Since 1992, the Mexican government has operated a program using the sterile insect technique to manage the Mexican fruit fly (Anastrepha ludens). New research shows the mass-rearing program that produces the millions of sterile fruit flies needed for managing wild populations can be made more efficient by employing a 4:1 ratio of females to males in the flies’ mating cages. (Photo credit: Jack Dykinga, USDA Agricultural Research Service, Bugwood.org)

By Paige Embry

Paige Embry

In the insect realm, males often have one role—sperm donor—and scientists have leveraged that role for pest management using the sterile insect technique (SIT). With SIT, scientists raise a bunch of pest insects in the lab, irradiate them so they’re sterile, and release the males into the wild where they can mate with fertile females and thus decimate the next generation. Over the last 60-plus years, scientists and pest management professionals have used this method for a variety of pests: mosquitoes, screwworms, tsetse flies, as well as an array of fruit flies, including Anastrepha ludens, the Mexican fruit fly. Refining the mass-rearing portion of the SIT for A. ludens is the subject of a new study published in July in the Journal of Economic Entomology.

For an SIT program to work, certain baseline conditions need to be met. First, you need to be able to raise gargantuan numbers of the pest in captivity. Second, the irradiated males need to be able to successfully mate with females and therefore displace fertile males. Third, the whole effort needs to be cost effective. “This is not always feasible for certain pests,” says Diana Pérez-Staples, Ph.D., research professor at Veracruz University’s Institute for Biotechnology and Applied Ecology in Mexico and senior author on the study.

Since 1992, the Mexican government has operated a program using the sterile insect technique (SIT) to manage the Mexican fruit fly (Anastrepha ludens). A critical part of any SIT effort is being able to readily discern male and female insects during the rearing process. The program in Mexico uses a strain of A. ludens in which the male pupae are brown (left) and female pupae are black (right). (Photo courtesy of Diana Pérez-Staples, Ph.D.)

Diana Pérez-Staples, Ph.D.

In 1992 the Mexican government launched a campaign against fruit flies that damaged citrus and mangoes, and the campaign has seen a lot of success, thanks in part to SIT.  As part of that eradication campaign, the  government built the Moscafrut facility, which can produce up to 300 million sterile flies—per week.

Now, the general method for raising A. ludens is well-established—but differs significantly from what happens in nature. In the wild, female Mexican fruit flies lay their eggs (oviposit) inside fruit, but Ivan G. López, lead author on the paper, says that the mass reared insects, “have been adapted to ovipositing on an artificial device that does not resemble an actual fruit. … Mass-reared females readily oviposit eggs into this device, but a wild female would probably not like it much.”

For this study, the researchers wanted to assess how changing the sex ratio of females to males from 1:1 to 4:1 in the rearing cages impacted their longevity, fecundity (the average number of eggs produced per day per female) and fertility (the percentage of live larvae that hatched). If you can produce the same number of eggs with fewer males, it’s more cost effective—fewer males to feed and more to send out into the fields. This study was possible because scientists had already developed a strain of A. ludens with different colored prepupae so the sexes could be sorted. (Being able to tell the difference between males and females is key for an SIT program. That way, you can sort out, irradiate, and release only the males.)

Ivan G. López

The scientists collected the eggs each day and counted them. That number, divided by the number of live females, gave the fecundity. They then incubated a portion of the eggs and the percentage that hatched gave the fertility.

The researchers found that, surprisingly, having fewer males increased the fecundity and fertility of the females. The authors speculate that, in the congested world of mass-rearing cages, fewer males might mean that both mating and egg-laying are less likely to be interrupted. The skewed sex ratio did lead to shorter lives for both sexes, but the authors note those losses aren’t enough to offset the gains in fecundity and fertility or the cost-savings from feeding fewer males found in the 4:1 female-to-male ratio.

This finding is good news for the facility—fewer flies to feed—and bad news for fruit fly pests, because there will be more sterile males to send out as birth control devices.

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.