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Putting the bite on Caribbean fruit flies.

Bite into an apple and you hear the crunch of your teeth through the skin and into the fruit. On a smaller scale, fruit fly maggots can be heard eating their way through grapefruit pulp.

We can't hear the maggots nibbling, but parasitic wasps can. One way these wasps hunt for a maggot meal for their offspring is by listening to the maggots eat.

Once a female wasp finds a maggot, she sticks her ovipositor through the fruit peel and lays an egg inside the maggot. Within a short time, the maggot leaves the fruit and develops into a pupa. Later, the wasp egg will hatch into a larva that will begin to eat the maggot pupa.

John Sivinski and scientific cooperators have released millions of these parasitic hunter wasps to suppress the Caribbean fruit fly in Florida. It's part of a 4-year pilot study to use the Diachasmimorpha longicaudata wasp to maintain and expand a buffer zone free of the flies, which attack citrus and other fruits.

The scientists released 50,000 to 150,000 wasps per square mile each week and were able to reduce fruit fly populations by 90 to 95 percent.

Maintaining a fruit-fly-free zone allows Florida citrus growers to export fruit to California, Japan, and other areas that refuse fruit unless it is grown in fly-free zones. Fruit flies are among a number of pests subject to such quarantine restrictions.

Previously, growers used the fumigant ethylene dibromide (EDB) to kill fly larvae in fruit, but the U.S. Environmental Protection Agency has suspended EDB because it was found to be carcinogenic.

Sivinski notes that other alternatives to EDB have also run into trouble. Citizens in urban areas have objected to aerial spraying of malathion, while citrus growers are uneasy about releasing sterile males to suppress fruit fly populations.

"They're concerned that if sterile flies are trapped, it could complicate certification of their groves as fly-free," says Sivinski, an entomologist with USDA's Agricultural Research Service. "We can tell sterile and wild flies apart, but growers are worried that identifying the flies could cause delays and jeopardize shipments."

So Sivinski entered into cooperation with Carroll Calkins of ARS, Richard Baranowski with the University of Florida, Donald Harris and Ed Burns with the Florida Division of Plant Industry, and Tim Holler of USDA's Animal and Plant Health Inspection Service.

They focused on using biological controls as a viable alternative for protecting fly-free zones. Their aim was to create buffer zones in urban areas next to commercial groves by releasing parasites to drastically lower fruit fly numbers. Urban backyard gardeners often raise guava, loquat, Surinam cherry, and other fruit trees. Fruit flies migrate from them into commercial citrus, after feeding on the fruits.

Growers first found the Caribbean fruit flies at Key West, Florida, in 1931. That prompted regulators to begin a quarantine that lasted until 1936, when populations declined. The fly resurfaced at Key West in 1959 and then disappeared again until 1965, when regulators found larvae in Surinam cherries in Miami Springs. Since then, the fly has spread to 30 counties throughout southern and central Florida.

Over the years, researchers have introduced 15 different species of biocontrol wasps to control the pest. Five of those species have become established, but the fly still remains a serious problem for several reasons, Sivinski says.

"Often the fly larvae are so deep in the fruit that the wasp' s ovipositor can't reach them," he says. "Also, the flies tend to reproduce faster than the wasp parasites, so the fly populations outstrip the parasites.

"That' s why we're releasing millions of the wasps at a time when fly populations are starting to build up. This gives the wasps a better chance of finding and parasitizing fruit fly larvae."

There are several factors working in favor of controlling the fly in Florida. For one, the flies infest grapefruit only at low levels--preferring loquat, guava, and other backyard fruits that can be separated from commercial citrus by maintaining the fly-free zones.

D. longicaudata has proven to be one of the more effective hunters of fruit fly maggots. Originally from India, Borneo, the Philippines, and other areas, this wasp now accounts for about 95 percent of all fruit fly larvae parasitized in the southern part of Florida.

In the pilot study, Sivinski and colleagues set up two test sites--on Key Biscayne, an island about a half mile offshore from Miami, and at Clewiston, on the southwest shore of Lake Okeechobee.

At Key Biscayne, during 36 weeks, they released 10 million D. longicaudata wasps per square mile over 2 square miles. At Clewiston, they released 3.75 million per square mile over a larger, 5-mile area, but they had to curtail releases after 15 weeks because of trouble in rearing the parasites.

Sivinski said they attempted to release from 4 to 10 times as many parasites as flies estimated to be in the areas, depending on fluctuations in fly populations. Originally, scientists put fly pupae containing parasites in the field; after that, they released adult wasps to hunt their own prey.

Within 5 weeks of the initial adult releases, fly populations at Key Biscayne were at least 95 percent lower than fly populations in nearby areas of South Miami where there were no parasite releases. In the Clewiston release area, fly populations were also substantially lower.

"It seems that the adult wasps were more effective in hunting for fly larvae," Sivinski said. "Putting parasitized pupae in the fields didn't seem to have much of an effect in Florida, although it has been used successfully in Hawaii."

In Hawaii, ARS scientist Tim Wong and colleagues used the braconid wasp D. tryoni to suppress Mediterranean fruit flies. In those studies, they released parasitized fruit fly pupae. Sivinski used irradiated fly larvae, to prevent any flies that might escape parasitism from developing into fertile fruit flies.

Sivinski is now expanding the scope of the research in tropical America, where he is working with cooperators in Mexico and Guatemala to set up a fly-free zone against the Mediterranean fruit fly. The work there combines parasite releases and sterile male flies that mate with wild females to produce infertile eggs that do not hatch.

"We're also looking in those countries for new parasites that do a better job of hunting for fruit fly larvae," Sivinski says. "That could make it easier and more cost effective to maintain the fly-free zones."
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Author:Adams, Sean
Publication:Agricultural Research
Date:Jul 1, 1994
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