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Issue No. 6, Article 1/May 10, 2012

Entomological Musings This Spring

Black cutworms. Despite the record-breaking warm weather in March and early corn planting across much of the state, reports of insect injury to stands of corn have been few and far between. Entomologists in several midwestern states have reported impressive flights of black cutworm moths this spring, but to date I've not received any reports of significant infestations. Don't be lulled into complacency just yet, though--cornfields in the 1- to 4-leaf stage of development should still be scouted for signs of leaf feeding and cutting.

Reasons for the lack of black cutworm injury reports may include the suppression/control offered by Bt hybrids, the effect of neonicotinoid seed-treatment insecticides (active ingredients thiamethoxam and/or clothianidin) and their systemic action, and early-season tillage and weed control followed by the early planting of corn across much of the state. It will be interesting over the next few weeks to see how this yet-unfinished story plays out.

Armyworms. Accompanying the impressive migration of black cutworm moths has been a noteworthy spring flight of armyworm moths. Some observations of larvae in wheat fields have been passed along to me, but no reports of economic numbers yet. Armyworm larvae will feed on other crops in addition to wheat, including barley, corn, oats, and rye.

The suggested threshold for armyworms in wheat is when six or more ¾ -to 1¼-inch-long larvae are found per linear foot of row. Full-grown larvae typically reach about 1½ inches in length. When scouting wheat fields for armyworms, be sure to look for evidence of head clipping. Armyworms tend to be heavily parasitized and are susceptible to diseases. Rescue treatments often are not required because natural enemies can keep armyworm numbers below economic levels.

Bean leaf beetles. Just as producers transition from corn planting to soybeans, observations of bean leaf beetles are becoming more common, especially in central Illinois. Bean leaf beetles overwinter in protected areas (beneath plant debris), and survival this past winter was undoubtedly quite good. Favorite overwintering sites include wooded areas adjacent to stands of alfalfa. In the spring, dormancy of the adults is broken, and they often move initially into alfalfa. Soybean seedlings in early-planted fields are most at risk to intensive feeding by the overwintering population.

Rescue treatments for bean leaf beetles in seedling soybeans are typically not warranted given the densities required to cause economic losses: 16 beetles per foot of row for the early seedling stage and 39 for the V2 stage and beyond. In light of the mild winter, I encourage producers to scout fields carefully for bean leaf beetles this spring. Pay particular attention to soybean fields that are planted first in your area of the state.

Corn rootworms. Ron Estes (senior research specialist) and Nick Tinsley (visiting research specialist) of the Department of Crop Sciences have made excellent progress establishing corn rootworm trials in our four standard plot locations (near DeKalb, Monmouth, Perry, and Urbana). Treatments in these experiments include various Bt hybrids and soil insecticides, both granular and liquid formulations. In July, roots will be evaluated for injury, and plots will be harvested in the fall. Results from the trials are published each year in the On Target report (ipm.illinois.edu/ontarget).

There is considerable curiosity regarding the timing of this season's corn rootworm larval hatch. Hatch occurs most often near the Memorial Day holiday weekend, but with the spring's unseasonably warm temperatures, I suspect it will be earlier this season, beginning across central Illinois by the end of next week (mid-May). Approximately 50% of corn rootworm larvae are expected to hatch when 684 to 767 degree days (base 52 °F) have accumulated since January 1 of a given year (biofix). Not all larvae hatch simultaneously; they will continue to hatch throughout May and into early June. If you observe corn rootworm larvae on seedling root systems during your scouting forays, please let me know; I would like to share your observations with readers.--Mike Gray

Mike Gray

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