Veronica Yurchak, Vegetable Specialist, UMD; vjohnso4@umd.edu
Congratulations to Mike Crossley and Grier Stayton for correctly identifying these as sap beetle larvae infesting this sweet corn ear. Sap beetles often enter sweet corn ears following other types of damage (corn earworm, bird damage, etc.), however infestations can occur in the absence of other damage as well. Adult sap beetles are typically black or brown beetles with an oval shape, and economically important species typically measure 3-6 mm (about 0.12-0.25 inch) long. Larvae are small, cream colored, maggot-like grubs with a brown head. They can measure up to 6 mm (about 0.25 inch) long and are active, moving quickly when exposed to light (Fig 1). Most insecticides used to control other corn pests, such as corn earworm, will also control sap beetles. However, sap beetles are not susceptible to the Bt proteins in transgenic corn, so ears should be monitored for sap beetles in transgenic sweet corn not treated with additional insecticide sprays.
Figure 1. Sap beetle larvae infesting sweet corn ear.
European corn borer larva and damage in sweet corn ear.
Guesses this week were interesting, as several incorrect suggestions were also submitted. To quickly differentiate sap beetles from the other guesses provided:
European corn borer: European corn borers have become an uncommon sight in most crops over the past two decades with the increased adoption of Bt field corn. Still, larvae may occasionally be found in areas with low Bt adoption. In sweet corn, sap beetle larvae will only infest the kernels, whereas corn borers are more often found tunnelling into the stalk. In ears, corn borers will typically tunnel into the cob, rather than feed only on kernels. Sap beetles will also typically be present in higher numbers and adult beetles will likely be seen as well.
Seedcorn maggot: Seedcorn maggot can be similar in size and initial appearance to sap beetles. However, their life cycle is very different. Seedcorn maggots are primarily an early season pest that attack sweet corn (and other crop) seeds and germinating seedlings. Adut flies lay eggs on the soil surface, and larvae burrow down into the seeds. They are therefore not likely to be found in sweet corn ears.