URBANA – University of Illinois professor of entomology and crop sciences Extension coordinator Mike Gray usually gets the first reports of the emergence of adult western corn rootworm during the July 4 festivities.


However, on May 29, Joe Spencer, an entomologist with the Illinois Natural History Survey, observed second instar corn rootworm larvae northeast of Urbana in his research plots. Spencer anticipates that the first beetle emergence will occur in approximately two weeks (June 11-12), which is very early.

“If the hot and dry weather persist in areas of the state more prone to corn rootworm damage --the northern two-thirds of Illinois -- heavy infestations of larvae could take their toll on small root systems,” said Gray. “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.”

Also during the last week of May, adult Japanese beetles were observed in southern, southwestern, and south-central Illinois. Ron Hines, a former University of Illinois Extension employee and crop consultant, saw Japanese beetles in Massac County on May 28. Robert Bellm, University of Illinois Extension commercial agriculture educator, observed Japanese beetles in Fayette and Madison counties over the Memorial Day weekend. He estimates that these sightings are about two weeks ahead of normal.

The suggested economic threshold for Japanese beetles in soybeans is based upon a defoliation level of 30 percent before bloom. Soybean fields that are under severe moisture stress may suffer greater yield loss due to defoliation, and this needs to be considered in making treatment decisions. Japanese beetles tend to concentrate their numbers along field margins. Scouts are encouraged to examine plants in five separate areas of the field’s interior.

“Don’t base rescue treatment decisions on quick looks of defoliation in border rows,” Gray advised. “With the mild winter we experienced followed by the hot and dry spring, I anticipate overall good survival of Japanese beetles and encourage vigilant scouting for this insect throughout the growing season.”

Gray also notes that potato leafhoppers are now frequently observed in stands of alfalfa and should be monitored using a sweep net. These insects can inflict significant yield losses, especially in dry years. Field perimeters are often the first areas to shown signs of injury. The regrowth of stands following a cutting should be examined carefully for leafhoppers because 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,” Gray said. He recommends consulting the information about the life cycle, biology, and management of the potato leafhopper on the Department of Crop Sciences Extension website: http://extension.cropsci.illinois.edu/fieldcrops/alfalfa/potato_leafhopper/.

Finally, producers are observing higher-than-normal numbers of beet and yellowstriped armyworms in both corn and soybean fields during the late days of May, probably due to the mild winter, warmer than average spring, and suitable migratory conditions this spring. Gray recommends scouting of both corn and soybean fields for these species for the next several weeks.

Armyworm species are associated mainly with damage to corn and wheat. However, in the Handbook of Soybean Insect Pests published by the Entomological Society of America (ESA), the fall armyworm, beet armyworm, and yellowstriped armyworm are listed as occasional pests of soybeans. ESA’s suggested economic threshold for these armyworm species in soybeans is “when larvae threaten to reduce stands below the optimum plant population, typically to six or fewer plants per row-ft (19.7 per row-m).”

Kevin Black, an insect and plant disease technical manager at Growmark, Inc., observed beet armyworms on corn in western and northwestern Illinois on May 29. The threshold for armyworms in seedling corn is when 25 percent of plants are damaged, larvae are three-fourths of an inch long or smaller, and some plants are being killed. The economic threshold for armyworms in seedling corn suggested by the ESA in its Handbook of Corn Insects is “when stand loss exceeds 10 percent.”

The handbook also states that “the yellowstriped armyworm 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 percent).”