Effectiveness of explosive chemical defenses in bombardier beetles against native and non-native frogs

Animals encounter various predators, including native and non-native species. While native species develop defensive strategies to counter native predators, these defenses vary in efficacy against non-native predators. Bombardier beetles (Coleoptera: Carabidae: Brachinini) employ an explosive chemical defense, discharging toxic chemicals at nearly 100°C. However, experimental evaluations of this defense against non-native predators are limited. To test its effectiveness, we investigated the defense efficacy of a native bombardier beetle species (Brachinus incomptus) against its potential predators, two native frog species (Pelophylax nigromaculatus and Fejervarya kawamurai) and an invasive non-native species (the American bullfrog, Lithobates catesbeianus), which co-occur with this beetle species in Japan. Behavioral interactions between these frogs and adult B. incomptus were observed under laboratory conditions. In predation trials, 90%, 100%, and 65% of P. nigromaculatus, F. kawamurai, and L. catesbeianus, respectively, rejected adult B. incomptus, suggesting that the beetles’ defenses were effective against both native and non-native frogs. When frogs were offered beetles that had been repeatedly stimulated until no further explosive discharges were elicited, 80%, 95%, and 10% of P. nigromaculatus, F. kawamurai, and L. catesbeianus, respectively, rejected the treated beetles. This treatment did not significantly affect defensive success against native frogs but notably decreased it against the non-native frog. Our findings suggest that explosive chemical discharges may make a smaller relative contribution to defense against the native frogs tested than against the non-native American bullfrog.