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Issue No. 6, Article 2/May 2, 2008

Insect Updates During the Holding Pattern

The delays in corn planting in Illinois in 2008, our slowest start since 1999, have most agricultural pest managers in something of a holding pattern waiting for drier and warmer temperatures so planting can begin in earnest. The prospects for insect pests of corn and soybean are largely unknown at the moment, although we can continue to provide updates of incoming pests. On the other hand, scouting for insect pests is sensible in the crops that are growing (i.e., alfalfa and wheat). Following are updates that capture what we know at the moment:

Alfalfa. The warm weather during the week of April 21 sped the accumulation of degree-days (base 48°F) since January 1 for estimating development of alfalfa weevils. As of April 29, 200 degree-days, a rough trigger for initiating scouting for alfalfa weevils, had accumulated along a line from just south of the Quad Cities in western Illinois through Henry, Bureau, southern LaSalle, Livingston, northern Ford, and Kankakee counties in eastern Illinois. As many as 400 degree-days (potential for significant leaf feeding) had accumulated along a line from Randolph County in western Illinois to White County in eastern Illinois. We have received reports that some alfalfa growers in Massac and Jackson counties have already sprayed insecticides to control alfalfa weevil larvae. Projected accumulations of degree-days for May 6 suggest that leaf-feeding injury caused by alfalfa weevil larvae should be obvious in alfalfa fields in the southern half of the state. To keep abreast of development of alfalfa weevils, go to "Daily Pest Degree-Day Accumulations" on the Water and Atmospheric Resources Monitoring site maintained by the Illinois State Water Survey.

It is important to point out that two distinct peaks of alfalfa weevil larval activity usually occur in southern Illinois, one from fall-deposited eggs and one from spring-deposited eggs. Hatching of overwintering eggs usually occurs when 200 degree-days accumulate beyond January 1. An early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 degree-days; a second major peak of third-stage larvae from spring-deposited eggs occurs after an accumulation of 575 degree-days. As the temperatures warm up again, alfalfa weevil activity will become much more obvious.

Corn. With so little corn planted, the primary insect of concern at this point is the black cutworm. Intense captures of adults (9 or more captured over two days) continue to be reported from Indiana, Kentucky, and Missouri as well as from a couple of locations in Illinois:

Massac County, April 25, according to "The Hines Report"

Piatt County, April 26, according to Doug Gucker, University of Illinois Extension

These and previous intense captures keep us on alert for cutting injury by mid- to late May. Some of the early intense captures in southern Illinois suggest even earlier dates for cutting injury, assuming corn is planted sometime soon. Be sure to prioritize scouting in fields where weeds, especially winter annual weeds, have been growing this spring.

Although the focus may be on watching the skies for black cutworms, as soon as corn gets planted our attention will turn to those unpredictable subterranean insects, especially white grubs and wireworms. Wet, cool soils usually make corn seedlings more susceptible to injury caused by below-ground insect pests because the insects can munch away while the corn is growing slowly. So don't walk away from seedling cornfields in the rush to plant soybeans. The effects of feeding damage by below-ground pests can last through May and into June.

Wheat. Large numbers of armyworm adults have been captured in traps in Kentucky, and 196 adults were captured during the week of April 22 (ending April 29) in a trap in Franklin County in the southern Illinois trap line ("The Hines Report"). The numbers of moths being captured rival or exceed the numbers captured in 2001, when the last significant armyworm outbreak occurred. So dense stands of wheat, grass pastures, and seedling corn should be the first targets for scouting for this defoliator over the next couple of weeks. The larvae will be small and difficult to find at first, so look for evidence of leaf feeding (notches chewed from the leaf edges) to note the presence of armyworms. On most sunny days, you'll have to look for the larvae on the ground, because armyworms feed mostly at night. When temperatures warm up, however, the larvae will develop quickly, so don't be caught unaware. Stay on top of the situation for as long as reports of large numbers of adults continue.

We know that some people recommend using insecticides during the spring to control cereal aphids (bird cherry-oat aphid, corn leaf aphid, greenbug, English grain aphid) in wheat. The premise is to prevent the spread of barley yellow dwarf (BYD) virus; however, the evidence to support this recommendation in the spring is not overwhelming. The best approach for managing cereal aphids in wheat and preventing spread of the BYD virus goes back to the previous fall, when resistant or tolerant wheat varieties should have been planted after "fly-free" dates. Use of insecticides to manage aphids often is recommended in the fall, rather than during the spring. However, in this day and age of high commodity prices, wheat growers might be persuaded to apply an insecticide unnecessarily in the spring for the promise of higher yields. This is a dubious practice, and wheat growers should beware.--Kevin Steffey

Author:
Kevin Steffey

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