High density brood of Australian gall-inducing Acacia thrips aid in fungal control

Kladothrips (Froggatt) is a genus of gall-inducing thrips that develop broods, and in some species, long-lived colonies within galls they form on phyllodes of Acacia in arid and semi-arid Australia. The gall interior is a stable environment for thrips in an otherwise inhospitable environment, but these conditions may also be favorable for fungal parasites. This fungal threat is corroborated by the observation that Kladothrips produce highly effective antifungal compounds. Here we investigated antifungal production in three Acacia thrips species, two gall-inducers: Kladothrips arotrum and Kladothrips tepperi, and one kleptoparasitic thrips, Koptothrips dyskritus. Using a spectrophotometer, the germination of a suspension of Cordyceps bassiana spores (an entomopathogenic fungus) can be detected by observing an abrupt increase in light absorption by the suspension. The addition of thrips exterior washes to these fungal spore suspensions resulted in significant delays in fungal germination for all three species. Foundresses in both Kladothrips species strongly delay fungal germination before their brood has matured. Young and maturing colonies with less than 50 adult individuals (the remainder of the brood are in juvenile stages of development) produced some antifungal effects but within the range produced by the foundress alone. Mature colonies (>100 adults) delayed germination for the duration of our observational window (48 h), suggesting a possible group size threshold for antifungal effectiveness. Koptothrips dyskritus antifungals were observed to be the strongest of the three species when <50 individuals were present. The strong antifungal abilities of the invading Ko. dyskritus would allow this species to invade older or damaged galls, which has been observed in the field. This pattern of antifungal activity in Acacia thrips suggests that effective defense against fungal pathogens is strongly associated with group size and colony maturity.