was among the most common weeds in the field. As you may recall, I mentioned that I observed spider mites infesting henbit in a field in Ford County in May, the first spider mite-infested field I visited this year ("Twospotted Spider Mites in Soybeans Add Another Level of Concern," issue no. 11, June 3, 2005). In both situations, and likely in others, the spider mites moved from the dying henbit onto healthy soybeans when the soybean plants were quite small.

Some people have reported that twospotted spider mites are more common in no-till fields than in tilled fields. This may have a direct correlation with the presence or absence of henbit or other weeds on which female spider mites overwintered. Adjacent to the spider mite-infested field (separated only by turn rows) was a field of soybeans that had been tilled this spring. There were no obvious signs of spider mite infestation in the tilled field, although the spread of the mites in the no-till field will likely be a source of future infestation if a miticide is not applied soon. You can see for yourself in these photographs. I faced south as I took the photo of noninfested soybeans and merely turned around to face north to take the photo of infested soybeans.


Field of soybeans in Champaign County not infested with twospotted spider mites (University of Illinois).


Field of soybeans in Champaign County infested with twospotted spider mites. Note the infested patches (University of Illinois).


Patch of soybeans in Champaign County infested with twospotted spider mites (University of Illinois).

If hot, dry weather persists, twospotted spider mites will thrive and infestations will spread. We urge everyone to scout thoroughly now before the symptoms become dramatic (i.e., dead or dying soybeans). Make certain you diagnose the problem properly (e.g., don't mistake a plant disease for a spider mite infestation). Spider mites and their webbing will be evident on infested plants. Spot treatments with chlorpyrifos (Lorsban 4E or Nufos 4E) or dimethoate will be effective only if spider mite infestations are confined. However, we strongly encourage you to examine entire fields rather than focus solely on the infested areas. If spider mites are becoming established on seemingly healthy soybeans, symptoms of their feeding injury will appear shortly thereafter.

Rain would help us a great deal, so let's all keep hoping.--Kevin Steffey

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