Corn Pith Weevil

For the past three years, my graduate student Eric Bohnenblust, Greg Roth, and I have been sampling Bt and non-Bt corn plantings around Pennsylvania to understand the value of Bt corn varieties targeting European corn borer for Pennsylvania growers. We have found that Bt varieties continue to do a great job controlling European corn borer, but generally speaking, European corn borer populations around the state are quite low, indicating there may be some value in growing high yielding non-Bt varieties. Importantly, though, there are some exceptions to the generally low populations we have found. Notably, York County seems to have consistently high populations of European corn borer, suggesting that this pest species continues to be relevant for some areas of the state.

To sample corn fields for European corn borer, we cut open hundreds of stalks per site, noting the number of larvae and tunnels and whether stalks were broken from tunneling. In our recent sampling efforts in York and Lancaster Counties, we encountered for the first time another insect pest in corn stalks, and occasionally this pest was responsible for causing the stalk to break. The pest is the corn pith weevil, Geraeus penicillus, occasionally called the corn stalk weevil. (This species is different than the corn stem weevil, which is an occasional early season pest of corn). This species has one generation per year, with adults being active in July, laying eggs on stalks up near the tassel or even in the tassel. These eggs hatch and larvae burrow into the upper nodes of plants. Notably, its tunnels are long, skinny and continuous unlike corn borer tunnels (Fig. 1), which are often short and intermittent and rarely occur in the upper portions of corn stalks. Larvae of this small weevil are distinct from European corn borer in that they do not have obvious legs and have a head capsule that is flattened and not round like that of a caterpillars (Fig. 2).

Fig. 2. A corn pith weevil larva that has been pulled out of its tunnel. Notice the lack of legs and a somewhat flattened (not round) head capsule, which distinguish the species as a beetle larva, not a caterpillar.

There is little doubt in my mind that this species is not an economic pest; it is tunneling too late in the season and in the upper reaches of stems where it is unlikely to influence yield. Not surprisingly, it seems to occur in both Bt and non-Bt varieties in equal abundance (Bt varieties targeting corn borer should not have activity against this beetle species). We found the species in York and Lancaster Counties, but not Centre. Because it can cause some stalk breakage, there is a chance that its damage could be misinterpreted as late season corn borer damage and I want to raise the issue to avoid any confusion. I would be curious if others find evidence of this species in your fields. Cut open some stalks and let me know what you see!