In last week's issue of the Bulletin (issue no. 6, April 30, 1999), I discussed the possibility of finding cereal leaf beetles in wheat. That has been confirmed by Robert Bellm, Extension Educator/Crop Systems at the Edwardsville Extension Center, who found cereal leaf beetle eggs in some fields in southwestern Illinois. Larvae should be evident soon. I also discussed armyworms in a previous issue of the Bulletin (issue no. 5, April 23, 1999). Although no one has reported finding armyworms yet, it is probably only a matter of time.|
Other insects found in wheat at this time of year are aphids, especially bird cherry-oat aphids. Matt Montgomery, Extension Assistant/Crop Systems at the Sangamon/Menard Extension Unit, found some bird cherry-oat aphids at the bases of wheat plants in a wheat field in southern Sangamon County on April 29. He also observed some lady beetles in the same field. Remember this: Lady beetles love aphids, the "candy" of the insect world. In fact, lady beetles usually prevent several species of aphids from reaching economic levels during most years in Illinois.
The bird cherry-oat aphid usually is the first species of aphids found in wheat. It is olive green with a red-orange band across the rear of the abdomen; the tips of its cornicles ("tail pipes" that protrude from the rear of the abdomen) are black. Other species of aphids in wheat are English grain aphids, which are green and have long, narrow cornicles that are entirely black; and the greenbug, the most threatening aphid species. The greenbug 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 sometimes 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 open.
Economically important outbreaks of aphids are uncommon in wheat in Illinois, but noting their presence and potential buildup is important. Seedling wheat can be severely injured by the feeding of virus-carrying aphids, but wheat in the boot or heading stage is seldom damaged economically by these insects. Greenbugs generally cause greater damage to wheat than the other aphids because they inject toxic enzymes into plants during feeding. Research regarding the effect of aphids on wheat yields suggests that the threshold is 12 to 15 aphids per tiller during seedling to boot stage. However, the presence of natural enemies often keeps aphid populations in wheat below economically damaging densities. In addition to lady beetles, a fungus disease and parasitoids also suppress aphid populations. In cool, wet springs, a fungus disease helps to keep aphid populations in check. The presence of aphid "mummies"swollen, copper- or tan-colored aphids (see Figure 4)reveals the activity of parasitic wasps. Hopefully, the effects of these natural enemies will help us out again this year.--Kevin Steffey