Corn flea beetles are occasionally a problem as the earliest sweet corn plantings of the season are in the seedling stage. The beetles vector the bacterium responsible for the disease Stewart’s wilt. Fortunately, many modern sweet corn varieties have a high degree of resistance to Stewart’s wilt. Additionally, many seed companies offer seeds that have been treated with fungicides and neonicotinoid (IRAC 4A) insecticides. These latter materials are systemic and confer protection from flea beetle feeding to seedling stage plants.
Direct feeding injury (chewing on leaf surface) from flea beetles is rarely significant, but can occur early in the season when days are warm and still but night temperatures are still low. This permits beetle activity during the day and little growth of seedlings during the cold nights.
The threshold of 6+ beetles/100 plants is extremely conservative and was originally meant to limit transmission of the Stewart’s wilt bacterium to susceptible plants. At this time, that threshold should only be observed if no seed treatment or soil applied insecticide was used AND the variety is not resistant to Stewart’s wilt. Should direct feeding injury become widespread in samples, treatment may be necessary. Later in the season, as corn is growing rapidly, flea beetles are not considered pests.