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Synopsis of Insects in Field Crops

May 29, 2003

Reports of insect problems by telephone and e-mail have been relatively infrequent in offices during the past couple of weeks. It's been fairly quiet. We suspect that a lot of people are assessing corn stands, planting soybeans, monitoring wheat fields, and cutting alfalfa. We seem to be in a lull period for insects in most of these crops, but some problems (current or potential) are worth considering.

Corn

Corn rootworms and stalk borers are discussed in other articles in this issue of the Bulletin. Other insects to watch for or be prepared for at this time of year are secondary insect pests, black cutworm, and European corn borer.

Secondary insects. After some initial reports of injury caused by white grubs and wireworms, not much has been reported regarding these perennial insect pests. Some replanting has been necessary, but an overall assessment of these pests is that they have caused fewer problems in 2003 than they have during the preceding 3 or 4 years. Maybe the increased use of insecticidal seed treatments has provided some benefit. However, that's difficult to assess without surveys or studies designed to ascertain the prevalence of the pests and the impact of seed treatments and soil insecticides.

Reports of southern corn leaf beetle activity continue to trickle in, and some fields have been damaged fairly severely. In addition to their usual leaf feeding, some of the beetles have chewed into stems and killed the plants outright. This is an insect sorely in need of some research effort. The thresholds being used in the field are based solely on best guesses, and management options have not been well defined.

Black cutworm. We kept you apprised of the moth flights this spring, but not many people have experienced economic levels of black cutworm damage. Remember that intense captures of adults in pheromone traps do not necessarily result in economic infestations of larvae. Black cutworm larvae may not have survived well with the cool, wet weather that has prevailed. Nevertheless, for those growers who have not planted corn yet because of wet soils, black cutworms remain a threat. Late-planted corn almost always is more susceptible to black cutworm infestation than early-planted corn.

European corn borer. Based on captures of adult European corn borers in traps in Massac, Pope, and Pulaski counties (see the Hines Report at http://www.ipm.uiuc.edu/pubs/hines_report/index.html), peak moth flight in southern Illinois appears to have occurred during the week ending May 20. Many fewer moths were captured at all four locations during the week ending May 27. Consequently, females undoubtedly have been laying eggs in southern counties, and larvae should be evident in plants tall enough to support them. Remember, corn borer larvae typically do not become established on corn younger than the 6-leaf stage. We have received no information about European corn borers elsewhere in Illinois, but it's likely that moths have emerged or soon will emerge in central counties. Keep your eyes open. The numbers of overwintering larvae were quite high in an area roughly from counties near St. Louis northeast into central Illinois. Refer to the article titled "The 2002 Survey for Second-Generation European Corn Borers in Illinois Is Complete" in issue no. 24 (November 1, 2002) of the Bulletin for a refresher about overwintering corn borers.

Soybeans

Bean leaf beetle. After a flurry of reports of bean leaf beetles feeding on soybeans a couple of weeks ago, all has become rather quiet. Many soybean fields remain to be planted, and the later soybeans are planted, the potential for infestations later in the season is reduced. This also would reduce the potential for bean leaf beetles to spread the bean pod mottle virus. Very shortly, the bean leaf beetles that are present now will have finished feeding, deposited their eggs, and died. First-generation adults will appear in about a month.

Wheat

Armyworm. As we indicated in articles in previous issues of the Bulletin, captures of armyworm moths have occurred throughout the spring. However, their numbers have not been alarming, and thus far, no one has reported finding significant numbers of armyworms in wheat fields. Keep looking, and let us know if you find any injury worth reporting.

Cereal leaf beetle. Entomologists at the University of Missouri have reported finding low to moderate numbers of adults and larvae in wheat fields throughout the state, so you should add these pests to your list as you scout for armyworms. Cereal leaf beetles rarely cause economic damage to wheat in Illinois, but extensive feeding on flag leaves by the larvae can threaten yield. Look for small, brown globs on the leaves. What appears to be drops of mud may be cereal leaf beetle larvae covered in their own fecal material, a protection against predators and parasitoids. (Think about it. Would you eat them?) Underneath the fecal material, cereal leaf beetle larvae are yellow. The adults are colorful metallic blue and orange insects, but they usually cause no economic damage in wheat. Treatment of wheat may be warranted if you find one or more cereal leaf beetle larvae per plant.

Alfalfa

Potato leafhopper. The cool, wet weather has slowed the drying of the first cutting of hay, especially in southern counties. Consequently, not many people have focused on scouting for potato leafhoppers. However, numbers of leafhoppers in northern counties, reported by Jim Morrison, Extension crop systems educator in Rockford, have been worth noting. He found slightly fewer than one leafhopper per sweep in a field in Lee County on May 22. The plants were at the bud stage and were 26 inches tall. The field was cut that day, so it's possible that numbers of leafhoppers will build back up to threatening levels soon. However, the cool temperatures have slowed leafhopper development. In addition, leafhopper injury usually is more prevalent when the weather is hot and dry. The threat posed by potato leafhoppers is minimal right now, but a change in weather conditions could bring them to our attention again.

Stay in touch. We are always thankful for your reports.--Kevin Steffey

Author: Kevin Steffey


The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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