Euxesta stigmatias Loew Diptera Otitidae

Natural History

Distribution. Cornsilk fly is found in tropical areas of the western hemisphere. It occurs throughout the Caribbean Islands, Mexico, Central and South America, south to Bolivia and Paraguay, and in Florida. Historically, it has been a pest in the United States only in southernmost Florida, Puerto Rico, and the Virgin Islands. However, in recent years cornsilk fly has become a pest in south-central Florida, and there are sporadic reports of its occurrence from some other states.

Host Plants. Both larvae and adults feed on a wide variety of plants, including such vegetables as sweet corn, potato, and tomato; such field crops as field corn, sorghum, and sugarcane; and such fruit crops as atemoya, banana, guava, and orange. However, sweet corn and field corn are highly preferred, and are the only crops to be seriously damaged by cornsilk fly.

Natural Enemies. Little is known about natural enemies. The eggs are consumed by earwigs (Dermap-tera), mites (Acarina), and minute pirate bugs (Hemi-ptera: Anthocoridae).

Life Cycle and Description. Phenology of these flies is inadequately documented, but they seem to be present throughout the year in southern Florida. The life cycle requires only about 30 days so apparently several generations occur annually. Overwintering potential is unknown.

  1. The eggs are deposited mainly at the tip of the ear, on or near the silk at the point of emergence from the ear. Young ears are most preferred; decaying ears are avoided. In the absence of ears, the eggs may be deposited at the base of leaves or elsewhere, but survival rates are much lower than when they are deposited in ears. The eggs measure about 0.85 mm long and 0.16 mm wide. They are white and cylindrical, with ends that taper to a broadly rounded point. There are reports of eggs being laid singly, in a row and fastened end to end, or in clusters of up to 40 eggs. Females may cluster on corn ears and deposit hundreds of eggs in individual ears. Duration of the egg stage is 2-4 days.
  2. Larvae are elongate and cylindrical, with a blunt, broadly rounded posterior that tapers to a pointed head which is equipped with a pair of mouth hooks. Mature larvae measure about 5.4 mm long. Larvae are whitish, and the ventral surface bears ridges and coarse spines. Duration of the larval stage averages about 7.5 days.
  3. Pupation commonly occurs on the corn silks inside the corn ears, but sometimes in the soil. The puparium is elongate and cylindrical, with the anterior end tapered to a blunt point and the body slightly flattened. It measures about 3.9 mm long and 1.4 mm wide. Initially, puparia are yellowish, become red-

Cornsilk fly larva.

Cornsilk fly larva.

dish-brown after a few hours and then dark-brown at maturity. Mean duration of the pupal stage is about 7.5 days.

Adult. The adult flies are metallic green or black, with reddish eyes, and wings banded with black. Males measure about 3.8 mm long, females about 4.2 mm. The adults feed on nectar, plant sap and glandular exudates, and also drink from dew and rain droplets. They tend to move their wings continuously, even while alighted. Adults frequent corn plant tassels and ears more than other plant parts. Mating occurs mostly at dusk and dawn.

The biology was described by App (1938), Seal and Jansson (1989,1993), and Seal et al. (1995,1996).

Damage

Damage is caused by the larval stage. Larvae feed on the silks, but the principal injury is caused to the developing kernels on the ear, where they often hollow out the kernels. Larvae may be found feeding along the entire length of the ear, or mostly at the tip. Larvae also bore into the cob, resulting in ear deformity, and introduce fungi beneath the ear sheath. Yield reductions of 95% are reported at peak levels of damage early in the season (Seal and Jansson, 1989; Seal et al., 1996). Significant damage occurs even when insecticides are applied.

Management

Sampling and management tactics are poorly developed. Sampling should commence before tasselling is

Cornsilk fly puparium.

Cornsilk fly puparium.

Adult cornsilk fly.

initiated. Adults can be detected in the late afternoon and early evening when they rest on the plants and mate on the tassel. During silking, silks should be checked for the presence of eggs. Growers in affected areas rely on frequent insecticide applications to maintain a toxic residue on the rapidly developing corn silk. Alternatives to insecticides are very few. Covering the tassel and ear with a bag can decrease the number of larvae found in ears, but as the silk must remain uncovered until the ear is pollinated, some oviposition is likely to occur on the young silks.

0 0

Post a comment