Sex pheromone component ratios and mating isolation among three Lygus plant bug species of North America

Abstract

The plant bugs Lygus hesperus, Lygus lineolaris, and Lygus elisus (Hemiptera: Miridae) are major pests of many agricultural crops in North America. Previous studies suggested that females release a sex pheromone attractive to males. Other studies showed that males and females contain microgram amounts of (E)-4-oxo-2-hexenal, hexyl butyrate, and (E)-2-hexenyl butyrate that are emitted as a defense against predators. Using gas chromatography-mass spectrometry, we found that female L. lineolaris and L. elisus have a 4:10 ratio of hexyl butyrate to (E)-2-hexenyl butyrate that is reversed from the 10:1 ratio in female L. hesperus (males of the three species have ~10:1 ratio). These reversed ratios among females of the species suggest a behavioral role. Because both sexes have nearly equal amounts of the major volatiles, females should release more to attract males. This expectation was supported because L. hesperus females released more hexyl butyrate (mean of 86 ng/h) during the night (1800-0700 hours) than did males (<1 ng/h). We used slow-rotating pairs of traps to test the attraction of species to blends of the volatiles with a subtractive method to detect synergism. Each species' major butyrate ester was released at 3 μg/h, the minor butyrate according to its ratio, and (E)-4-oxo-2-hexenal at 2 μg/h. The resulting catches of only Lygus males suggest that (E)-4-oxo-2-hexenal is an essential sex pheromone component for all three species, (E)-2-hexenyl butyrate is essential for L. elisus and L. lineolaris, and hexyl butyrate is essential for L. hesperus. However, all three components are recognized by each species since ratios of the butyrate esters are critical for conspecific attraction and heterospecific avoidance by males and thus play a role in reproductive isolation among the three species. Because L. hesperus males and females are known to emit these major volatiles for repelling ant predators, our study links defensive allomones in Lygus bugs with an additional use as sex pheromones.