Variation in propensity to defend by reproductive gall morphs in two species of gall-forming thrips

Abstract

Six species of Australian phleaothripine gall forming thrips on Acacia have two morphs. One morph, referred to as a soldier, has reduced antennae and wings and greatly enlarged fore femora, which are thought to be adaptations for gall defence. For most species, female soldier morphs have reduced reproductive output relative to dispersing females and these species have been regarded as eusocial. We examine defensive behaviour of soldier morphs using in situ presentation of gall invading kleptoparasites. We show that the gall-morph of Oncothrips tepperi attack kleptoparasites more often (N = 87 interactions, n = 11 attacks) than does the gall-morph of Oncothrips morrisi (N = 231, n = 1). This difference in proclivity for defence is coincident with a lower reproductive output by the gall-morph in O. tepperi compared to O. morrisi. Our results also show an almost complete absence of attack behaviour by the gall-morph of O. morrisi. Lastly, we show that there is no difference in proclivity for defence between soldier and foundress morphs of O. tepperi. These observations taken together call into question the suitability of the term 'soldier' as applied to the gall-morph of the gall-forming thrips of Australia.

Antifungal activity in thrips soldiers suggests a dual role for this caste

“…These outcomes further eroded the idea that forelimb morphology is linked strongly to the soldiers' role in gall defence. While our experiments could not address previous findings that soldiers showed little proclivity to engage the enemy [4], this behaviour, in the context of our data, suggested that, although soldiers clearly repel other insect invaders, their apparent reluctance to engage one kind of enemy may be because resources are allocated to fight another: micro-organisms. This is supported by the antimicrobial assays, which show that soldiers strongly suppress fungal growth and, significantly, can do so at much lower densities than dispersers.…”

Australian Thysanoptera – biological diversity and a diversity of studies

“…In contrast, other species induce small galls within which the foundress lays only a few eggs, and these give rise to wingless adults of both sexes with enlarged forelegs and prothorax. These wingless adults function as soldiers, fighting to protect their gall from invading caterpillars and kleptoparasitic thrips (Crespi & Abbot 1999;Perry et al 2002), and in some species the females have reduced reproductive capacity Chapman et al 2002).…”

The Evolutionary Ecology of Eusociality in Australian Gall Thrips: a ‘Model Clades’ Approach

“…The soldiers themselves appear to make an additional trade-off. Thus, the reproductive output of thrips soldiers ranges widely , and soldier defensive efficacy appears to be negatively correlated with the degree to which soldiers reproduce (r = −0.88, P = 0.0489, N = 5 by simple correlation; r= − 0.84, P = 0.076 using independent contrasts, Perry et al 2002Perry et al , 2004. Given evidence for a relationship between soldier reproduction and defensive ability, the mechanism and source of the trade-off remain unclear.…”