Kansas Corn, Sorghum: Chinch Bugs Attacking Stands

Wheat harvest around the district is progressing with some producers done! The rains have really helped the continued development of the corn and sorghum which is significant for combating against insect infestations.

All life stages of chinch bugs seem to be extremely active at the present time in both corn and grain sorghum.  Nymphs and adults started migrating out of wheat fields at least two weeks ago, moving into any adjacent corn or grain sorghum fields.  Those smaller-reddish nymphs have grown considerably since then, and are now either late instar nymphs or adults.

Many of these recently matured adults are now mating and have even started egg deposition. These eggs are, and will continue to be, hatching which means more bugs and thus more feeding on these plants.

Fortunately, most corn is large enough to withstand considerable feeding by chinch bugs, but the milo may have some concerns as it is smaller. Recent rains have certainly helped greatly enhance the growing conditions, which increases the plant’s tolerance for chinch bug feeding.

Eggs may be laid anytime from the end of April to early October, although in each generation the majority are laid over a two to three week period. Each female lays up to several hundred eggs.

Female reproductive potential varies greatly from year to year and may be affected by the host plant quality during development. The time required for egg hatch ranges from one to two weeks and depends on temperature and location. Eggs in warm locations with good solar exposure hatch first.

Chinch bugs puncture vascular tissues to extract plant juices and secrete digestive enzymes that cause the break-down of surrounding plant tissues. Feeding punctures also can allow pathogens to enter the plant. Consequently, damaged plants present a variety of symptoms including stunting, yellowing, wilting, and necrotic lesions.

Older nymphs are larger and cause more damage than younger ones. The effect of nymphal feeding depends to a large degree on the health and nutritional status of the plants. Growth stage and water balance are critical because small or drought-stressed plants have less ability to tolerate or recover from chinch bug feeding damage. The chinch bug feeds on a wide variety of grasses, including many cereal and forage crops.

The risk of first generation damage is greater where sorghum is planted next to thin stands of wheat. Seedling sorghum is most vulnerable, and seven to 10 bugs per plant will cause stunting, poor root development and stand reduction. Larger plants can tolerate more bugs, but severe infestations can cause stunting, lodging, and yield loss.

Timely applications of foliar insecticides can be used to rescue corn or sorghum fields invaded by migrating chinch bugs on the edge of the fields to help prevent movement into the field. Most currently approved materials have good efficacy against chinch bugs, if three factors are considered.

First, it is important to use the full recommended rate of the selected insecticide, preferably delivered in 20 to 40 gallons of water per acre. High gallonage ensures good plant coverage and enhances the movement of material into protected plant parts such as leaf sheaths, which increases the probability of contact with bugs.

Second, the material should be delivered with properly adjusted and calibrated equipment. Fortunately, insecticides targeting the sugarcane aphid will likely provide some control of late ­season chinch bugs as well.

Third, the timing of the insecticide application is critical. Early morning applications are preferred because winds tend to be calmer (reducing drift), temperatures are cool (reducing volatilization of chemicals), and a large proportion of the chinch bug population will be on the plants and exposed to the application. Treat promptly as migrations begin and before significant numbers of bugs enter the field.

Most grain sorghum is much less developed than corn and won’t be able to tolerate as many chinch bugs as the larger corn plants.  Treating plants much after the V6/V7 (corn 6 and 7 leaves) growth stages is not as effective as treating smaller plants.

Like corn, good growing conditions significantly help sorghum plants withstand chinch bug feeding.  So we are fortunate that we have received moisture to help the growth of milo fields.  However, if dry conditions return, chinch bug feeding can significantly weaken stalks and cause lodging later in the season.

No natural enemies of chinch bugs can reliably reduce or control large populations when conditions are favorable for their development. Predation by quail and other birds can be conspicuous, but likely has little effect on bug population growth.

For more information on chinch bugs, management decisions, and insecticide recommendations, K-State Research and Extension has an excellent publication: “Chinch Bugs” that is available ONLINE here or at any of our Post Rock Extension District Offices at Beloit, Lincoln, Mankato, Osborne or Smith Center.

If you have more questions on insect management in your crops, give me a call at any of our Post Rock Extension District Offices.

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