All You Need to Know About Cob Flies in Sweet Corn

cob flies larvae in sweet corn

In Florida, cob flies (phorid flies) are sporadically found in sweet and field corn ears grown during the late spring and in both dry and rainy summer seasons.
Photo by D. Owens

For several years, cob flies have been reported damaging field and sweet corn in the summer around the Everglades Agricultural Area, causing damage to ears less than seven days after first silk. Damaged silks are reddish brown to brown. Heavily damaged silk appears wet and slimy and may be a site of secondary infection by pathogens. Severe tip damage can occur on ears that are still a week to 10 days from harvest. Field corn ears infested with phorid larvae have the top 25% to 50% of the cob bare of kernels at harvest.

Phorid fly larvae feed on silks, cobs, and kernels as do silk flies. Because larvae develop faster and leave the ear earlier than silk fly, it is possible phorids are partly responsible for the damage often attributed to silk fly.

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Adult cob flies are tiny, about half the length of silk flies. Adult phorids are light brown with dark brown to black markings on the dorsal side of the abdomen. Corn-feeding phorids have no dark bands across their wings, distinguishing them from silk flies. Phorid flies have a very distinctive motion moving quickly in a “herky-jerky” fashion.

Larvae can be distinguished from silk flies by the tapering shape of the posterior abdomen. In contrast, silk fly larval abdomens terminate bluntly, and two dark-colored, peg-like spiracular plates can be seen at the end of the abdomen. Phorids retain their white coloration throughout the larval stage, while silk flies darken to a yellow color in the third instar. When disturbed, silk fly larvae will flex and jump several centimeters. Phorid larvae do not exhibit this behavior.

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cob flies adult

Adult cob flies are about half the length of silk flies.
Photo by G.S. Nuessly

Survival and Spread

Cob fly eggs are deposited in the silks at corn husk tips. There have been reports of fly eggs deposited on husk leaves before silks emerge; these might be either cob fly or silk fly eggs.

On emergence, the larvae enter the ear to feed on silks, cob, and kernels. Cob fly larvae can be observed as a larval “mass” that feeds through the silks and destroys them before reaching the ear tip. Larvae develop and damage silks quickly enough to interfere with pollination. Once they reach the ear, larvae feed on the cob and developing kernels at the tip. Cob flies vacate the ear 10 to 14 days after first silk, leaving a visible shiny mucous trail on the drying silks. Silk flies do not leave a mucous trail when they exit ears.

Cob fly larvae pupate in shallow soil. Pupae are light brown, somewhat triangular and noticeably flattened, with ridges across the surface.

Management Methods

Scouting corn for cob flies should begin at tassel-push. Treatment may be justified prior to first silk or as early as ear shoot emergence, if phorid adults are present. Contact insecticides used for silk fly management might provide cob fly control.

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