Bombardier beetles repel invasive bullfrogs

Abstract

Invasive non-native predators negatively affect native species; however, some native species can survive the predation pressures of invasive species by using pre-existing antipredator strategies or evolving defenses against invasive predators. The American bullfrog Lithobates catesbeianus (Anura: Ranidae) has been intentionally introduced to many countries and regions, and has impacted native animals through direct predation. Bombardier beetles (Coleoptera: Carabidae: Brachininae: Brachinini) discharge chemicals at a temperature of approximately 100 °C from the tip of the abdomen when they are attacked by predators. This "bombing" can successfully repel predators. However, adults of a native bombardier beetle Pheropsophus (Stenaptinus) occipitalis jessoensis have been reportedly found in the gut contents of the introduced bullfrog L. catesbeianus in Japan. These records suggest that the invasive bullfrog L. catesbeianus attacks the native bombardier beetle P. occipitalis jessoensis under field conditions in Japan; however, the effectiveness of the bombing defense against invasive bullfrogs is unclear. To test the effectiveness of the bombing defense against bullfrogs, we investigated the behavioral responses of L. catesbeianus juveniles to P. occipitalis jessoensis adults under laboratory conditions. Contrary to previous gut content results, almost all the bullfrogs (96.3%) rejected bombardier beetles before swallowing them; 88.9% rejected the beetles after being bombed, and 7.4% stopped attacking the beetles before being bombed. Only 3.7% successfully swallowed and digested the beetle. All of the beetles collected from non-bullfrog-invaded sites could deter bullfrogs, suggesting that the pre-existing defenses of bombardier beetles played an essential role in repelling bullfrogs. When treated beetles that were unable to discharge hot chemicals were provided, 77.8% of bullfrogs successfully swallowed and digested the treated beetles. These results indicate that bombing is important for the successful defense of P. occipitalis jessoensis against invasive bullfrogs. Although invasive bullfrogs have reportedly impacted native insect species, P. occipitalis jessoensis has an existing defense mechanism strong enough to repel the invasive predators.

Keywords: Bombardier beetles; Brachinini; Carabidae; Chemical defences; Introduced predators; Invasive alien species.

Figure 1. A bombardier beetle, an invasive bullfrog, and a native frog.

(A) An adult bombardier beetle Pheropsophus occipitalis jessoensis. (B) A juvenile bullfrog Lithobates catesbeianus. (C) An adult pond frog Pelophylax nigromaculatus. These photographs were taken at the same site and microhabitat on the same date. Photo credit: Shinji Sugiura.

Figure 2. Temporal sequence of the bullfrog Lithobates catesbeianus rejecting a control adult Pheropsophus occipitalis jessoensis.

(A) 0 ms. (B) 375 ms. (C) 900 ms. (D) 2,200 ms. (E) 2,575 ms. (F) 2,625 ms. (G) 2,650 ms. (H) 3,475 ms. The bullfrog spat out the beetle after taking it into its mouth. Bombing by the beetle was audible and the ejected vapor (E) was observed just before the bullfrog spat out the beetle (see Video S2). Credit: Shinji Sugiura and Tomoki Date.

Figure 3. Behavioral responses of the invasive bullfrog Lithobates catesbeianus and the native pond frog Pelophylax nigromaculatus to adults of the bombardier beetle Pheropsophus occipitalis jessoensis.

Swallow: bullfrogs or frogs successfully swallowed control beetles. Reject before bombed: bullfrogs or frogs stopped attacking control beetles before being bombed. Reject after bombed: bullfrogs or frogs rejected control beetles after being bombed. The graph showing data for Pe. nigromaculatus was taken from Sugiura (2018). Photo credit: Shinji Sugiura.