Defenses of whirligig beetles against native and invasive frogs.

Many native insects have evolved defenses against native predators. However, their defenses may not protect them from non-native predators due to a limited shared history. The American bullfrog, (Anura: Ranidae), which has been intentionally introduced to many countries, is believed to impact native aquatic animals through direct predation. Adults of whirligig beetles (Coleoptera: Gyrinidae), known for swimming and foraging on the water surface of ponds and streams, reportedly possess chemical defenses against aquatic predators, such as fish. Although whirligig beetles potentially encounter both bullfrogs and other frogs in ponds and lakes, the effectiveness of their defenses against frogs has been rarely studied. To assess whether whirligig beetles can defend against native and non-native frogs, we observed the behavioral responses of the native pond frog, (Anura: Ranidae), and the invasive non-native bullfrog, . , to native whirligig beetles, and , in Japan. Adults of whirligig beetles were provided to frogs under laboratory conditions. Forty percent of . and . were rejected by . , while all whirligig beetles were easily consumed by . . Chemical and other secondary defenses of . and . were effective for some individuals of . but not for any individuals of . . These results suggest that native whirligig beetles suffer predation by invasive non-native bullfrogs in local ponds and lakes in Japan.