Thrips Wars!

Two males of Elaphrothrips tuberculatus fight it out on the left, while the object of their desire guards her egg-mass on the right. Figure from Crespi (1986).

All around, little dramas are taking place every day, conflicts as intense as the plot of any daytime soap opera. And like most daytime soap operas, the main focus of these dramas often comes down to who is shagging whom. Most people only known thrips as small annoying insects that damage garden plants and crops, but some thrips may engage in remarkable behaviours.

Elaphrothrips is a genus of thrips found almost throughout the tropics (though it is absent from Australasia). They are found on dead leaves, where they feed on fungal spores. Well over a hundred species have been named in Elaphrothrips, though Mound & Palmer (1983) pointed out that many of these may be turn out to be synonymous as individual species can vary significantly in appearance. Males may have thick forelegs with strong tubercles on the femora, while the forelegs of females are usually slender and lack tubercles. Indeed, the sexes are different enough that at one point they have been mistaken for separate genera. The males themselves may vary significantly in size, with larger males having correspondingly larger legs and spines.

A lot of these differences are related to the Elaphrothrips' mating behaviour. The best-studied of the Elaphrothrips species is E. tuberculatus, a widespread species in eastern North America and the largest North American thrips species. Elaphrothrips tuberculatus prefer dead oak leaves that are still hanging in clusters from the tree, where females lay eggs in clusters on the leaves and then stand guard over them. The females are themselves guarded by males, but the males may be challenged by others who want to take the female for themselves. Battles between male Elaphrothrips most commonly take the form of the two males lining up alongside each other, as in the drawing at the top of this post, and then one or each begins batting at the other with his elongate abdomen. Alternatively, one male may attempt to reach under his opponent's abdomen with his own, and then try to flip his opponent over. Crespi (1986) noted that challenging males were more likely to try to flip their opponent than defending males, perhaps because the success rate of flipping attempts was very low, making this tactic more of a gamble. Flipping could also act as a defense against a third attack strategy, in which one male would climb up onto the back of his opponent and use the tubercles on his forelegs to stab at his opponent's thorax. Larger males were more likely to stab their opponents than smaller males, which of course have less developed leg spines. However, a smaller male may also get around larger male through sneaking behaviour, mating with the female before her guarding male realises he is there.

Whichever male mates with the female, one thing is certain: he will only have daughters. Thrips have a haplodiploid sex determination system like that of ants and bees, with males developing from unfertilised ova and females from fertilised ones. Elaphrothrips tuberculatus adds another wrinkle to the system that only females hatch from eggs. Male offspring, on the other hand, develop inside their mother and are born live (Crespi 1989). Nevertheless, an individual female may have both male and female offspring, as she may change her reproductive mode between broods to be a live-bearer or an egg-layer!

REFERENCES

Crespi, B. J. 1986. Size assessment and alternative fighting tactics in Elaphrothrips tuberculatus (Insecta: Thysanoptera). Animal Behaviour 34: 1324-1335.

Crespi, B. J. 1989. Facultative viviparity in a thrips. Nature 337: 357-358.

Mound, L. A., & J. M. Palmer. 1983. The generic and tribal classification of spore-feeding Thysanoptera (Phlaeothripidae: Idolothripinae). Bulletin of the British Museum (Natural History): Entomology 46 (1): 1-174.