Whence the Medfly?

"The Mediterranean fruit fly may be the most beautiful pest in the world," says entomologist Bruce McPheron.

It's her offspring that are objectionable. Hatched inside a fruit and capable of reaching half an inch long, medfly larvae slash their way through the fruit's pulp, making mush and sucking out the juice. They'll eat peaches, citrus, apples, apricots, guavas, olives, coffee beans—more than 200 fruits.

close up of a fly

"When people see this mess, they are disgusted and throw away the fruit. This is how most of the infestation in the U.S. is assumed to occur," McPheron says, "through people discarding infested fruit. How and from where the fruit enters the country in the first place is unknown."

By comparing the genes of 2,000 flies caught worldwide, McPheron and his colleagues hope to solve those mysteries and end California's yearly medfly plague.

So far Hawaii is off the hook. Through DNA fingerprinting, two distinct genetic markers have been identified. Says McPheron, "All of the 50 flies analyzed from California since 1975 have the same genetic marker form. All of the flies from Hawaii have the alternative marker form." The Hawaiian-type flies are also found in Venezuela and Brazil, while California, Central America, and the Andean countries share the other genetic type. Both types live in Argentina and in Africa.

Currently the researchers are trying to locate additional genetic markers, looking for sections of mitochondrial DNA that show variability. They are also analyzing DNA microsatellites, areas of the DNA that in other organisms have shown variation among individuals.

"Two markers are not enough to deal with this problem," says McPheron. "We can eliminate Hawaii, Venezuela, and Brazil, but we cannot narrow the field to one location, or even tell if each infestation is truly new or a resurgence of a previous, incompletely eradicated event."

In fact, it is conceivable (though not probable), McPheron says, that there was only ever one beautiful medfly imported into California, and that every medfly found there since is descended from her.

Bruce A. McPheron , Ph.D., is associate professor of entomology in the College of Agricultural Sciences, 535 Ag Science & Industry, University Park, PA 16802; 814-865-3088. He is collaborating with Water S. Sheppard of the U.S.D.A. Agricultural Research Station in Beltsville, MD, and Gary J. Steck of the Florida Department of Agriculture and Consumer Services in Gainesville. Reported by Andrea Messer.

Last Updated June 01, 1996