Insect Management Challenges in 2001: Will 2002 Be Different?

Michael Gray

Michael E. Gray
Professor and Extension IPM Coordinator

Phone: (217) 333-6652

Kevin Steffey

Kevin L. Steffey
Professor and Extension Specialist in Entomology

Phone: (217) 333-6652


Illinois producers faced numerous insect management challenges in 2001. The following were a few of the highlights:

  • Corn rootworm larvae caused more problems in 2001 than they had in recent years, and the adults exhibited very unusual behavior during the summer.

  • European corn borers made their presence known late in the summer; the numbers of larvae found during our fall survey were the largest they have been since 1997.

  • Soybean aphid infestations flourished in some areas of northern Illinois; early-season densities of this new insect pest may be up in 2002.

  • Armyworm infestations in wheat, corn, and grass pastures and hay fields were historic in nature, causing significant anxiety in May and June.

Based upon our knowledge of these numbers and behaviors in 2001 and upon past occurrences, we offer commentary about the potential for their return in 2002 and recommend the most current management strategies.

Corn Rootworm Densities Reached Alarming Levels in 2001

Rootworm adults collected on the 49th and 50th floors of the Prudential Building in Chicago! "Swarms" of rootworm adults in yards and gardens in metropolitan areas! Children afraid to play outside because of "alarming" numbers of rootworm adults! These phrases sound like headlines from the National Enquirer or promotional slogans for very bad horror movies. However, we heard these comments from reliable people who witnessed some of the strangest behavior displayed by western corn rootworms in years. In addition, we received a multitude of "normal" reports of very high densities of western corn rootworm adults in cornfields and extensive damage caused by rootworm larvae. All of this information has raised our level of concern about the potential for rootworm problems in Illinois in 2002. Although predicting rootworm problems is far from an exact science, everyone involved with corn production in Illinois should be on heightened alert next spring and summer.

Our first inclination that 2001 would be a "rootworm year" occurred in mid-May when heat-unit accumulations in the soil suggested that rootworm larvae would hatch earlier than normal. The weather was warm and dry, so we anticipated good-to-excellent survival of larvae and a potential for poor performance of some rootworm-control products. Larry Bledsoe, an entomologist with Purdue University, found first instars on May 16 in Tippecanoe County, Indiana, the earliest confirmed hatch of corn rootworms in Indiana since 1985. We received the first report of a heavy infestation of rootworms in mid-June from individuals who had sifted soil and dissected corn roots to look for larvae in a field in western Illinois. By early July, people had begun observing extensive rootworm larval damage in cornfields throughout the northern half of the state. By mid-July, reports of significant rootworm larval damage were relatively common. Several corn producers reported being dissatisfied with the performance of the products they had used for rootworm control. We also learned that rootworm larval damage was severe in the untreated control plots in our rootworm insecticide efficacy trials near DeKalb, Monmouth, and Urbana. Rainfall totals for the months of May, June, and July were strikingly different for each of these experimental sites in 2001: DeKalb (May- 3.34 inches, June-2.64, and July-0.96), Monmouth (May-9.65 inches, June-4.03, and July-3.41), and Urbana (May-3.83 inches, June-2.83, and July-3.64).

Several soil insecticides and seed treatments, including many established products, failed to provide consistent root protection. Because of the large densities of corn rootworm adults this year, producers should anticipate escalating pressure for the 2002 growing season. Continuous corn, most at risk to corn rootworm larval injury in 2002, includes those fields that were planted late in 2001; many first-year cornfields in east central Illinois also remain at a heightened level of risk for 2002.

European Corn Borer Densities Rebound in 2001

Regional results of the 2001 pre-harvest European corn borer survey are presented. With the exception of some southern counties, the 2001 level of European corn borers exceeded 2000 densities in each region of Illinois. Very significant increases in infestation levels occurred in 2001 from the previous year for northeastern (+45.4 percent), east central (+33.2 percent), and northwestern counties (+20.4 percent). The overwintering population of European corn borers is worth noting for the northeastern region (1.98 borers per plant) of Illinois. This is an increase of 1.65 borers per plant from 2000. Densities of overwintering borers also are above average for east central (1.34 borers per plant) and northwestern (1.40 borers per plant) counties. If diseases (Beauveria bassiana and Nosema pyrausta) fail to reduce densities of overwintering larvae, producers in these regions of the state should expect to see a spring flight of moths that is larger than in recent memory. This, coupled with early planting, could result in economic levels of the first generation of European corn borers. However, stormy spring weather during the mating and egg-laying period of moths could cause the first-generation population to crash.

On October 16, 2001, the United States Environmental Protection Agency (EPA) announced that it had approved the use of "corn genetically modified with Bacillus thuringiensis" (Bt corn) for another seven years. This was good news for corn growers throughout the United States who have to contend with infestations of European and southwestern corn borers.

Soybean Aphids Here To Stay

During the growing season of 2001, soybean aphids continued to capture the attention of producers in many states scattered throughout the north central region of the U.S. On May 10, 2001, soybean aphids were collected from their overwintering host, buckthorn (Rhamnus spp.), in northwestern Illinois (Whiteside County). This confirmed that soybean aphids (egg stage) were able to successfully spend the winter on buckthorn, setting the stage for subsequent colonization of their secondary host, soybean plants. Six weeks later (June 20), survey teams discovered soybean aphids within soybean fields (V2 to V4 stages) in seven Illinois counties: Cook, Grundy, Kane, Kendall, Lake, McHenry, and Will. Numbers of aphids found were low, ranging from one to 20 per 30 plants examined. However, soybean aphid densities can "explode," and by July 2, this invasive insect had been reported in numerous counties: Boone, Bureau, Champaign, Cook, DeKalb, Grundy, Kane, Kendall, Lake, LaSalle, McHenry, Peoria, Stephenson, Vermilion, and Will. These types of observations also were being reported by entomologists in other surrounding states.

Although densities of soybean aphids reached impressive levels in certain Illinois soybean fields last year, natural enemies (multicolored Asian lady beetles, green lacewings, and syrphid fly larvae) and heavy rains helped to keep this new pest in check. During the fall of 2000, entomologists had considerable difficulty finding soybean aphids on their overwintering host, buckthorn. Unfortunately, this past fall, finding soybean aphids on buckthorn was less problematic. David Voegtlin, an entomologist with the Illinois Natural History Survey, found fall migrants on buckthorn in a wooded lot next to a soybean field in southern Cook County on October 2, 2001. This may suggest that during the spring of 2002, densities of soybean aphids in soybeans might capture our attention earlier than in 2001. Management guidelines and thresholds will be refined during the winter after consultation with entomologists in other north central states.

The Armyworm Outbreak of 2001

Although we have experienced outbreaks of armyworms in the past, the magnitude of the outbreak in 2001 surpassed all others in recent history. One pundit referred to the outbreak as "Biblical in proportion" and many others reported that the ground in infested fields appeared to be moving. Although we may never know all of the reasons for the intensity of the outbreak, it's safe to say that environmental and ecological conditions in 2001 were excellent for the survival and development of armyworms.

Armyworms do not survive the winter in Illinois; rather, they fly into our state early in the spring on prevailing wind currents. One early indication of an "armyworm invasion" is the capture of large numbers of moths in light traps and pheromone traps, although few people pay attention to armyworms during that time of year. Upon their arrival in Illinois, females seek grasses on which they lay their egg masses. Wheat fields, grassy no-till cornfields, and grass pastures and hay fields are well-known recipients of their egg laying.

After hatching, armyworm larvae begin feeding on foliage. The larvae pass through six instars, requiring about three to four weeks to complete development, depending upon temperature. Early instars are very small, so feeding injury is barely detectable. Research has shown that the sixth instar, which lasts for approximately seven days, causes about 80 percent of the damage. Because of this and because they feed mostly at night, armyworm damage seems to appear suddenly.

A full-grown armyworm larva is 1� to 1 3/5 inches long and green-brown, with varying degrees of black mottling and white flecks. Two orange stripes along each side and two dark stripes on the back are characteristic. A distinct black band is present on each proleg. The head is yellow-brown with a brown, netlike pattern of dark lines.

When numbers of armyworms are large, as they were in 2001, the larvae can defoliate wheat, corn, and grasses very quickly. After they deplete a food source, armyworms move en masse toward other food sources. The common name "armyworm" is attributable to their behavior of moving as an "army" of caterpillars from one food source to another. After safe to say that environmental larvae complete their development, they pupate, eventually emerging as adults. The adults mate to initiate a second generation in Illinois.

During most years, natural enemies suppress populations of armyworms. Parasitoids and pathogens-fungi, microsporidians, and viruses -often suppress armyworm numbers below economic levels. Tachinid flies and several parasitic wasps lay eggs on armyworm larvae. The parasitoid larvae hatch and feed within the armyworm larvae. Cotesia marginiventris is a parasitic wasp that commonly attacks armyworms. In July of 2001, many people reported finding white, cottony masses, which were the cocoons of the braconid wasp Glyptapanteles militaris. Unfortunately, the effects of natural enemies on armyworm populations in 2001 were not realized until after the armyworms had caused significant crop damage.

Management of armyworms is not particularly difficult in wheat and corn. Several effective insecticides are labeled for use in these crops, although harvest intervals need to be considered. In addition, Bt corn, although usually planted for control of European and southwestern corn borers, is very effective against armyworms. However, the challenge faced by many producers in 2001 was the lack of effective insecticides for controlling armyworms in grass pastures and hay fields. Hopefully, the dearth of choices of insecticides for grass pastures and hay fields will be rectified in the near future.

It is not likely that armyworms will reach outbreak proportions in 2002. However, after our experience in 2001, it would be wise to remain vigilant early in the season.

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