Ormia depleta — THE BENEFICIAL FLY


What is this fly?

There are thousands of species of flies in Florida. They range from the size of a pinhead to about half an inch long. Some of them are pests, some are beneficial, and most are innocuous. Ormia depleta is a fly species native to South America where Florida’s pest mole crickets are likewise native.

Where is this fly?

Ormia depleta flies were imported from Brazil by University of Florida/IFAS entomologists in the 1980s. From 1988–1992, releases were made in many counties, especially on golf courses under a project sponsored by the Florida Turfgrass Association. Over 10,000 flies were reared and released. By December 1994, there seemed to be a continuous population of this fly in at least 38 Florida counties, from Alachua south to Collier and Miami–Dade. But in the counties north of about Orange, the flies seem to die off in the winter and repopulate during the next year. Ormia depleta seems to be incapable of surviving cold winters in the north. Releases of this fly in states north of Florida have shown no evidence of success.

Adult flies are nocturnal and parasitize pest mole crickets

Adult flies are active at night, especially soon after sundown, and that is when mating occurs. The female flies, when they are gravid, are attracted to the song of pest mole crickets. Eggs hatch inside them into tiny fly larvae (maggots). The female flies deposit these living larvae on and close to the singing mole crickets at night. The larvae burrow into male and female mole crickets to about an equal extent. The larvae develop in about seven days at 70 degrees Fahrenheit, taking longer at lower temperatures, and emerge when they are fully developed to pupate in the ground. The pupal stage takes about 11 days at 70 degrees Fahrenheit. The host mole crickets die. To those 7+11 days must be added a few days for maturation of the adult flies, and it can be seen that the fly is capable of completing its this life cycle in little more than three weeks at warm temperatures. This can be contrasted with a full year for pest mole crickets. Highest populations of adult Ormia depleta seemed to occur in May–June in Bradenton when they were measured there. This same seasonality is likely to occur throughout central and southern Florida. What it may imply is that female flies were more successful in finding host mole crickets in April and May than at other times of year.

Host-specificity and effect of this fly

Female Ormia depleta flies are attracted only by the song of pest mole crickets. They are harmless to all other organisms. Maximal proportion of mole crickets (trapped in April–May in central Florida) determined to be parasitized by Ormia depleta has been measured as about one third. Doubtless there is a small effect in other months. So these flies are capable of reducing pest mole cricket populations in central (and southern) Florida by AT LEAST a third. At that to the effect achieved by Larra bicolor and Steinernema scapterisci, and it is easy to see how the combined effects have been VERY substantial.

What does the fly cost, and how can its effects be enhanced?

Introduction of the fly into Florida was successful and it is now widely distributed in central and southern Florida doing its job to control pest mole crickets. Those aspects of research are complete and there is no further cost. No one sells the flies because the cost of production is too high. But why pay for them anyway when they are widely distributed in 38 counties?
We have no doubt that adult flies need nectar resources, just like Larra bicolor. But we do not know what are those nectar resources (because the adult flies are nocturnal and research on their nectar-feeding has been difficult). The mole cricket research program at UF/IFAS ended with the retirement of senior personnel.