The impact of kleptoparasitic invasions on the evolution of gall‐size in social and solitary Australian Acacia thrips

Many species of gall‐inducing Acacia thrips are attacked by kleptoparasitic thrips who enter the gall, destroy the occupants, and then use the gall for producing their own offspring. The hypothesis tested here is that pressure exerted by kleptoparasites (genus Koptothrips ) not only provoked the evolution of soldiers in the gall‐inducing clade, but have also influenced the evolution of gall size and morphology. Various size dimensions of invaded galls were compared to those of uninvaded galls using data from six gall‐inducing species and their kleptoparasites. For the non‐social gall‐inducing species ( K. ellobus and K. nicholsoni ) invaded galls showed no significant size differences from galls that had not been invaded. For the four social gall‐inducing species ( K. habrus, K. intermedius, K. waterhousei and K. morrisi ) invaded galls were significantly narrower and/or shorter than uninvaded galls. Galls of social species that had not been invaded and contained adult soldiers were significantly larger than galls where soldiers were still at a larval stage, suggesting that gall size is related to gall age in these species. An hypothesis is proposed that links the timing of invasion by kleptoparasites to size of the host gall: induction of a smaller gall by host founders will reduce the period of vulnerability to invasion (before soldiers become adults) for social thrips by allowing foundresses in these smaller galls to begin laying soldier‐destined eggs relatively sooner.