The literature about grape colaspis extends well back into the 1800s. However, it's only recently that concern about this pest has become more acute throughout central Illinois. Following is some background about the insect and some possible explanations for the ups and downs of its population densities over time.
Description and life cycle. Grape colaspis, Colaspis brunnea, has one generation per year. The adult colaspis, which appears around mid- to late June, is about 1/8 to 1/6 inch long and resembles, according to many, a "brown northern corn rootworm beetle." Found feeding on soybean trifoliates, corn leaves, and corn silks, the adults have a series of lines on their elytra (wing covers). These lines actually are a series of small "punctures" arranged in rows. Feeding injury by the adults, which has not been documented to be economically significant, appears "jigsaw puzzle-like" in shape. Adults mate several times throughout the growing season, and females deposit elongate, white to yellow eggs in clusters of about three dozen in the soil. Larvae hatch about 1 to 2 weeks later. The young larvae have orange head capsules and white to gray bodies. Grape colaspis larvae resemble small white grubs, although they are much smaller than grubs of Japanese beetles or Phyllophaga species.
Adult grape colaspis.
Three grape colaspis larvae next to a Japanese beetle grub larva. Note the significant difference in size. (Photograph courtesy of Mike Hellmer, Pioneer Hi-Bred International.)
In the fall, grape colaspis larvae move downward and outward in the soil profile and begin to feed on the roots of available plants. In September, as soil temperatures begin to decline, the larvae migrate 8 to 10 inches deep in the soil, where they overwinter. When the soil warms up in the spring, larvae return to more shallow regions of the soil profile. They begin to strip away fine roots and gouge "channels" in the roots of available plants (typically corn, but also soybean), sometimes causing significant stand reduction in fields of commercial corn hybrids. Grape colaspis damage to corn inbreds can be disastrous in seed-production fields. In early to mid-June, the larvae excavate earthen cells in which they pupate and transform into adults.
Grape colaspis injury in a seed-production field in Cass County in 2003.
Corn plants in a seed-production field injured by grape colaspis. Note the height differential between healthy and injured plants.
Some grape colaspis history. Grape colaspis was troublesome off and on throughout the 20th century and was especially noted on corn following red clover up to about the mid- to late 1950s. The pest appeared occasionally after the 1950s, making a late-1980s appearance in drought-stressed soybean fields. (Although we remember that spider mites caused damage to soybeans in 1988, grape colaspis larvae caused significant damage to soybeans earlier in that same year.) In the mid-1990s, grape colaspis larvae seemed to reappear with what some might call a vengeance. Although some people believe that grape colaspis larvae afflicted corn without being diagnosed for several years, problems with grape colaspis were verified in 1998 and thereafter. Widespread injury occurred in 1999 and 2000, and concern about this pest has escalated accordingly. In 2001 and 2002, grape colaspis problems abated somewhat.
Effect of soil moisture on grape colaspis larvae. So why the difference in densities of grape colaspis between the late 1990s and 2001? Nobody really knows for certain, but we can speculate. Dale Richard Lindsay published a thesis entitled The Biology and Morphology of Colaspis flavida in the 1940s (Colaspis flavida being the former scientific name for grape colaspis). Lindsay noted some interesting points about various portions of the grape colaspis's life cycle. He noted that eggs, pupae, and larvae were very susceptible to "desiccated conditions." In other words, dry conditions had a significant detrimental impact on grape colaspis survival during each immature growth stage. The larval stage, especially during the first week of the larval stage, was deemed most at risk when exposed to dry conditions. I mapped various rainfall patterns for Sangamon and Menard counties for different periods of the growing seasons in 1998, 1999, and 2000 and noticed some interesting patterns. When rainfall patterns during July to September were near average, grape colaspis problems were notable the following year. For instance, rainfall in Menard County in 1998 and 1999 appeared to be fairly close to a 10-year average. Subsequent grape colaspis problems in 1999 and 2000 were fairly widespread and significant. Conversely, rainfall during July through September in 2000 was significantly less than the 10-year average. Subsequent grape colaspis problems in 2001 were relatively uncommon. It is possible that dry soil conditions between July and September in 2000 may have resulted in significant mortality of young grape colaspis larvae. Adequate soil moisture between July and September in 1998 and 1999 may have encouraged survival of grape colaspis larvae. It is interesting to note that rainfall from July through September in 2002 was very similar to the rainfall that occurred during July through September in 1998 and 1999. We do not know, however, if there are cumulative effects on grape colaspis mortality from consecutive "dry" years.
Anticipation and management of grape colaspis. For now, our best recommendations for management of grape colaspis are to adopt practices that result in the most rapid, uniform emergence of seedlings in fields with a history of grape colaspis problems, as well as practices that result in fast-growing, vigorous root systems. Any valid option that can give the seedling a head start, allowing it to outpace colaspis damage, is to the producer's advantage. If we become better at predicting this troublesome pest, prophylactic use of effective soil insecticides or insecticidal seed treatments would be warranted.--Matt Montgomery