We investigated how change in plant community composition brought about by annual grass-specific herbicide application affects terrestrial arthropod communities, with special emphasis on the mutualists of the endangered Fender’s blue butterfly, Plebejus icarioides fenderi (Family: Lycaenidae). Larvae of this species form facultative mutualisms with ants, who chase away potential predators of the larvae. We used pitfall trapping to compare ant communities between control and herbicide-treated plots through time. The extent to which major changes in plant community composition affect the mutualistic ant community differed among years. Our findings may have relevance for management decisions if the focus of the conservation effort has strong ecological interactions with greatly affected non-target species
