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FRUIT FLIES IN CALIFORNIA Dr. E. F. Legner, University
of California, Riverside CLICK on Links to view: The fruit flies of the family Tephritidae constitute a
group of agricultural pests of worldwide importance, as they attack a wide
range of fruits and vegetables. The most
important are the several species of Dacus
and Ceratitis, which occur in many
countries of warm temperate and subtropical climates; Anastrepha, an American genus occurring from Mexico and the West
Indies through South America; and Rhagoletis,
with a more restricted host range, occurring in the north temperate
region. The Mediterranean fruit fly,
although eradicated periodically from the state of Florida where it had "peninsular" distribution, is
presently firmly established in southern Mexico where it is temporarily
contained by a massive sterile-male and parasite release effort by the U. S.
Department of Agriculture. Eventually
this species may move north and pose
a continuous threat along the Mexican border. Another chronic threat has been the permanently established
population in the Hawaiian Islands, from which periodic accidental invasions
of California are thought to occur.
Recently, Carey & Dowell (1989), Greathead & Waage (1983),
Gilstrap et al (1987), Wharton (1989) and Wong & Ramadan (1990) have
noted that further biological control efforts are definitely justified
against fruit flies.
Some investigators believe that the Medfly is already
permanently established in California and that unless the current eradication
effort is greatly increased, it is just a matter of time before it will
spread throughout the state (Barinaga 1990).
The Malathion baits currently in use against may not be potent enough
for fast eradication, as it is recognized that Medflies will not eat the bait
unless that is the only substance placed in their cages (Citrograph
1990). Under outdoor conditions they
may prefer to seek out clean ripening fruit. As it becomes increasingly apparent that fruit flies
pose continuous threats to California's agriculture through periodic
invasions of our borders, there is an urgent need to consider the application
of alternative methods to chemicals in eradication and control programs. The
implementation of effective biological controls at the sources of an invasion
as well as within the state boundaries where breeding may occur, offers an
environmentally sound, non-polluting alternative. There is a need to (1) develop and improve techniques for the
search, procurement and evaluation of natural enemies of fruit flies in their
natural ranges (parasites, predators and pathogens); (2) introduce and study foreign natural
enemies and evaluate their respective effectiveness under field conditions in
Hawaii, southern Mexico, and California; (3) develop a mass production scheme
for periodic releases of introduced natural enemies in infested areas of
California, and to observe their effectiveness under natural conditions; and
(4) build a culture bank of natural enemies for use in conjunction with other
eradication and control methods (e.g., sterile-male releases) during periodic
invasions of this pest and in anticipation of its possible permanent
establishment in the State of California. The need for investigation into the biological
control of fruit flies in Hawaii, Mexico and California is ever more
important as it becomes recognized that insecticides, although offering
expedient and predictable results under certain conditions, are often
inadequate and at least perceived as dangerous, if not physically dangerous
to wildlife and humans alike. As
problems involving insecticidal residues and insect resistance to chemicals
continue to increase, many programs directed at the control of fruit flies
must ultimately be modified with increased dosages and costs to such an
extent that they invariably arouse the concern and ire of naturalist and
conservationist organizations. A case
in point is the fire ant eradication program. By 1959 extensive damages to wildlife and domestic animals had
positively been attributed to the effects of several insecticides used in the
program (Clawson 1959). Fire ant
control was finally declared unsuccessful in 1960, and in some states, fire
ant numbers were actually reported to have increased since the eradication
program began (Byrd 1960, Cottam 1959).
Presently, a new effort to control fire ants is being attempted with
natural enemies imported from Brazil and Argentina. Principal Fruit Flies in
California
Biological
Control Efforts against Fruit Flies The biological control efforts against fruit flies of the
genus Tephritidae have been extensive over the past half century, a thorough
review being given in Clausen (1987).
However, as it becomes increasingly apparent that the Mediterranean
fruit fly, Ceratitis capitata (Wiedemann), and Mexican
fruit fly, Anastrepha ludens (Loew) pose a continued threat
to California's agriculture through periodic invasions of our borders, there
is an urgent need to consider the application of alternative methods to
chemicals in eradication and control programs. The implementation of
effective biological controls at the sources of an invasion as well as within
the state boundaries where breeding may occur, offers an environmentally
sound, non-polluting alternative.
There is an urgent need to (1) search for, procure and initially
evaluate natural enemies of Mediterranean and Mexican fruit flies in their
natural ranges in central Africa and southern Mexico (parasites, predators
and pathogens); (2) introduce and
study foreign natural enemies in the adult stage, and evaluate their
respective effectiveness under field conditions in Hawaii, southern Mexico,
and if applicable, California; (3) attempt development of a mass production
scheme of resident California fruit flies (e.g., walnut husk fly) to serve as
acceptable hosts for Medfly natural enemies for use in laboratory study and
periodic colonization efforts in infested areas of California, and (4) to
test the feasibility of building a culture bank of Medfly and Mexican fruit fly
natural enemies on resident California fruit flies for use in conjunction
with other eradication and control methods (e.g., sterile-male releases,
adult fly baiting) during periodic invasions of these pests and in
anticipation of their possible permanent establishment in the State of
California
The medfly invaded Hawaii in 1910 and soon thereafter
the Board of Commissioners hired Filipi Silvestri to again search for natural
enemies of this fly. It was determined
by experts of the day that collections should concentrate in Western
Africa. Therefore, Silvestri traveled
for eight months through West and East Africa and South Africa. He found only six specimens of the medfly
on the entire journey, but reared many parasitic insects from other
fruit-infesting tephritids collected along the way. He managed to establish four species in Hawaii: Opius
concolor Szepligeti, Biosteres tryoni (Cameron), Coptera
silvestrii Kieffer and Dirhinus anthracina Walker. Two more
missions over the next 30 years were sent out in hopes of obtaining parasitic
insects, but only Tetrastichus giffardianus Silvestri and Biosteres fullawayi (Silvestri) were established. Other biological control programs were undertaken in several
countries where the medfly was firmly established, but these programs have
not been well documented, and the extent of control of any of the parasitic
species is virtually unknown, the notable exception being Hawaii. Even in Hawaii control was never
noteworthy and the medfly problem was finally overshadowed by the
introduction of Dacus dorsalis Hendel. For North America the answer to the medfly
invasions starting in 1929 was complete eradication by means of fruit
stripping and poisoned bait sprays. The success of these early and subsequent biological
control programs against the medfly has been variable (Gilstrap & Hart
1987, Wharton & Gilstrap 1983). In
Hawaii, a cooperative biological control program initiated in 1948 involved
the release of 32 entomophagous species to combat both medfly and the
oriental fruit fly. Three parasitic
species, Biosteres longicaudatus (Ashmead), B. vandenboschi
(Fullaway), and B. oophilus (Fullaway) became widespread
and abundant (Bess et al. 1961). During
1966-1968, parasitization of the medfly and the oriental fruit fly was high
(ca. 70%); it was mainly due to the
egg-pupal parasite, B. oophilus (Haramoto & Bess
1970). During 1978-1981, Biosteres oophilus was still the predominant parasite as it accounted for
ca. 80% of the total parasitization.
Occasionally the larval-pupal parasite, Biosteres longicaudatus
and B. tryoni (Cameron) achieved a parasitization of 32 and 8%,
respectively (Wong et al. 1984).
Extensive fruit collections done between 1949-1985 showed that
Jerusalem cherry, coffee and peach were among the most important hosts of the
medfly. These fruits yielded more
than 100 larvae/Kg of infested fruits (Liquido et al. 1990; Nishida et al.
1985). The fruits that yielded a high
number of medfly larvae were elliptical or spherical and yellowish or
reddish. They had a diameter of 1-7
cm and a weight of 1-30 grams. Most
of these hosts belonged to five plant families: Myrtaceae, Rutaceae, Rosaceae, Sapotaceae and Solanaceae
(Liquido et al. 1990). In Costa Rica a classical biological control program
was initiated in 1955. During
1979-1980 parasitic insects were collected from <10% of C. capitata
populations. These were two
introduced braconids, B. longicaudatus and B. oophilus, and two
indigenous cynipids, Ganaspis carvalhoi (Dettmer) and Odontosema anastrephae (Borgmeier) (Wharton et al. 1981). An exploration for natural enemies of the
medfly, conducted in West-Central Africa during 1980-1982, showed that C. capitata
occurred in low frequency in coffee plantations in Cameroon. Parasitization of tephritids in coffee by
braconids ranged from 10-56% (Steck et al. 1986). In Guatemala infestation of C.
capitata was serious in coffee and
tangerine. The rest of the fruits
were mainly infested by Anastrepha
spp. (Eskafi 1988, 1990). Parasitization rate of C. capitata
and Anastrepha spp. was low,
ranging from 0.04 to 7.95%. The most
common parasitic species recovered from both flies were B. longicaudatus and Doryctobracon crawfordi (Viereck) (Eskafi 1990). The behavior of the ectoparasite Muscidifurax raptor
(Girault & Sanders) in searching for the potential host C. capitata
pupae was analyzed under laboratory conditions. The searching efficiency of M. raptor females decreased
with increasing density. The
proportion of avoidance of superparasitism was 0.615. The response to a high parasite density
was to increase the proportion of males in the progeny, as males searching
for mates interfered and decreased the searching activity of the females
(Podoler & Menzel 1977, 1979).
The medfly was susceptible to the Mexican strain of the nematode Steinernema feltiae. Emerging adults
and pupae were not susceptible to the nematode, but the third instars (prior
to pupating in the soil) suffered high mortalities (50-90%) when exposed to
high nematode concentrations (150,000 - 500,000 nematodes/cup) (Lindegren
& Vail 1986). Field exposure of
mature larvae to a dose of 500 nematodes/cm2 yielded high
mortality of C. capitata (Lindegren et al. 1990). In addition to the hymenopterous parasites
and insect pathogenic nematodes, arthropod predators such as ants could play
an important role in reducing fruit fly populations. Under laboratory conditions, the Argentine
ant, Iridomyrmex humilis (Mayr) caused 50% mortality of
medfly pupae after a 10 min. attack.
However, ant predation could be important only in localized areas; it
is not adequate to regulate medfly populations (Wong et al. 1984). Typically, the most effective natural enemies of an
insect occur in regions where the pest originated. The natural range of the Mediterranean fruit fly is the
sub-Saharan central African region, including the Island of Madagascar. Although no information is available from
Madagascar, a number of promising natural enemies have been discovered in
Central Africa (Table 1; Bianchi & Krauss 1936, 1937; Gilstrap & Hart
1987, Greathead 1976, Silvestri 1914, Steck et al. 1986, van Zwaluwenburg
1936, 1937, Wharton, 1989, Wharton & Gilstrap 1983). However, because of technological
difficulties associated with transportation and culture, only two species attacking Ceratitis capitata have been successfully translocated out of central
Africa. A concentrated effort to
locate natural enemies there might yield the kind of species capable of
regulating this pest at low densities, as it has been known to be rare in
that general region since the early 1900's (Silvestri 1913). The parasitic Hymenoptera are believed to
be the most effective natural enemies of fruit flies. At least six species of fruit flies in the
genus Ceratitis are known from
central Africa, and numerous parasitic Hymenoptera have been reported active
on them at very low host densities (Table 1, Silvestri 1913, Clausen 1978, F.
Gilstrap, pers. comm.). These have
not been tested by entomologists in California because the Mediterranean
fruit fly has been quarantined.
Therefore, promising species of natural enemies for Medfly might be
found among these related species.
Also, there has been no concentrated effort to locate disease
organisms, such as viruses, bacteria and fungi, which might prove invaluable
in eradication campaigns. Table 1. Known parasitic species attacking
fruit flies of the genus Ceratitis
in their natural range in Central Africa ____________________________________________________________________________________________________________ Parasite species Host stage Parasite species Host stage
attacked
attacked
___________________________________________________________________________________________________________
_____________________________________________________________________________________ Mexican fruit fly. Anastrepha ludens (Loew)--Some
of the natural enemies of oriental and Mediterranean fruit flies have shown
activity on Anastrepha spp. in
southern Mexico, and may be influential in partial biological control of that
species (Aluja et al. 1990). However,
there have been no formal attempts to obtain natural enemies from other areas
where different species of Anastrepha
occur, such as South America Two species of parasitic insects are already proven and
available as biotic insecticides (augmentive releases) against Medfly. These are Diachasmimorpha longicaudata
and D. tryoni, which have been used with some success in Mexico and
Hawaii (USDA 1988, Wong et al. 1990a,b).
The use of these parasites in lieu of Malathion during the
establishment phase of specific natural enemies from central Africa, would
greatly aid their survival and while providing some economic control of
Medfly. Misc. fruit flies--Several
species of Rhagoletis are very
important pests of cultivated cherries in North America and Europe, with some
species having been considered as subjects for biological control, despite
the low economic threshold.
Infestation rates of less than 0.2% are currently required for
commercial marketing of cherries in the United States. Four species of parasitic insects associated
with the Oriental fruit fly, Dacus dorsalis Hendel, were introduced
against such fruit flies. These
included Opius longicaudatus compensans
(Silv.), Opius longicaudatus farmosanus
(Full.), Opius oophilus Full., and Opius
longicaudatus novacaledonicus Full.
These parasites were introduced from Hawaii and released against Rhagoletis indifferens Cueran and Rhagoletis
fausta Osten Sak in Oregon and
Washington in the 1950's (Clausen 1956b).
However, none became established probably because they all originated
in tropical regions. A parasite of R. cerasi,
the European cherry fruit fly, was imported against the eastern cherry fruit
fly, R. cingulata Loew during 1959-64 in New Jersey, without successful
establishment. Other species
including Biosteres sublaevis Wharton, Coptera occidentalis and Phygadeuon
wiesmanni are under investigation
in California and Oregon (Croft & AliNiazee 1999). |
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