Life cycle and description

• Adult stage:Green-bodied flies with four dark bands on the wings. Typically they have a blue metallic thorax and reddish-brown eyes. Adult flies are 0.5cm long.  Adults are often found in pairs and are active runners. The banded wings slide together with quick strokes as they move on the foliage of the maize plant. Adults feed on pollen, nectar, plant sap and glandular exudates. Mating generally occurs at dusk and dawn.
• Egg stage: Small (0.16 × 0.9mm), cylindrical, white, and tapering to rounded points. Eggs of E. stigmatias are deposited in silk channels, around armyworm and earworm feeding holes in the husk, or at the base of leaves or the tassel. Eggs are usually deposited in clumps of 10 to 40, although several females may deposit eggs in the same ear resulting in large egg masses.  Duration of the egg stage is typically between 2 and 4 days.
• Larval stage: Pale yellow/creamy/white legless maggots. Maggots are cylindrical and elongated with rounded posterior that tapers to a pointed head with a pair of black mouth hooks. Mature larvae are typically 1cm long when fully grown. There are three larval instars. Maggots flip vigorously when disturbed. Larval development lasts 10 to 21 days.
• Pupal stage:Larvae pupate in brownish-red or dark brown cocoons. Pupation can occur in the soil or silk channels.  Pupation takes 7 to 11 days.
• General life cycle of E. stigmatias is completed in 24 – 32 days.
• Captive adults supplied with food and water can live up to 116 days.

Confirmation

Removal of husks will expose maggots that flip vigorously when disturbed. Ovipositing females can be seen on ears below the hanging silks.

Why and where it occurs 

Adults often oviposit in feeding sites of ear borers. Ear maggots are more prominent in tropical regions and are not known to survive freezing temperatures.

Host range 

Maize and sweet corn are the preferred hosts and are the only ones that suffer serious damage. Other hosts include sorghum, sugarcane, potato, tomato, atemoya (custard apple), banana, guava and orange.

Geographical distribution 

This insect occurs throughout tropical America. It is a serious threat in the highland valleys of the Andean region.

• E. major is found in Guatemala, El Salvador, Honduras, and Nicaragua.
• E. sororcula is found from El Salvador and Nicaragua down to Panama.
• E. stigmatias is found in the USA (particularly Florida), Mexico, Central America, Caribbean and South America.

Damage

• Mechanism of damage:In the lowlands, the larvae will feed in the whorl and bore into the growing point causing dead heart. Multiple tillers may form which do not bear ears.

  In the highlands, the ovipositing adult easily enters ears with previous damage from birds or insects such as the larval fall armyworm (Spodoptera frugiperda) or the corn earworm (Helicoverpa zea). Decaying ears are avoided. The larvae will eat the silks. In developing ears this will affect pollination, reducing yield. The insides of kernels may be eaten, leaving only the seed coat.

• When damage is important: Maximum damage occurs if young seedling are affected, resulting in dead heart, or when ears are attacked immediately after silking, resulting in poor or incomplete kernel development.

• Economic damage: Severe infestation can result in yield reductions of 100%.

Management principles

Control

• In the tropical lowlands where the whorl and seedlings are affected, use of early maturing varieties, early planting and well fertilized fields will reduce damage due to ear maggots. Removing infested plants can also minimize damage.
• In lowland regions, granular insecticides can be applied to the whorl ten days after planting. Dermal contact should be avoided.
• As larval and pupal stages are sheltered in the ear, insecticide application should focus on the control of adult stages.

Monitoring

• Fields should be monitored for the adults before crop silking. Monitoring is most effective in the early morning or early evening.
• Insecticide application should occur in the cool of early morning or late afternoon. This is when the adult fly is most exposed on the tassels and upper leaves. Insecticide application may not be effective, however, as flies are known to disperse and re-colonize fields rapidly.
• The most effective insecticides against this pest are organophosphates and pyrethroids.
• Some maize cultivars (hybrids) are moderately resistant to corn silk flies due to high levels of maysin in the corn silk.
• Egg predators include earwigs, mites and minute pirate bugs.

References

Granados, G. 2000. Maize Insects in Tropical Maize - Improvement and Production. Mexico, D.F.: CIMMYT.

King, A.B. and J.L. Saunders. 1984. The Invertebrate Pests of Annual Food Crops in Central America. London: Overseas Development Admin.

Nuessly, G.S. and J.L. Capinera. 2006. Cornsilk Fly. University of Florida Institute of Food and Agricultural Sciences. http://creatures.ifas.ufl.edu/field/cornsilk_fly.htm (30 January 2007).

Nuessly, G., K. Pernezny, P. Stansly, R. Sprenkel and R. Lentini. 2006. Corn Silk Fly: Euxesta stigmatias, Otitidae. Florida Corn Insect Identification Guide. University of Florida Institute of Food and Agricultural Sciences. http://fciig.ifas.ufl.edu/frcsilk.htm (24 January 2007).

Ortega, A. 1987. Insect pests of Maize: A Guide for Field Identification. Mexico, D.F.: CIMMYT.

Contributors: Gabrielle Turner, David Bergvinson, and Biswanath Das