No. 11/June 5, 1998
Grape Colaspis Larvae
Causing Problems in Many Fields
During the weeks of May 25 and June 1, we received numerous reports frompeople who have observed grape colaspis larvae causing significant injury tocorn. Most of the calls have come from western Illinois, but the problem hasbeen observed at least as far east as Macon County. All callers report thesame thing: Injured plants are wilting, some of them are dying, the roots arestubby, and the root hairs have been stripped away. These are classic injurysymptoms caused by feeding of grape colaspis larvae.
Following are some questions and answers about diagnosing grape colaspisproblems in corn and some discussion about the life cycle and management ofthis pest.
The plants look as if they have been injured by herbicide carryover. How do Iknow the injury is caused by grape colaspis? Grape colaspis larvae feed onroot hairs and may eat narrow strips from the roots. Denuded roots don'tobtain moisture and nutrients efficiently. Injury symptoms above groundinclude stunting, wilting, purpling of the leaves and stem (indicating aphosphorous deficiency), and browning of the tips and edges of the leaves.Severe infestations may cause plant death and reduced plant populations. And,of course, the real diagnostic sticker is the presence of the larvae. Althoughthey sometimes are difficult to find because they are so small, carefulsifting of the soil will reveal the culprits.
When I examined the injured plants, I found what appeared to be small whitegrubs. How do I tell the difference between small white grubs and grapecolaspis larvae? The grape colaspis larva (Figure 5) is 1/8- to 1/6-inchlong, slightly curved (comma shaped), and has a plump, white body with a tanhead and prothoracic shield, or plate just behind the head. Its three pairs oflegs are short. Bunches of hairs arise from bumps on the underside of theabdomen. Early instar white grubs are more C- or U-shaped, and they lack thecharacteristic bumps on the underside of the abdomen. Also, the legs of whitegrub larvae are relatively long.
Figure 5. Grape colaspis larva (from the1994 Handbook of Soybean Insect Pests, Entomological Society of America, Lanham, MD).
The stand is in rough shape. Will the plants recover? How much longer will thelarvae feed? Injury to corn caused by grape colaspis larvae is more severewhen weather conditions retard the growth of the seedlings. Corn growth mayhave been less than ideal within the recent past, but current weatherconditions have been more conducive for corn growth. We hope that some of thewilted plants will recover. The answer to the question about how much longerthe larvae will feed depends on temperature, too. Usually grape colaspislarvae finish feeding by mid-June, but warmer soil temperatures couldaccelerate their development. By contrast, cooler temperatures could slow themdown.
What is the life cycle of these critters? The grape colaspis completes onlyone generation per year in Illinois. It passes the winter as a small larva inthe soil 8 to 10 inches deep. Larvae become active early in the spring, feedon the roots of host plants, and complete their development from mid-June toearly July in the Corn Belt. Pupation occurs in an earthen cell 2 to 3 inchesbelow the soil surface. Adults emerge from the soil during June in thesouthern states and during July in the Corn Belt. The tan adult is oval andabout 1/6-inch long, with rows of tiny punctures on its wing covers, makingthem appear ridged. Females lay eggs in the soil near host plants, includingpatches of smartweed and bull nettle. Eggs hatch in 7 to 14 days. Newlyhatched larvae feed on roots during the latter part of summer and early fall.
Why am I seeing this problem in corn planted after soybeans? I thought grapecolaspis caused problems in corn planted after red clover. Early studies andobservations about grape colaspis indicated that grape colaspis problems weremuch more prevalent in corn planted after red clover than in any other croprotation sequence. However, these studies were conducted decades ago whensoybeans did not take up much of our acreage, and we don't grow that much redclover anymore. Obviously the grape colaspis had to adapt to our more modernagricultural system, so we now observe grape colaspis problems more often incorn after soybeans, and occasionally in soybeans planted after corn.Unfortunately, the grape colaspis, like a lot of "occasional" pests in corn,has not been studied for a long time, so our knowledge base is dated.
So what can I do about the problem? Are there any rescue treatments?Unfortunately, guidelines for management of the grape colaspis are few and farbetween. No insecticide is registered as a rescue treatment, and it's doubtfulthat anything would work anyway. Most subterranean pests are not controlledeffectively with rescue treatments after the crop has emerged. The managementconsideration that needs to be addressed for a field infested with grapecolaspis is whether the stand is acceptable or replanting is justified at thislate date. If a grower decides to replant, the use of soil insecticide toprotect the new stand probably is not justified. The larvae will finishfeeding soon, and no soil insecticide is registered for control of grapecolaspis.
Kevin Steffey (firstname.lastname@example.org) and Mike Gray (email@example.com), ExtensionEntomology, (217)333-6652