Wireworms as Pests of Field Crops

Wheat wireworms, Agroites mancus (Say), and the eastern field wireworm, Limonius agonu s (Say), are the predominant species that attack corn and other field crops in Pennsylvania. Wireworm damage to crops often is confined to certain areas of a field, and stand losses can vary from zero to 75 or 80 percent. However, high crop losses are not common in Pennsylvania. Wireworms can be best described as minor pests with the potential of causing high crop losses.

The two major species prefer grasses as host plants; therefore, the greatest damage occurs in crops planted in fields that were in grass sod the previous year or two. Major crop losses are to corn small grains, and potatoes. The larvae live and develop in the soil. They injure plants by eating the newly planted stems, and by boring into stems, roots and tubers.

Wireworm ©Marlin Rice Iowa State University

Description

The adult beetles are commonly called click beetles because of their ability to snap their thorax when placed on their backs or held between the fingers. The adults of destructive species are reddish brown, trim, slender, hard-shelled beetles about 1/3-inch long. This is the only stage found above ground.

Wireworm eggs are seldom seen since they are deposited in the soil. A female beetle deposits several hundred very tiny, pearly white, round eggs among soil particles. Eggs generally are laid singly and are widely scattered.

Newly hatched larvae are white but have dark brown jaws. They are about 1/16 inch long. After about a month, they become hard and have a shiny yellowish brown color. The wireworms normally seen on damaged plants are from 1/4 to 3/4 inch long, yellowish brown, cylindrical and, slender. They have three pairs of short legs.

The pupae seldom are seen but are white and about 1/2 inch long. They are soft and very fragile. They resemble the adult beetles in shape and size.

Life History

Wireworms overwinter mainly in the soil as partially grown larvae and as adult beetles. The adults (click beetles) deposit their eggs in the soil during the summer. The larvae (wireworms) feed on underground parts of plants for 2 to 3 years before reaching maturity. Mature larvae pupate in the soil during late summer and the new adult beetles remain in the soil until the next summer.

There is one generation every 2 to 3 years. However, considerable overlapping of generations occurs, thus adults and larvae of various sizes and ages are present each year.

The relatively slow development of wireworm larvae extends the time that crops planted into an infested field will be subjected to damage. Unless wireworms are controlled, moderate to heavy damage can be expected for 2 to 3 years.

Damage

Wireworm larvae feeding on corn plants (soil washed off). Dennis Calvin collection.

Corn plant showing a foliage effect of wireworm injury. Dennis Calvin collection.

Wireworms damage corn plants in several ways. Soon after planting they may eat the seed of cut off the seedlings below the ground level. A few weeks later, when the plants are larger, they will tunnel into the underground portion of the stem and cause the plant to wilt and die. After the plants are about 18 inches high, they will feed on the roots and either tunnel or scar the larger roots.

Wireworms are a major pest to potatoes. They ruin the tubers by eating small, round tunnels.

Control

Prevention of wireworm damage requires treatment before or at planting time. There are no practical or effective ways to control the pest after the crop is planted.

The most serious problems occur in fields that previously were in grass sod for several years prior to planting. A few isolated fields in the Commonwealth have a history of wireworm problems. These fields should be monitored for wireworms either the previous fall or prior to planting in the spring. Monitoring should be carried out when the soil temperature is 45 F or above. The soil should not be dry. To sample, dig down about 10 inches and lift the shovel of soil for examination. Round the soil sample off to approximately 6 inches in diameter. Sift through at least 20 shovels of soil from different locations in the field to check for wireworms. An average of one wireworm per shovel of soil indicates a population of more than 20,000 wireworms per acre. A suggested economic injury level for corn is an average of 2 or more wireworms per 10 shovels of soil. At this level, it probably will be profitable to use a preventive treatment. If the number is less, a treatment is not suggested. Wireworm infestations tend to be concentrated in certain areas of the field. Treatment can be limited to the infested areas when identified and marked. Potatoes should not be planted in a field with a high wireworm level. The field should be treated before either corn or small grains are planted.

Check the Agronomy Guide or consult with your pesticide supplier or county agent for details of pesticide use.

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: Stanley Gesell, Extension Entomologist Dennis Calvin, Professor

Updated 1983