Myrmecophiles, i.e., organisms associated with ants live in a variety of ecological niches in the vicinity or inside ant colonies and employ different strategies to survive ant encounters. Because different niches are characterized by different encounter rates with host ants, strategies used to avoid ant aggressions should depend on these niches. This hypothesis was studied with three rove beetle species of the genus Pella, which are myrmecophiles of the ant Lasius fuliginosus and the non-myrmecophilous relative Drusilla canaliculata. Behavioral tests in the field revealed that Pella species are better adapted to interactions with ants than D. canaliculata, but that they use species-specific strategies in ant interactions. Pella cognata and Pella funesta avoid encounters with ants by swift movements. Chemical analyses of the defensive tergal gland secretions showed that P. cognata has replaced the aggression inducing undecane by the behaviorally neutral tridecane. P. funesta repels the ants by releasing the panic alarm pheromone sulcatone from its tergal gland resulting in an “ant free space” around the beetles. Finally, Pella laticollis uses a specific and unique appeasing behavior. Behavioral and chemical data did not reveal any indication for the mimicry of the ants' cuticular hydrocarbon profiles by any of the beetle species. It is discussed that the employed strategies correlate with the ecological niches of the beetles. P. cognata and P. funesta are living along ant trails with ample space to escape and the employed strategies are probably sufficient to escape from dangerous conflicts. In contrast, P. laticollis lives in refuse areas of ant nests with frequent ant encounters, and its appeasement strategy allows it to stay at the encounter site.
Movie of behavioral interactions between Pella laticollis and Lasius fuliginosus. P. laticollis shows the appeasing behavior “duck down”, which is almost never observed in the other two Pella species described here. During “duck down”, the beetle stops and crouches to the ground with the abdomen bent upwards. The ants' reaction most often consists in intense antennation of the beetle's abdomen. After this, the beetle or the ant leaves the place. This is shown several times in the video (MPG 9305 kb)
Movie of behavioral interactions between Pella cognata and Lasius fuliginosus. P. cognata actively avoids encounters with ants. Nearly, each ant contact is prompted by immediate and swift changes of the beetle's direction (MPG 7173 kb)
Movie of behavioral interactions between Pella funesta and Lasius fuliginosus. P. funesta avoids aggressive encounters with ants by releasing tergal gland secretion. In this video, the beetle releases tergal gland secretion when it is molested by a large number of ants. The secretion contains sulcatone, a panic-alarm-inducing pheromone. As a consequence, ants actively avoid P. funesta (MPG 6558 kb)
Suppl. 5Total ion chromatograms of volatiles released by Drusilla canaliculata and three Pella species from the tergal gland. Numbers in chromatograms refer to the numbers in Table 2. GC, 30 m HP 5; 60°C/3 min; 60–300°C/3°C/min; hold 30 min/300°C (PDF 111 kb)
Suppl. 6Relative peak areas of different cuticular hydrocarbons (mean ± SD), which were found in extracts of single specimens of Lasius fuliginosus, Drusilla canaliculata, Pella cognata, P. funesta, and P. laticollis (PDF 31 kb)