Researchers in Japan have identified a chemical within the male reproductive organs of the West Indian sweetpotato weevil (Euscepes postfasciatus) involved in causing females to be unreceptive to mating for an average of 9.5 days after an initial mating encounter. Further research on this dynamic could influence the use of sterile insect technique in managing the insect. (Photo credit: Juliana Cardona-Duque, University of Puerto Rico, Bugwood.org)

By John P. Roche

West Indian sweetpotato weevils (Euscepes postfasciatus) are costly pests of sweetpotato plants (Ipomoea batatas) in the Pacific, the Caribbean, and Central and South America. Entomologists in the Ryukyu Islands within the Okinawa Prefecture of Japan are implementing a sterile insect technique (SIT) program for eradicating West Indian sweetpotato weevils.

SIT uses release of sterile male insects to reduce production of offspring by females, and it offers the advantage of reducing the use of chemical insecticides. Previous observational studies found that, after mating with males, females of some weevil species, including E. postfasciatus, were unreceptive to mating for a period of time. An understanding of how exactly mating triggers this unreceptivity in female weevils has important implications for SIT control strategies—the longer that females are unreceptive, the fewer young that are produced.

To increase an understanding of the effects of male matings on female receptivity, Chihiro Himuro and colleagues at the Okinawa Prefectural Plant Protection Center investigated the role four chemicals from male weevils had on female mating receptivity, and they reported their findings in March in Annals of the Entomological Society of America.

Previous studies that showed female unreceptivity following mating were observational, and the physiology behind this pattern of unreceptivity was not known. Himuro and colleagues examined this pattern in a controlled experimental procedure to gain insight into the physiological mechanism of female unreceptivity.

Himuro and colleagues used West Indian sweetpotato weevils from a stock population that originated from insects captured in the wild in Okinawa. They reared the experimental weevils on sweet potato roots for five generations. They then exposed females to chemicals present in the seminal fluid of male weevils to test if any of the chemicals influenced receptivity in females. The four chemicals were from the seminal vesicles and three accessory glands of males, as follows:

1. accessory gland B solution
2. seminal vesicle solution
3. accessory gland A solution
4. accessory gland C solution

They tested the chemicals by removing samples from virgin male weevils, injecting them into the abdomens of females using glass capillary tubes, and then recording how long each female remained unreceptive to mating.

The investigators found that females that received injections of accessory gland B solution showed significantly longer periods of unreceptivity than control females that received saline. Conversely, the period of non-receptivity of females receiving the other three solutions were not significantly different from that of control females.

“We were surprised that males control female sexual activity,” Himuro said, “and that we can artificially induce a long refractory period [a sexually inactive condition] in ‘virgin’ females using our injection method.”

Interestingly, the investigators found that females that received injections of accessory gland B solution showed an average period of unreceptivity of 9.5 days. This was a significantly longer period of unreceptivity than seen in control females, but it was considerably shorter than the average of 13.5 days observed in females that mated with males naturally. This indicated that more than just accessory gland B might be contributing to female unreceptivity and that the specific concentrations and combinations of chemicals found in natural seminal fluid contribute to producing longer unreceptivity.

There are several potential evolutionary benefits males could enjoy from chemically inducing post-mating unreceptivity in females. By reducing sperm competition with other males, a male would increase the number of eggs from a female that it fertilizes. Inducing unreceptivity could also reduce the energetic costs to males of having to spend more time mating with a female or having to spend time guarding the female to prevent matings with other males.

For their next steps in this research, the investigators will further explore the mechanisms of unreceptivity and their implications for refining SIT strategies. “We will use simulation models to study the effectiveness of control options,” Himuro said. “We will also study what kind of male is better for SIT—that is, are males that induce long refractory periods in females better for SIT, or not? If males that induce long refractory periods in females are better for SIT, then we will try to use artificial selection to produce males that induce long refractory periods.”

John P. Roche is a science writer and author with a Ph.D. in the biological sciences. He has served as a senior scientist and adjunct professor at Boston College, as an editor-in-chief of periodicals at Indiana University and Boston College, and as a science writer at Indiana University and the University of Massachusetts Medical School. He has published more than 170 articles and has written and taught extensively about science. For more information, visit http://authorjohnproche.com/.