Fungus fighting soldiers cast new light on the evolution of sociality

Scientists have discovered that the solider caste of a tiny insect species fights harmful fungal attackers rather more effectively than their traditional insect enemies, according to research published today in Biology Letters.

Thrips of the species Kladothrips intermedius - found in plant galls - are able to produce powerful antifungal compounds capable of neutralising Cordyceps bassiana, a harmful fungus pathogen which kills the miniscule bugs.

The research was prompted by the puzzling observation that soldier thrips seemed unusually reluctant to fight when placed together with their traditional enemies, another thrips species called Koptothrips.

Soldier Kladothrips have enlarged forelimbs and are able to crush enemy Koptothrips to death in a fight - however, not only are they reluctant to do this in a fight, but previous work has shown that their ability to do so is no better than the colony foundress, raising the question of why a soldier caste exists at all.

However, according to lead author Christine Turnbull of Macquarie University, "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."

The findings cast an intriguing new light on the evolution of sociality in insects.  While traditional theories have focused on the importance of soldier castes' physical battles with invaders and other enemies, this research suggests that fighting invisible microbial attackers may have been just as important, as Christine Turnbull adds:

"While it is unknown whether specialised fungal pathogens have been major selective agents in the evolution of the soldier caste in general, they were probably present when sociality first evolved and may have been the primordial enemies of social insects."