Biological Control of Mole Crickets By Ormia depleta (Brazilian Red-eyed Fly)
Ormia depleta (Wiedemann) (OR-mee-a dep-LATE-a) is a
species of tachinid fly
(tach-INN-id) native to South America. In the 1930s in Amazonian Brazil, its larvae were found
to be parasitoids of
Scapteriscus mole
crickets. An unsuccessful attempt was then made to introduce the fly into Puerto Rico as a
biological control agent for the West Indian mole cricket. In
the 1980s in Paraguay, adult female Ormia depleta were found to be attracted to
songs of tawny and southern mole crickets when these songs
were produced artificially by a sound-emitter developed by the
UF/IFAS mole cricket program. This gave a method of
trapping the female flies, and was used by the UF/IFAS mole cricket program in southern Brazil
in 1984-1989. It was found that
larvae could be dissected out of the
gravid flies that were trapped, then placed on
mole crickets, and would burrow into and parasitize the mole
crickets. These larvae developed in about a week, killing the
mole crickets in the process. The larvae pupated in soil, and
adult flies emerged from the pupae about 11 days later.
Survival of the newly-emerged adult flies was very poor in cages, and repeated attempts failed to
get the flies to breed in a laboratory in Brazil and (under a USDA importation permit) in a
quarantine facility in Gainesville, Florida.
In 1987, thanks to studies made on Ormia ochracea at the University of Florida,
methods finally were devised for getting Ormia depleta to breed in captivity. Following
that success, flies were released near Gainesville in April 1988, and at Bradenton in October 1988;
they established populations at both places. Ormia depleta was dubbed the
"Brazilian red-eyed fly" by the popular press. From 1989-1992, releases were made
in several other counties, many of them on golf courses under a project sponsored by the Florida
Turfgrass Association. By December 1994, there seemed to be a continuous population of flies in
at least 37 counties, from Alachua to Dade. There was no information about establishment in
counties north of Alachua, nor was there information about the proportion of
mole crickets killed by the fly.
Ormia depleta is believed to be a specific natural enemy of Scapteriscus
mole crickets because the gravid females need to hear the song
of Scapteriscus borellii or Scapteriscus vicinus in order to find, and lay their
larvae on, those hosts. The female flies also will lay larvae on
other mole crickets very close to the singing mole crickets.
Native species of Ormia in the southern USA have never been found to attack
Scapteriscus mole crickets for similar reasons: they require to hear the song of
Gryllus or Neoconocephalus to find these hosts. Therefore Ormia depleta
is safe to non-target organisms. The adult flies neither bite nor sting people and, because they are
active at night (when mole crickets sing), they are unlikely to be seen by people.
The optimal use of Ormia depleta is as a classical
biological control agent. It is labor-intensive and thus expensive to rear in the laboratory, and
the chances of reducing the labor costs very much are slight. However, when released in the field
in central and southern Florida it has shown the capability of establishing permanent populations.
These established populations will kill mole crickets, and they do so at no recurrent cost.
Therefore, in places where this fly has established populations, there is no point in releasing more.
Two puparia of Ormia depleta next to a mole cricket in which they developed, and
killed, when they were larvae.
It may be that there will be a northern limit to the area the fly can occupy permanently
because of cold winter temperatures farther north destroying
honeydew sources on which adult flies depend for their survival. If
this should be the case, then it may prove worthwhile to release flies in the northern area when the
weather warms up each spring to the point where honeydew is available. Flies to be moved north and
released could be: (1) flies caught in traps in the south and shipped north for release, or (2) flies
reared in laboratories. These possibilities might allow for commercialization of the fly in northern
areas.
Populations of the fly currently extend from Alachua County to Dade County but may not
be distributed to the best advantage in that area. For example, in early summer 1993 at
Bradenton, the fly population was high and mole cricket populations were extremely low; other
areas had much higher mole cricket populations but unknown
fly populations. It may be that honeydew sources are inadequate to
maintain adult flies in some areas, and that this might account for pockets of high mole cricket
populations. This deficiency might be overcome by provision of additional nectar sources, which
is biological control by habitat manipulation. Identification has
not yet been made of honeydew sources used by the fly outdoors, and this will take quite a lot of
research effort of unknown complexity. What is needed is a list of
honeydew sources used in successive months of the year for north,
central and south Florida, subdivided into native plants (for any kind of habitat) and of ornamental
plants (for landscaping on golf courses and in gardens).
Biological control of mole
crickets by Ormia depleta is an extremely low-energy form of pest control, since
only the research phase requires energy input. This is true in general of
classical biological control. An assumption is made here that
releases of the fly, to get populations established, were part of the research phase. That
assumption is valid for releases made in 1988-1993 in Florida because the outcome of none of the
releases could be predicted: all were experimental.
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