Soldiers are often deemed crucial in the defense of their social insect colonies from foreign invaders. One such insect – the tiny slim winged thrips, Kladothrips intermedius – lives and feeds on acacias, forming an abnormal growth of plant tissue that they then control. The enlarged forelimbs of this species' soldiers were thought to aid in defending its turf against its foe, the specialized invader Kladothrips dyskritus. However, previous observations of combat between these species yielded conflicting results, casting doubt on the utility of K. intermedius' forelimbs for defence and generating speculation about alternative roles for the soldiers. Captivated by these diminutive crusaders, Christine Turnbull from Macquarie University, Australia, and her Canadian and Australian collaborators set out to resolve these inconsistencies. Having previously identified anti-microbial defence tactics in other insects, they opted to evaluate the interaction between K. intermedius and micro-organisms, in addition to its contests with its traditional nemesis, K. dyskritus.

First, Turnbull and her colleagues assessed the role of K. intermedius' fortified forelimbs in defence. The team collected both thrips species from Acacia plants in South Australia, tossing one defender (K. intermedius) and one attacker (K. dyskritus) into tiny tubes to fight to the death, an established technique in studying these warring factions. They then threw sex into the mix, wondering whether male and female soldiers had different odds of victory. The team established the sex of every K. intermedius defender and paired each individual in combat with a K. dyskritus invader, taking detailed images and measurements of the soldiers' body segments, legs and wings post-battle. They discovered that K. intermedius males have shorter forelimbs and are more slender than their female conspecifics, but they did not find a significant difference in combat performance between the sexes. Contrary to the assumption that enhanced forelimbs impart the upper hand in battle, the bulkier builds and heftier limbs of females did not win them any more victories than the more petite males. Forelimb size within a sex also had no bearing on battle outcome, adding to the ambiguity of their value in combat.

To test their hypothesis that these warriors may be vital for guarding against micro-organisms, the team collected K. intermedius soldiers, as well as members of the docile working class. Washing groups of individuals from each class to obtain the compounds that they secrete onto their body surfaces, the scientists then tested the effects of the soldier and worker rinses on spores of the specialized fungus Cordyceps bassiana. Recording the fungal growth using optical density measurements, the team found that the rinses from the soldiers were much more effective at suppressing fungal growth than those from the non-warring workers.

Rather than serving their colony solely with brute force, these researchers believe that K. intermedius soldiers also expend considerable resources contending with micro-organisms. It is known that social insects host a diversity of microbes, from pesky parasites that reduce insect reproductive capacity to beneficial bacteria that provide essential nutrients to their insect host, suggesting coevolution in these relationships. The work of Turnbull and colleagues suggests that micro-organisms have played as important a role as macroorganisms in the evolution of social insect soldiers. As these scientists might advise, don't count out the little guy!

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