Many of you will recall that grape colaspis larvae caused some significant damage last year in fields of corn planted after soybean in west-central and western counties. Well, they're back. Late in the week of May 8, I received my first report of grape colaspis injury from Todd Burrus with Burrus Power Hybrids in Arenzville in Morgan County. Since then, at least three other people have reported injury caused by this pest, one from as far east as Christian County in central Illinois. Some of the damage has been rather severe, necessitating replanting in one situation. In an article Mike Gray and I wrote about secondary insect pests of corn, we speculated that grape colaspis and other "minor" insect pests may become more commonplace if we continue to plant early and if mild winters persist. The reoccurrence of grape colaspis and other insects (discussed in the article "Report from the Field") lends some credence to our speculation. 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?
The following is a quote from my write-up about grape colaspis in the Entomological Society of America's Handbook of Corn Insects published in 1999: "The grape colaspis is a sporadic pest most often found in corn planted after red clover or mammoth clover, and occasionally in corn planted after sweet clover, alfalfa, or soybeans." Apparently we need to reconsider that statement about "occasionally in corn planted after . . . soybeans." Very little acreage is devoted to clover production in Illinois these days, so that preferred host is no longer readily available. It is possible 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.
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.
Above-ground symptoms of injury caused by grape colaspis 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?
Unfortunately, no soil insecticides currently are labeled for control of this pest. Assuming we could predict the occurrence of grape colaspis, application of a soil insecticide would constitute an economic risk with no guarantee of positive results.
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. And as I suggested with my answer to the previous question, the use of a soil insecticide during replanting offers little in the way of guaranteed control.
If we hear from reliable sources that certain products are providing either suppression or better control, we'll let you know. Thus far, reports have been negative.
Will the experimental Bt-corn hybrids for rootworm control be effective against grape colaspis?
Good question, but we don't know. We intend to address this question in some of our efficacy trials either this year or next year.
If you encounter this pest, good luck. It's going to be another "tough nut to crack."--Kevin Steffey