The corn earworm, also known as the tomato fruitworm and the cotton bollworm, feeds on a number of crops including corn, tomato, cotton, green beans, clover, vetch, lettuce, peppers, soybeans, and sorghum.
The most severe infestations of corn earworm generally occur in the southern United States. Losses due to the corn earworm in field corn has been estimated at 2.5% annually, with losses in the southern United States ranging from 1.5-16.7%. Losses in sweet corn may be as high as 50%.
Description:
Corn earworm larvae vary greatly in color ranging in from light green or pink to dark brown or nearly black. Alternating light and dark stripes run the length of the body. Double dark stripes can usually be seen down the center of the back are usually present and the underside of the larva is typically light colored.
Distinct tubercles are present with two or three large hairs protruding from each. Larvae pass through five growth stages and are 1½ inches long at maturity. Pupa are oblong, ¾ to 1 inch in length and reddish or light brown. They can be found in the soil near the host plant at a depth as great as 6 inches.
The adults are nocturnal moths about ¾ inch long with a wingspan of 1½ inches. The forewings are buff colored with dark lines or spots near the tips. Eggs are laid singly on the leaves of preferred food plants. Pubescent leaves are preferred, but corn tassels and silks are even more attractive.
Eggs are hemispherical, ribbed, and less than 1/16 inch in diameter. White when first laid, they develop a red ring around the circumference in 24 hours and show the black head capsule of the larva just before hatching.
Life Cycle:
Although corn earworm populations usually die out during the winter in most of the state, pupae in the soil survive the winter in the soil in far southern Illinois most years. In addition, populations can overwinter in other portions of the state when the winter is mild. Survival is greatly reduced when winter temperatures are less than 30°F.
Generally, corn earworms do not overwinter in Illinois north of Interstate 70. Despite some local overwintering, summer populations of this insect in Illinois arise primarily from the immigration of moths from southern states in late spring and early summer.
These moths arrive on weather fronts and lay their eggs in susceptible crops. Adults are usually found in June. Egg laying takes place primarily in the evening hours, with each female capable of laying 500 to 2000 eggs during her 2-week life span.
Eggs hatch in 3 to 4 days at an average temperature of 77°F. Larvae feed on whorl stage corn and other host plants for a period of 3 to 4 weeks before burrowing in the soil to pupate. Two full generations of earworm development can occur each year after the spring/summer migration into the state.
Second generation larvae and moths occur during pollination. Population densities usually peak in late summer. Moths lay eggs on the silks of corn plants. Even though moths may lay more than one egg per ear, there is generally only one larva per ear because they are cannibalistic.
Larvae will migrate down the silks to the ear tips within 1 hour of hatching. Larvae feed on the developing kernels under the protective husk for the entire larval stage. When larval development is complete, the larva chews through the husk and exits the ear. It then drops to the ground, burrows into the soil, and pupates.
Injury:
Injury caused by the corn earworm ranges from destruction of the host crop to cosmetic damage that may cause a crop to be unmarketable. Larvae feed on the pods of snap beans. Their burrowing through the pod or chewing through the pod wall to feed on developing seeds causes cosmetic damage.
These injuries can result in an unusable product for both the fresh and processing market. Although the corn earworm will feed on buds and flowers of tomato plants and may also bore into the stems, it prefers green fruits. Larvae often enter fruit under the calyx at the stem end and remain unnoticed until extensive damage is done.
The deep cavity made by larval feeding is usually contaminated with wastes, becomes watery and soft, and serves as an entry point for diseases. Fruits that are damaged early in their development are most likely to rot before harvest.
Although the corn earworm is a severe pest of field corn in southern states, in Illinois its economic importance in corn is limited to sweet corn and seed corn. The corn earworm feeds not only on the whorl, tassel, and silks, but on the kernels of the ear itself. Severe feeding on the leaves gives the plant a ragged appearance.
Feeding on kernels at the tip of the ear creates an avenue of entry for diseases, especially molds in seed corn. Similar damage in sweet corn results in an un-salable product, especially for the fresh market. Home gardeners and even some sweet corn processors often accept some damage by corn earworm because feeding is commonly limited to ear tips, which can be cut off before processing or home use.
Unfortunately, the presence of damage and a live earworm beneath the husk is usually not acceptable in the fresh produce market.
In seed corn production, damaged kernels represent yield loss. Just as importantly, removing damaged kernels from seed lots results in additional losses because substantial amounts of undamaged seed corn are also discarded during the mechanical sorting process.
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