Flea Beetle

Chaetocnema pulicaria

The corn flea beetle is the most common species found in Pennsylvania attacking field corn. Several other species can be seen, but they are seldom found in numbers high enough to cause significant injury to the crop. The pest also attacks other field crops that are grown in the state, including soybeans, sorghum, small grains, sweet corn, and several vegetable crop species (particularly those in the nightshade and cabbage families).

The Problem in Pennsylvania

Flea beetles can be found feeding in cornfields throughout the summer, although few fields have economic infestations in a given year. The frequency of economic damage increases in years when average monthly temperatures for January, February, and March total 85 or more. A higher percentage of adults survive the winter under these conditions.

The pest is primarily a problem during the early growth period of corn seedlings (emergence to fourth leaf stage). Although the pest can kill young seedlings by causing significant defoliation, this level of injury is very rare. Flea beetles are a concern in corn production because they can transmit Steward's disease (bacterial wilt). The insect picks up the disease when feeding on diseased plants and then carries it to new hosts. It harbors the bacteria in its gut, providing a mechanism for the disease organism to survive the winter. Injury to field corn caused by disease inoculation has been reduced in modern hybrids because of their moderate to high levels of resistance. Therefore, most cornfields escape economic injury from the disease, although some hybrids are more susceptible to the disease than others. In general, sweet corn varieties tend to be much more susceptible to the disease.

Description

The adult corn flea beetle is shiny black, very small (less than 1/16 inches long), and more or less rounded, with large hind legs for jumping. These hind legs give the insect its common name. When approached or touched, the insect will jump off the plant in a similar fashion to a grasshopper. The larvae vary in appearance depending on species. In general, they look similar to the larvae of the corn rootworm, but have a more slender shape. Their body is milky white and cylindrical with a dark head capsule and three sets of legs, located immediately behind the head. Some species' bodies tend to taper toward the head. On rare occasions, root feeding by larvae can cause economic damage to corn. Eggs are white, oblong, and very small.

Life History

Most flea beetle species overwinter in the adult stage. Adults reside in field rows, roadsides, and other protected areas to overwinter. In the spring, the adults migrate out of their overwintering site as soon as adequate vegetation is available for feeding and egg deposition. Eggs are deposited on plant leaves or in the soil around the root systems of host plants. The larvae hatch and feed on the root systems of host plants. The feeding and developmental characteristics of flea beetles are not well studied. Most species complete one or two generations during the growing season. However, most damage to corn plants is done by the overwintering adults. This damage typically occurs during the first few weeks after the young corn plants emerge.

Damage

Flea beetle damage symptoms vary between the type of hosts attacked. In general, feeding on grasses (including corn) appears as very narrow areas where the green leaf tissue has been removed leaving a clear membrane (epidermis of leaf). The feeding scar will run parallel to the leaf veins and sometimes zigzags across the vein into the next vein, giving the feeding scar a jagged appearance. This feeding damage varies from that of the adult corn rootworm since they tend to eat out large areas along the margin of the leaf, leaving the epidermal layer. If bacterial wilt is introduced by the pest's feeding, an irregular lesion can be seen beginning at one end of the flea beetle's feeding scar. When plants are severely infected with the disease during the seedling stage, they will wilt, die, and dry up. Late- season infections will show up as lesions on the leaf and possibly accelerate corn dry down.

In broadleaf hosts, the injury appears as more or less circular holes in the leaf. This injury is said to have a "shot hole" appearance. These holes are the result of adults feeding on the tissue between the leaf veins, which are arranged in a network on broadleaf plants.

Control

Management is seldom required for this pest; however, there are a few tactics that can help reduce the likelihood of problems. Fields kept free of weeds in and around the field are less likely to be injured by the pest. Avoid early planting dates that slow seedling development. In cases where early planting is necessary, the field should be watched closely for developing infestations (from emergence to the fourth leaf stage), and a rescue insecticide treatment can be applied if needed. It is not economical to apply a preventative treatment just for this pest. However, if a soil insecticide will be applied for other more common pests, then select a material also effective against the flea beetle to prevent injury. Refer to the current issue of the Penn State Agronomy Guide for recommendations on flea beetle management.

Warning

Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

Authored by: Dennis Calvin, Associate Professor of Entomology