Eggs-ecution Date : Release of Sterile Mexflies May Mate the Pest Out of Existence

El Cajon’s first shipments of sterile Mexican fruit flies arrived Thursday for five days of incubation before being released as the vanguard of an effort to mate the pest out of existence locally.

Sixteen million sterile Mexflies will be released weekly in El Cajon through the summer during the second phase of the state’s eradication effort.

Three aerial sprayings of malathion during the past month were designed to kill most of the fertile wild flies, and the steriles are supposed to take care of the rest within two to three generations.

The first release is scheduled for Tuesday.

“The sterile flies are just as important as the spraying and need to be just as effective to eradicate this pest,” said Bill Routhier, area manager of the California Department of Food and Agriculture.

Two shipments of pupae--the fly’s third growth stage--arrived Thursday at the state’s Mexfly headquarters in Santee, including one batch that had been scheduled to arrive late Wednesday. Each shipment has 4 million pupae.

Mexfly officials were irritated that the shipment was bumped from a Continental Airlines flight that ran out of luggage space, but the delay caused only minor damage to the pupae and will have no significant effect on the overall eradication effort, said John Worley, director of the Mexican Fruit Fly Rearing Facility near Mission, Tex., the only place in the world that raises sterile Mexflies.

About 10% of each batch never reach sexual maturity for a variety of reasons, but the delay probably ruined another 10% of the pupae, Worley said.

Any interruption of the lab-raised flies’ carefully controlled growth stages increases their mortality rate and diminishes the adult insects’ flying ability, sexual aggressiveness and life span, Worley said.

Mexfly officials chastised the airline for leaving the first shipment behind and have decided to pay a priority shipping cost of $20 per 50 pounds of pupae to ensure that no more batches are delayed, Worley said.

Several pupae from each shipment are placed in small cages to measure the percentage of pupae that emerge into adults, the rate of emergence and the overall physical quality of the adult flies, Routhier said.

The first shipments of pupae--irradiated and dyed orange to distinguish the steriles from wild flies--were placed in paper bags and small cardboard buckets where they will incubate for five days in one of the many mobile trailers that make up the temporary Mexfly headquarters near Gillespie Field.

After hatching, the adult flies will feed briefly on a gelatinous mixture of sugar and yeast in the buckets. They will be released from trucks moving slowly through the infested areas, Tuesdays through Fridays.

Although more than 200 million flies will be released in El Cajon during the summer, they will pose little problem for local residents and pets, Worley said. The flies are not aggressive and do not bite but might be attracted by the strong odor of wet paint and the shade of carports to escape the summer heat, he added.

Sixteen million pupae--or 1 million per square infested mile per week--are scheduled to arrive weekly in Santee. Trapping has been intensified to monitor the success of the sterile mating program.

In Compton, officials released the first batch of sterile flies Tuesday, where 14 square miles were found to be infested with the Mexfly this spring.

Mexflies, natives of northern Mexico, destroy citrus by laying eggs in the fruit. The eggs develop into larvae, which feed inside the fruit before crawling out to burrow into the ground and develop into mature flies.

They are prolific breeders during their typical four- to six-week life span. Females are capable of laying 1,000 to 2,000 eggs.

Each shipment of sterile flies contains males and females, but the success of the eradication program is based primarily on the sterile males numerically overwhelming the wild females so only non-viable eggs are laid.

Mexflies prefer grapefruit but also lay eggs in other types of citrus.