Issue No. 5, Article 1/April 24, 2009
Once again we find ourselves in a holding pattern, waiting for corn planting to begin in earnest. As has always been the case when corn planting is delayed, potential insect problems are not a high-priority thought for most producers. Nonetheless, it never hurts to stay current with insect activity (but we'll keep it brief).
Although we still have not received any reports of alfalfa weevil activity, accumulated degree-days (above a base temperature of 48°F) from January 1 through April 21 suggest that larvae could be active anywhere south of a line from Carthage (western Illinois) through Peoria to Danville (eastern Illinois). As of April 21, 400 degree-days had accumulated throughout the southern tip of the state (Alexander, Hardin, Jackson, Johnson, Massac, Pope, Pulaski, Saline, Union, and Williamson counties), suggesting that third instar larvae are imminent. At the very least, light feeding on alfalfa tips should be evident, with heavier feeding injury (leaf skeletonization) appearing soon, assuming densities of alfalfa weevil larvae are large enough to cause noticeable injury. As usual, we recommend first scouting fields that warm up quickly (e.g., south-facing slopes), just to get a sense of potential for alfalfa weevil injury. As of April 21, 300 degree-days had accumulated south of a line from Calhoun County (western Illinois) through Fayette County to Crawford County (eastern Illinois). Larvae from spring-deposited eggs will be noticeable soon. Projected warm temperatures will speed up alfalfa weevil development significantly.
You can check out more recent degree-day accumulations for alfalfa weevil development. Once again, we direct you to the fact sheet on our IPM website to see photos of injury and to review scouting and management tips.
People voluntarily operating black cutworm pheromone traps have continued to capture moths over the past week. Doug Gucker, University of Illinois Extension in Piatt County, reported an intense capture (14 moths) on April 20, suggesting a projected cutting date of May 21. This date will become sooner as temperatures warm up because black cutworm development will accelerate. Obviously, we fully expect corn to be in the ground by then, so staying current with projections for black cutworm injury will become important in the near future. Fields that have a pretty good weed cover (especially winter annual weeds) should be targeted for initial scouting trips. Assuming we don't have a hard freeze between now and then, black cutworm larvae will begin feeding on weeds after they hatch from eggs, and they will continue to grow. When the weeds are killed by herbicides, larger larvae will begin feeding on corn seedlings, so the injury may seem to appear all of a sudden.
Kelli Bassett (Pioner Hi-Bred International, Inc.) reported finding bird cherry-oat aphids and greenbugs in wheat fields in west-central Illinois during the past week, and she has learned that large numbers of aphids have been found in southern Indiana fields. Other aphids likely to be found in wheat are corn leaf aphids and English grain aphids.
There has been very little research to indicate that economic yield losses result from aphid feeding (sucking plant fluids) in Illinois wheat. Of greater concern with aphids in wheat is their ability to transmit viruses that cause barley yellow dwarf (BYD) disease. The potential for transmission of the viruses by aphids that colonize wheat in the fall has elicited the use of preventive tactics, such as chloronicotinyl insecticide seed treatments and foliar-applied insecticides in the fall. However, the guidelines for controlling aphids in wheat in the fall are not very well developed in Illinois.
When numbers of aphids begin to increase in the spring, there is potential for yield loss to occur from their feeding injury. As a rule of thumb, insecticide may be warranted when numbers reach or exceed 25 to 50 aphids per stem (depending on species: 25 greenbugs, 30 corn leaf or bird cherry-oat aphids, 50 English grain aphids), up to the boot stage. Insecticides are not recommended for aphid control from the dough stage to maturity.
One other insect of note is the armyworm. There have been reports of large numbers of armyworm moths captured in traps in Kentucky and Missouri, so we need to consider armyworm larvae as potential near-future threats to wheat. The key for finding armyworm larvae quickly is to target thick stands first. The females tend to deposit more eggs in thick stands than in thin ones, so larval activity is usually most evident in thick stands first. It will be difficult to find the small larvae, so look for evidence of leaf feeding. Such feeding on lower leaves relatively early in the spring causes no economic losses. But evidence of leaf feeding will alert you to the potential for more serious defoliation as the larvae grow and consume more leaf tissue. Be particularly watchful as the injury progresses up the plant; armyworm larvae often feed from the bottom up. If the larvae begin feeding on the flag leaves, yield losses can occur. Read more about armyworms in the fact sheet on our IPM website.
Much can, and will, happen weather-wise between now and when insect pests will be more threatening, so the recommendation to monitor crops regularly and frequently early in the season stands the test of time. Even if insect pests get off to a good start this year, inclement weather (that is, inclement for the insects) can lead to epizootics caused by disease pathogens. We have seen this happen in the past with alfalfa weevils (a fungal pathogen) and armyworms (a virus). Population crashes under such circumstances can significantly reduce the threat, resulting in savings if an insecticide is not necessary.
Don't forget to let us know what's happening in your neck of the woods. We sincerely appreciate such reports that we can share with everyone.--Kevin Steffey