No. 18 Article 1/August 6, 2010

A Strange Summer for Field Crop Entomologists

The peculiar summer for field crop entomologists continues. The summary here offers tidbits of information for a variety of insects observed in several areas of the Midwest.

Corn rootworms. I think the emerging consensus is that the western corn rootworm population is very low this season. I haven't received a single report of excessive lodging in producers' fields where western corn rootworm larval injury was suspected. In addition, adult densities are very low, even in our trap-crop plots. Perhaps we will see these numbers increase over the next several weeks. Of interest are the numerous reports of large densities of southern corn rootworm adults this summer. Why has this occurred? Are these large numbers of adults simply a result of a significant migration that occurred from southern states earlier this spring? Is the successful overwintering of adults occurring further north than in previous years? If so, will this trend continue and southern corn rootworms then become a more numerous insect throughout Illinois?

We have concluded our annual root digs for the summer. Although we had some root injury in our experiments in DeKalb and Urbana, pressure was not evident at the Perry and Monmouth sites (see below for a view of Monmouth control roots).

Undamaged check (control) roots from the Monmouth corn rootworm product efficacy experiment.

Soybean aphids. Although there are a few reports of soybean aphid numbers beginning to increase in some soybean fields in northern Illinois, overall, soybean aphids continue to be a no-show for much of the state in terms of economic impact. As soybeans begin to move through the R4 (full pod) and R5 (beginning seed) stages of development and densities start to reach the economic threshold, the return on investment for a rescue treatment becomes much more inconsistent. I encourage producers to scout their fields for soybean aphids, particularly in the northern third of the state; if densities reach 250 aphids per plant, management decisions will need to be made, particularly if late-planted fields are at more susceptible developmental stages.

Soybean defoliators. There continue to be reports in many areas of Illinois of some soybean defoliation caused by a variety of caterpillars, such as green cloverworms (a healthy larva is pictured below), soybean loopers, and woollybears. Green cloverworms and other lepidopteran defoliators are susceptible to many parasitoids and diseases (see below for a photo of diseased larvae), and these natural enemies often suppress densities to levels that do not cause economic loss. The general defoliation threshold used for many years is 20% (Figure 1) for soybeans between bloom and pod fill. For the great majority of fields, this threshold has not been reached; however, producers are encouraged to scout their fields and make the necessary management decisions if the threshold is reached.

Green cloverworm larva (note the parasitoid eggs near the head).

Diseased green cloverworms (note the three pairs of abdominal prolegs).

Figure 1. Soybean defoliation–level guide.

In last week's Bulletin, I discussed the large number of celery leaftier moths that have been observed in central and northern Illinois, in addition to some other areas of the Midwest, including Iowa, this season. Tom Hillyer (Hillyer Agriservice, West Liberty, Iowa) was kind enough to provide excellent photos of celery leaftier larvae that he observed feeding in southeastern Iowa soybean fields (see below). He found them to be most numerous in the lower canopy of early-planted soybeans, in contrast to green cloverworms, which were most abundant in the upper canopy. Tom indicated that as the canopy closed and humidity increased, the number of celery leaftier larvae declined.

Celery leaftier larva. (Photo courtesy of Tom Hillyer, Hillyer Agriservice, West Liberty, Iowa).

Why are we seeing celery leaftiers in such large numbers this season? And why are they feeding on soybeans? Within greenhouses, where this insect is referred to as the greenhouse leaftier, hosts include azalea, begonia, chrysanthemum, carnation, geranium, petunia, rose, snapdragon, and violet. Hosts outside of greenhouses are most typically celery, beets, and lettuce. The name "leaftier" refers to the larval behavior of tying leaves of the host plant together using silk. Multiple generations are reported in southern areas of the United States. Will this insect pest become more common in upcoming years? The answer remains unclear.--Mike Gray

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