Let's review some of what we know and some of what we don't know about grape colaspis.|
What types of fields are most susceptible to injury caused by grape colaspis?
As reports on grape colaspis damage in cornfields and soybean fields begin to arrive again this year, we must consider the possibility that the grape colaspis has adapted to modern corn/soybean rotation to survive. If such an adaptation has occurred, we can expect to encounter grape colaspis problems more frequently in the future. The literature indicates that patches of smartweed and bull nettle are attractive egg-laying sites. We wonder if other weeds might be attractive, too. Very little modern information is available, but we hope current research projects will begin to shed some light on this pest. Last year, researchers conducted sweep net sampling for grape colaspis. This data may help producers predict the likelihood of injury in their fields, but remember that there is currently no economic threshold available for grape colaspis (Figure 1).
What does injury caused by grape colaspis look like?
Grape colaspis larvae feed on root hairs and may eat narrow strips from the roots. Denuded roots cannot obtain moisture and nutrients efficiently. Injury symptoms above ground include stunting, wilting, purpling of the leaves and stem (indicating a phosphorus deficiency), and browning of the tips and edges of the leaves. Severe infestations may cause plant death and reduced plant populations. Injury is more severe when weather conditions retard the growth of the seedlings.
Aboveground symptoms of injury caused by grape colapsis larvae.
Fine roots stripped from roots by grape colaspis larvae.
What do grape colaspis larvae look like?
The larva is 1/8 to 1/6 inch long; it is slightly curved (comma shaped); and it has a plump, white body with a tan head and prothoracic shield (the plate just behind the head). Its three pairs of legs are short. Bunches of hairs arise from bumps on the underside of the abdomen. In essence, they resemble miniature white grubs.
Grape colaspis larvae around corn roots.
What do we know about their life cycle?
The grape colaspis completes only one generation per year in Illinois. It overwinters as a small larva in the soil 8 to 10 inches deep. Larvae become active early in the spring, feed on the roots of host plants, and complete their development from mid-June to early July. Pupation takes place in an earthen cell 2 to 3 inches below the soil surface. Adults emerge from the soil in July. Females lay eggs in the soil near host plants, including patches of smartweed and bull nettle. Eggs hatch in 7 to 14 days. Newly hatched larvae feed on roots during the latter part of summer and early fall.
Are any soil insecticides registered for control of grape colaspis larvae?
Regent 4SC is currently labeled for grape colaspis at a rate of 0.13 lb ai/acre. Capture 2EC is not registered for grape colaspis but may suppress the population at a rate of 0.3 oz per 1,000 ft of row.
Seed treatments are not labeled for control of grape colaspis larvae. The University of Illinois and the Illinois Natural History Survey are conducting insecticide efficacy trails again this year to evaluate their effectiveness in controlling grape colaspis. The results of last year's grape colaspis, white grub, and wireworm insecticide efficacy trials are available on the University of Illinois IPM web site. Menard County results are available here and results from Logan County are available here.
So what can be done if grape colaspis larvae are causing significant stand reduction?
Rescue treatments probably would not be effective against grape colaspis larvae. Therefore, replanting is the only option in fields with significant stand reduction.--Susan Ratcliffe and Kevin Steffey