To sample corn fields for European corn borer, hundreds of stalks per site were cut open, noting the number of larvae and tunnels and whether stalks were broken from tunneling. In their recent sampling efforts in York and Lancaster Counties, they 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).
There is little doubt 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). This species was found 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.