Using weapons instead of perfume: chemical association strategies of the myrmecophilous bug Scolopostethus pacificus (Rhyparochromidae)

Abstract

A vast diversity of parasites associates with ants. Living in and around ant nests of these organisms must overcome ant colony defenses. As ant defensive behavior is mainly mediated by species-specific cuticular hydrocarbons (CHCs) or alarm pheromones, and ant-associated parasites can either crack their hosts’ chemical communication code by modifying their own CHC profiles or use pro-active strategies like chemical weaponry for distraction and repellency. While the chemical nature of ant–parasite interactions has been intensively studied for highly host-specific parasites, the chemical-deceptive strategies of the rather rare ant-resembling heteropterans are unknown. To gain insight into this system, I studied the bug Scolopostethus pacificus (Barber 1918) which can be found near the nests of the ecologically dominant and aggressive velvety tree ant (Liometopum occidentale, Emery 1895). Using behavioral, chemical, and molecular approaches, I disentangled the relationship of S. pacificus and its host ant. Chemical profiling of the bug and the ant revealed that the bug does not make use of CHC insignificance or mimicry, but instead uses a cocktail of volatile compounds released from its metathoracic glands that likely moderates encounters with its aggressive host. Feeding trials with armed and artificially disarmed bugs revealed a defensive function of the gland exudates. Targeted molecular gut barcoding showed that S. pacificus does not feed on L. occidentale. These results suggest that chemical weaponry, rather than a chemical code-cracking CHC matching or chemical insignificance, enables S. pacificus to get along with and live in close proximity to its host ant.