Image by Gilles San Martin

Frogs are well known for their insatiable appetite for insects. However, recent evidence suggests that in some situations, the hunter has become the hunted. This appears to be the case with the diving beetle and their larval offspring, which have been found to actively hunt and feed on tadpoles. Although relatively small insects, their voracious appetite may be playing a role in reducing wild frog populations.

One night while conducting fieldwork, a dozen adult diving beetles were witnessed surrounding, attacking, and quickly dismembering a tadpole. Seeing the group of beetles attacking and eating a live tadpole, I quickly got my camera out to record the event. Although the larvae of the diving beetle are carnivorous predators, adults are considered scavengers due to their small mouth parts, making this find very interesting.

In a different area of the pond, a lone adult diving beetle was discovered with its head buried deep inside and feeding on a tadpole three to four times its size. Unlike the typical adult behavior of tearing prey into small pieces, this beetle was feeding in a similar way as their larvae, which pierce and inject digestive enzymes into their prey to suck their liquefied remains. I later realized that this pack predatory behavior and feeding method of adult diving beetles had not been previously documented.

Although these behaviors were interesting, I was concerned since it occurred in a managed conservation site created to protect an endangered frog species. I investigated further and discovered that, despite making up a very small number of the ponds, most tadpoles within the managed site and the surrounding area were found in ponds without diving beetles, suggesting these predatory insects may be negatively effecting this endangered species. After discussing my findings with my colleague, John Gould from the University of Newcastle, we both realized these beetles were even more ingenious and brutal than they could have imagined.

Image by Evanherk from Wikipedia

While collecting recently deposited frog egg masses, also called frogspawn, Gould noticed tiny unidentifiable eggs scattered throughout some of the samples. He brought the spawn back to the lab and a few days later discovered that not only were the eggs laid by a species of diving beetle, but that the larvae had begun to hunt down and eat the tadpoles once they started to hatch. “These larvae have a ravenous brutality,” Gould said. “The larvae were eating up to three tadpoles in an hour, often throwing away the remains of one that was half eaten to hunt down a new one that swam by.”

Image by Gilles San Martin

After bringing back more spawn from the field, he found that the diving beetle larvae would hatch within 24 hours as the tadpoles, suggesting that the beetles could be synchronizing their reproduction with the frog so that the hatching of their offspring coincides with that of the tadpoles. “This behavior makes a lot of sense as these spawns are laid by a frog species in very short-lived puddles of water lasting only a few days,” says Gould. “With such a short time before the ponds dry, there is often very few food resources available, making the tadpoles an enticing meal for the newly hatched beetle larva.”

Diving beetle larva a few days after hatching from frogspawn samples collected from the field. Credit: John Gould.

Closeup of the head of a diving beetle larva showing the opened mandibles. Credit: John Gould

These findings have implications for amphibian conservation since most conservation projects ignore and are not typically concerned with threats posed by small insects. Although amphibian conservation plans typically expect some losses from natural predation, small insect predators such as diving beetles may seriously effect efforts, particularly for populations already on the threshold of extinction where the survival of each offspring into adulthood is critical to save the species from extinction.

Written by Jose Valdez

Research articles:

Valdez J.W. (2019). Predaceous diving beetles (Coleoptera: Dytiscidae) may affect the success of amphibian conservation efforts. Australian Journal of Zoology. doi: https://doi.org/10.1071/ZO19039

Gould. J., Valdez J.W., Clulow J., Clulow, S. (2019). Diving beetle offspring oviposited in amphibian spawn prey on the tadpoles upon hatching. Entomological Science. 22 (4) 393-397. https://doi.org/10.1111/ens.12381