A new study of two significant pest fruit fly species finds that the size of males influences female mating choice in one of the species, but not in the other—important knowledge for fine-tuning management efforts for both species via the sterile insect technique. Here, female Mexican fruit flies (Anastrepha ludens), one of the species in the study, are shown. (Photo by Jack Dykinga, USDA Agricultural Research Service, Bugwood.org)

By Andrew Porterfield

For sterile insect techniques to control pests, the sterile insect must be attractive. For two species of Anastrepha fruit flies, the West Indian fruit fly (A. obliqua) and the Mexican fruit fly (A. ludens), the size of the male in one was important for a female’s selection, while size did not matter in the other species.

Andrew Porterfield

Both flies are serious fruit pests in Central America and parts of Florida, California, Texas, and similar citrus-growing areas of the United States. The insects also travel with fruit to their final destinations, expanding their territory even further.

Female flies are extremely prolific. A single A. ludens female, for example, can lay 40 eggs at a time and about 2,000 over her life span. So, reducing those numbers via the sterile insect technique is a key component of controlling pest populations. When mating, Anastrepha males create leks, locations where they gather to attract females for mating. For the fruit flies, these leks are created by emitting pheromones on the underside of host (and non-host) plants, and attracting females. Sterile insect technique (SIT) has consisted of introducing sterile males, who mate but produce no offspring, thereby reducing subsequent populations of flies.

Male size has often been assumed to be a significant factor to females selecting a male. But several studies have produced conflicting evidence in which smaller males were often selected. To build a better understanding of the Anastrepha species and mating choices and size, researchers from the University of Veracruz in Mexico tested various male sizes and their affect on female mate choice.

Led by Diana Pérez-Staples, Ph.D., research professor at the university’s Institute for Biotechnology and Applied Ecology, the researchers set up field cages, testing large wild males against smaller laboratory-bred sterile males and large laboratory-bred sterile males against small wild males. The team found that, for A. obliqua, large males had an advantage. However, they also found no such advantage for large A. ludens males. Their results were published this month in the Journal of Insect Science.

For the experiments, wild insects were collected from infested fruits around Chiapas, Mexico. Laboratory insects were obtained from a biofactory and sterilized with chemical and radiation treatments. Insects were collected at pupal stages. Once they emerged from pupae, the flies were kept in mesh cages and observed in 3-by-2-meter field cages with host trees (orange and mango). For each species, cages contained about 25 wild females, 25 small males, and 25 large males. For A. obliqua, 306 copulations were observed, while 342 copulations were observed for A. ludens.

For A. obliqua, the proportion of copulations showed a competitive advantage for large wild males when competing against small laboratory males. For wild against wild males, the larger flies had the advantage, as well. However, for A. ludens no such advantage was observed.

“For species that form leks, such as A. ludens and A. obliqua, selective pressures and variance in mating success is high due both to male-male competition for adequate calling sites as well as female choosiness,” Staples and her colleagues write.

These findings, in near-field conditions, show that SIT strategies at least for A. obliqua should include the mass production of large sterile males, since they were chosen more often by females. No such selection may be necessary for SIT among A. ludens.

The study also demonstrates that “it is important not to assume that male mating success is dependent on a large size for all pest tephritids” (flies in the family Tephritidae), the researchers note. “For A. ludens, other factors, such as male-male competition, lek site selection, and pheromone quality may be more important determinants of male copulatory success.”

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.