Invasion by Medfly Defies Logic, Scientists Say : Infestation: Experts discover peculiar patterns in the spread of the stubborn fruit fly. Some suspect sabotage by biological terrorists.

As the battle against the Mediterranean fruit fly enters a new year and aerial pesticide spraying covers still more neighborhoods in Orange and Los Angeles counties, state and federal officials remain baffled by several aspects of a stubborn infestation.

“There are some peculiar things happening,” said Caroll Calkins, a researcher in the U.S. Department of Agriculture’s Insect Behavior Unit in Gainesville, Fla.

For instance, some of the scientists are puzzled by the small number of Medfly larvae that have turned up. It is in this insect stage that the Medfly does its greatest damage to fruit and vegetables. The scientists believe an infestation this size should produce more evidence of larvae.

Others wonder why 23 out of 30 fertile flies trapped on the infestation’s frontier during the last month have been females, and have been found in only one of the three kinds of Medfly traps. Agriculture officials said they cannot explain why more males aren’t being trapped.

And in a final riddle, many of the Medflies have been trapped just outside previous spray zones, forcing steady expansion of the area undergoing malathion treatments. Some experts said the pattern is as illogical as it is frustrating.

At a meeting in Los Angeles on Dec. 6, the scientific panel questioned whether the unexplained patterns of infestation might lend credence to a letter distributed by a group that calls itself the Breeders. In the letter, sent to newspapers and Los Angeles Mayor Tom Bradley, the group claimed to be spreading Medflies to protest California agricultural practices.

Roy Cunningham, a Department of Agriculture Medfly expert and chairman of the panel, said Friday that the panel is taking this possibility seriously but still feels there is no proof: “We don’t see any hard evidence (that the letter writer) has the wherewithal to do what he has threatened.”

The Breeders letter was turned over to the Los Angeles Police Department’s Criminal Conspiracy Section. Department spokesman Cmdr. William Booth said investigators have “made some progress.”

At the advisory panel meeting, according to one member who was present, one adviser suggested that the finding of females in just one type of trap, known as a McPhail trap, could be evidence of sabotage. In interviews, scientists have also said the small numbers of larvae, which indicate a natural breeding population of Medflies, and the nagging pattern of new flies emerging just outside spray zones could fuel suspicions that the flies are being spread by the Breeders.

At the same time, other, more benign scenarios are being circulated in an attempt to explain the infestation’s quirks. For instance, James Carey, a member of the science panel, said the small number of larvae detected does not necessarily mean that the fly is being spread unnaturally, rather than by breeding and moving.

It is his opinion that the Los Angeles infestation could date back several years and that the bug has become established in the region. He is a minority voice on the panel on that point.

Calkins said the lack of larvae could show that the infestation was small and isolated, and located away from traps.

Authorities believe the current infestation began in Whittier, where a Medfly was first trapped on Sept. 26. They believe this because larvae were detected there, indicating the existence of a breeding population.

As the infestation spread across the Los Angeles Basin to parts of Brea, La Habra and Fullerton in Orange County, however, scientists did not find nearly as many larvae.

The life cycle of a Medfly begins with the laying of eggs in a piece of fruit or a vegetable. When the eggs hatch several days later, the larvae feed on the fruit, turning it into an inedible mush. The bugs drop to the ground and continue developing in a pupal stage, and the adult flies emerge later.

Three types of traps are being used to lure Medflies--the Jackson, Steiner and McPhail traps. Almost all of the flies are females, and are showing up in the McPhail traps. It would be expected that males would be caught in the Jackson trap, which contains a sex lure that has proven effective in past Medfly invasions.