No. 7/May 8, 1998
Other Insects in Wheat at This Time of Year
As you scout for armyworms, you likely will encounter other insects in wheat, some of which can pose a threat to optimal yield. Cereal leaf beetles and aphids could be active in wheat right now. In fact, entomologists in Kentucky have reported that aphids are appearing in seedling corn in western counties, especially in corn that has been planted into abandoned wheat acres. They also reported that cereal leaf beetles are active.
Cereal leaf beetle.
Several aphid species may be found in wheat. English grain aphids are green and have long, narrow cornicles ("tailpipes" that protrude from the rear of the abdomen) that are entirely black. The bird cherry-oat aphid is olive green, with a red-orange band across the rear of the abdomen; the tips of its cornicles are black. The greenbug, the most threatening aphid species, is bright green, with a darker stripe along the middle of its back. The tips of the cornicles are black. Entomologists have never been able to associate economic yield losses in wheat with infestations of bird cherry-oat aphids; however, both English grain aphids and greenbugs are capable of causing yield losses under the right circumstances. Cool temperatures may hold back the parasitoids that usually suppress early season populations of aphids in wheat. If aphids begin building their colonies in the absence of natural enemies, their numbers could escalate rapidly, so keep your eyes peeled.
Adult cereal leaf beetles emerge from overwintering quarters and move to wheat, where they feed before they begin laying eggs. An adult cereal leaf beetle is hard-shelled, about 3/16-inch long, with metallic blue wing covers and head, and red-orange legs and prothorax (the area just behind the head). Recently deposited eggs are elliptical, yellow, and smaller than a pinhead. Just before hatching, they turn almost black. Eggs are deposited singly or inrows of three or four, but never in clusters. They usually are found close to the mid-rib on the upper surface of a leaf. The larva resembles a slug or a small glob of mud. This "glob" is an accumulation of fecal matter carried around by the immature cereal leaf beetle. This behavior probably is a defensive mechanism that discourages some predators and parasitoids from attacking the larval stage of this pest. However, at least three parasitic wasps are natural enemies of the larvae. Another small wasp parasitizes cereal leaf beetle eggs, lady beetles prey on the eggs, and one tachinid fly parasitizes the adult. Consequently, natural enemies occasionally prevent densities of cereal leaf beetles from exceeding the economic threshold.
Adult cereal leaf beetles feed for about 2 weeks before they begin laying eggs. Eggs hatch in about 5 days, and larvae usually require 10 days to become full grown. After the larvae finish feeding, they move to the ground, pupate in the soil, and emerge as beetles after 2 to 3 weeks.
The larvae feed upon the green epidermal tissue of leaves, causing injured leaves to appear silver. Severely damaged fields look "frosted." The potential for yield loss depends upon the stage of growth of wheat plants, location of larvae on the plants, and the density of the pest. Severe damage to the flag leaf can reduce yields by 25 to 30 percent. An insecticide treatment may be justified when the combination of eggs and larvae average three or more per stem. Larvae feeding on the flag leaf cause more yield loss than larvae feeding on lower leaves of the plant.
If we hear about any developing infestations of either aphids or cereal leaf beetles in wheat, we will provide suggestions for insecticides. In the meantime, keep scouting.
Kevin Steffey (firstname.lastname@example.org), Extension Entomology, (217)333-6652