Severely damaged seedlings may die, while less seriously damaged plants often suffer a reduction in vigor and stamina. Hot, sunny weather is conducive to feeding activity, while cool, damp weather slows feeding and favors crop growth. Hot and dry weather may cause damaged seedlings to wilt and die, and partial to complete crop loss can result. Once the crop advances beyond the seedling stage, serious damage usually does not occur, since vigorously growing mustard can outgrow the beetle defoliation. No major effects on plant vigor have been noted from the feeding of the larvae on plant roots.
Cultural methods can help reduce plant losses caused by flea beetles.
A firm seedbed that is well tilled and adequately fertilized will help plants outgrow beetle damage during the susceptible early season stages. A few flea beetles or scattered shot-holing are not necessarily cause for alarm. However, if flea beetles are numerous on plants and feeding holes are present in most cotyledons, immediate control will likely be required. Flea beetles are shiny black, jumping beetles about inch long. Diamondback moth caterpillars attain a length of inch and are light yellowish-green to green.