Ant parasitoidism in checkered beetles: Phyllobaenus obscurus developing inside intact cocoons of two species of the Ectatomma ruidum species complex.

Known parasitoids of ants include species from several families of flies, wasps, strepsipterans, nematodes, and mites. Curiously, while myrmecophily is heavily biased towards Coleoptera, one of the most diverse and speciose insect orders, no beetles specialized as parasitoids of ants have been recorded, although the parasitoid habit has evolved at least 13 times within this order. Here we report on observations that strongly suggest that a checkered beetle species behaves as a parasitoid of ant brood. A total of 146 colonies or part of colonies of three species of the Ectatomma ruidum species complex (E. ruidum sp. 2, 3, 4) were excavated in several sites along the Pacific coastal plains of Oaxaca, Mexico, during three collecting campaigns (2015-2017). Overall, 11060 adults, 5795 cocoons and 2185 larvae were examined. Upon dissection, four intact, ethanol-preserved cocoons contained ant prepupae/pupae parasitized by characteristic campodeiform beetle larvae (prognathous head, three pairs of segmented legs, no prolegs, body with sparse but long pubescence), and a fifth cocoon presented a round exit hole. Sequencing of the COI gene fragment support assignment of the larva to Cleridae, consistent with the genus Phyllobaenus (Cleridae). An active, pink-colored larva that emerged from a cocoon containing the remains of the ant host pupa, was reared to adulthood and could be identified as Phyllobaenus obscurus (Gorham). Beetle larvae were found inside intact cocoons of two species: E. ruidum sp. 3 and sp. 4. The prevalence of parasitism is extremely low, representing less than 0.6% of available cocoons. Predatory during both adult and larval stages, checkered beetles are broadly known as predators of wood-boring and cone-boring beetles, and some species are facultative parasitoids of solitary bees or wasps or, very rarely, specialized in predating social insects. We assert that the novel discovery of clerid-ant brood parasitoidism within the subterranean host colony deviates yet further from any adaptation known to date among clerids.