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Issue No. 9, Article 1/June 1, 2012

Scattered Insect Observations from Around the State

Western corn rootworms. On May 29, Joe Spencer with the Illinois Natural History Survey observed second-instar corn rootworm larvae in his research plots northeast of Urbana. Joe, a well-known entomologist who specializes in insect behavior, has worked since the mid-1990s on characterizing western corn rootworm dispersal and their feeding behavior. His research has contributed significantly to our improved understanding of the rotation-"resistant" western corn rootworm, often referred to as the western corn rootworm variant.

Joe anticipates the first beetle emergence to occur in about 2 weeks (June 11–12). This is very early; typically I begin to get reports of adult emergence during 4th of July festivities. If the hot and dry weather persists in areas of Illinois more prone to corn rootworm damage (the northern two-thirds), heavy infestations of larvae could take their toll on small root systems. Because of the early hatch and accelerated development of larvae this spring due to very warm temperatures, even Bt hybrids may sustain more root injury than expected. The lack of soil moisture in many fields will speed up larval development due to the more rapid heating of dry soils. If you observe severe root injury caused by western corn rootworms during the next few weeks, please share your observations with me and I will report them in the Bulletin.

Japanese beetles. Adult Japanese beetles have been observed in southern, southwestern, and south-central Illinois counties this last week of May. Ron Hines, formerly with University of Illinois Extension and now a crop consultant, saw Japanese beetles in Massac County on May 28. Robert Bellm, U of I Extension commercial agriculture educator, observed them in Fayette and Madison counties over the Memorial Day weekend. He estimates these sightings are about 2 weeks ahead of normal.

The suggested economic threshold for Japanese beetles in soybeans is based on a defoliation level of 30% before bloom. Special consideration should be given if fields are under severe moisture stress and soybeans are in the early stages of development. Japanese beetles tend to concentrate their numbers along field margins. It would be prudent during scouting to examine plants in several areas of the field interior; five separate areas are a common recommendation. Don't base rescue treatment decisions on quick looks at defoliation in border rows. With the mild winter followed by a hot and dry spring, I anticipate overall good survival for Japanese beetles and encourage vigilant scouting throughout the growing season.

Potato leafhoppers. Potato leafhoppers can now be commonly seen in stands of alfalfa and should be monitored using a sweep net. This pest can inflict significant yield losses in alfalfa, especially in dry years. Field perimeters are often the first areas to show signs of injury. The regrowth of stands following a cutting should be examined carefully for leafhoppers. As few as 0.2 leafhoppers per sweep in alfalfa 0 to 3 inches high can significantly stunt further plant growth. Don't assume that the dry weather is solely responsible for delayed plant development following a harvest.

For more information about the life cycle, biology, and management of the potato leafhopper, see this page on potato leafhoppers on the Department of Crop Sciences Extension website.

Beet armyworms and yellowstriped armyworms. Both beet and yellowstriped armyworms have been observed in producers' corn and soybean fields during the late days of May. Armyworm species are probably most often thought of as damaging corn and wheat, but the fall armyworm, beet armyworm, and yellowstriped armyworm are cited as occasional pests of soybeans in the Handbook of Soybean Insect Pests (Entomological Society of America). The economic threshold suggested for these species in soybeans is "when larvae threaten to reduce stands below the optimum plant population, typically to 6 or fewer plants per row-ft (19.7 per row-m)."

Kevin Black, insect and plant disease technical manager with Growmark, observed beet armyworms on corn in western and northwestern Illinois on May 29. The threshold for armyworms in seedling corn is when 25% of plants are damaged, larvae are ¾-inch or smaller, and some plants are being killed. The suggested economic threshold for armyworms in the Handbook of Corn Insects (Entomological Society of America) for seedling corn is "when stand loss exceeds 10%." For the yellowstriped armyworm, the handbook says it "seldom is a serious pest of corn in the Corn Belt. Control with insecticides is not economical unless feeding would cause heavy damage (defoliation greater than 50%)." Reasons for the greater occurrence of these armyworm species this spring are likely the mild winter, the warmer-than-average spring, and suitable migratory conditions this spring. Scouting for these species is encouraged in both corn and soybean fields for the next several weeks.--Mike Gray

Author:
Mike Gray

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