Issue No. 7, Article 2/May 8, 2009
Time to Scout for Wheat Insects: Pests and Predators Are Present
Insect pests of wheat, along with their predators, are evident in many wheat fields, particularly in southern Illinois. Robert Bellm, crop systems extension educator, Edwardsville Extension Center, has surveyed some wheat fields in southwestern Illinois and observed bird cherry-oat aphids, cereal leaf beetles, predators (lady beetle larvae), and parasitoids (parasitic wasps). The good news is that economic thresholds have not been reached for either aphids or cereal leaf beetles. Thus far, the natural enemies appear to be taking their toll on these pest species. Now is a good time to take a look in your wheat fields and familiarize yourself with some economic thresholds. Any insecticide treatments that are applied should be based on thorough scouting efforts and only after the economic threshold has been reached. Unnecessary treatments would not only reduce the natural enemy densities but also may not pencil out from an economic vantage point.
Robert observed both adults and larvae of the cereal leaf beetle. This insect is native to Europe and Asia; it made its first appearance in the north-central U.S. in Michigan in 1962 and has since spread to many states. The cereal leaf beetle completes one generation per year. The adult is an attractive beetle (3/16 inch long) with metallic bluish-black wing covers (elytra) and reddish-orange legs and thorax. However, the adults are not the primary concern; nearly all of the economic damage is caused by larval injury to the upper leaves of wheat plants. The "sluglike" larvae remove epidermal tissue in elongate strips from leaves over about a 10-day feeding period. Wheat fields with extensive injury may appear "frosted." The sluglike appearance of the larvae is caused by their encapsulation with moist fecal material, which is believed to deter predators and parasites. The current economic threshold for this insect pest suggests that a treatment may be warranted when a combination of eggs and larvae averages 3 or more per stem. The eggs (1/32 inch long) are elliptical and yellow to burnt yellowish-orange. The eggs can be located along the midvein of the upper leaf surfaces and are laid either singly or in short chains.
Cereal leaf beetle adult (photo courtesy Robert Bellm, University of Illinois Extension).
Cereal leaf beetle larva (photo courtesy Robert Bellm, University of Illinois Extension).
Robert also found bird cherry-oat aphids relatively easily. These aphids are olive green to blue-black with reddish patches at the base of their cornicles (tail pipes). These aphids do not inject toxins into plants; however, they can vector the barley yellow dwarf virus, which can lead to yield loss. The economic threshold suggests that treatment is warranted when densities exceed 30 aphids per stem during the seedling to boot stage of development. Lady beetle larvae were evident in many of the wheat fields, and they are voracious aphid predators. When considering the need for an insecticide treatment, pay attention to densities of these beneficial insects. Are predator densities increasing or decreasing over time? If lady beetle densities are increasing, you can usually save the expense of an insecticide application.
Bird cherry-oat aphids, nonparasitized (top) and parastized (bottom) (photo courtesy Robert Bellm, University of Illinois Extension).
Lady beetle larva (photo courtesy Robert Bellm, University of Illinois Extension).
We thank Robert Bellm for sharing these observations and look forward to other reports from our readers.--Mike Gray and Kevin Steffey