Preliminary Results from Grape Colaspis Insecticide Efficacy Trial

June 2, 2000
Grape colaspis injury reports continue to flood our office. In most instances, significant stand reductions in cornfields are reported. However, in some locations (Hull, Illinois, Pike County) some soybean fields also have been severely damaged by grape colaspis. In 1988, there were hundreds of reports of grape colaspis significantly reducing stands of soybean fields. That's right; grape colaspis larvae also can feed on soybean roots. Some producers in western Illinois are contemplating replant decisions for soybeans. As with corn, rescue treatments are not an option in soybeans. So frustrations with grape colaspis and other grubs continue this spring. Don't forget that most published reports indicate that grape colaspis larvae may continue to feed on roots from mid- to late June. Let's hope we see pupation sooner and not later this year.

As mentioned in last week's Bulletin (issue no. 9), we evaluated a grape colaspis insecticide efficacy trial in Menard County near Tallula on May 23 and 24. The team was led by John Shaw, Illinois Natural History Survey, and included many summer interns and several graduate students from the Departments of Crop Sciences and Entomology. For a full description of life- cycle information on grape colaspis, please consult issue no. 8 of the Bulletin. As producers are well aware of this spring, no soil insecticides are currently labeled for this insect pest. Obtaining meaningful insecticide efficacy data for grape colaspis has been a challenge due to the sporadic and unpredictable nature of the target insect.

Presented in Table 1 are the results from our trial near Tallula. Please note that, in addition to grape colaspis data, we also present information on annual white grub abundance. All insecticide treatments were replicated four times. For each replicate by treatment combination, we removed all the soil from a 1-meter trench of soil to a depth of approximately 6 inches. Live corn plants, grape colaspis larvae, and white grubs found in the 1-meter trench were counted. This process was very time-consuming due to the abundance and small size of grape colaspis larvae. Although the plants in many of the plots were stunted and purple, limiting our observations to those above ground and blaming all of the injury on grape colaspis would have been a mistake. In addition to grape colaspis larvae, annual white grubs were very numerous. In troubleshooting fields with suspected grub injury, keep in mind that other factors such as herbicide injury, environmental stresses, and hybrid type may contribute to purple discoloration of plants.

Considerable variation in plant number and grape colaspis and white grub abundance occurred among the insecticide treatments. Only Regent 4SC (8.0 plants per meter) had significantly more live plants compared with the untreated control (4.13 plants per meter). None of the insecticide treatments were significantly different from the untreated control with regard to the average number of grape colaspis larvae found per meter of row. This observation seems to give more credence to the belief that soil insecticides have not fared well against grape colaspis infestations. Keep in mind that this experiment was evaluated only 3 weeks after insecticides had been applied; yet grape colaspis larvae and white grubs were very abundant. Many products were able to significantly reduce the density of white grubs; however, grubs could be found in every treatment. With regard to the optimum placement of a soil insecticide for grape colaspis control, an in-furrow application may have a slight advantage (see Counter 20CR results). The bottom line of this experiment is that grape colaspis larvae and white grubs will remain a challenge for some time to come. This will be especially true if mild winters and very early planting become the norm. Remember, no soil insecticides are registered for grape colaspis larvae. Results from this experiment should not be used to justify the application of a product for grape colaspis control. For white grub control, use only a labeled product.--Mike Gray

Author: Mike Gray