Stalk Borer

Papaipema nebris

Stalk borer inside a soybean stem
Stalk borer inside a soybean stem. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
As their name implies, stalk borer larvae bore into stalks and stems and feed on the plant from the inside, causing considerable damage or death to the plant in the process. They overwinter as eggs and because of this are more serious in continuous corn and no-till fields which have abundant oviposition sites. The eggs hatch in May and June and the larvae continue to feed into July.
Larvae have distinct longitudinal stripes and a large dark purple region behind the head.
Stalk borer inside a stem
Stalk borer inside a stem. Photo courtesy of James Solomon. Licensed under CC BY 3.0.
Stalk borer larvae have two feeding patterns. The first pattern is to enter at the base of the plant and then work up, which often kills the plant or results in "dead heart". The second pattern is to start at the top and feed through the whorl into the stalk. This can cause the top of the plant to be deformed or stunted, reducing yield.
Weeds and grasses are often the primary host for stalk borers, with corn a secondary host as the larvae increase in size. Removing primary hosts helps to protect the corn. Tilling the field in the fall removes many oviposition sites, making the field less susceptible to damage the following year.
Many Bt traits provide some level of protection against stalk borer.