Praying Mantises Can't Stand 'Beetle Bombs'

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Praying mantises are perhaps the most distinct and fearsome predators of the insect world. Their ambush-style attacks are comparable to those of lions and tigers. But, as a recent study published to PeerJ reveals, despite their size and ferocity, praying mantises fall short when it comes to predating on bombardier beetles. Why? As their name suggests, these beetles have an explosive trick up their sleeves, or rather, their bums.

Shinji Sugiura, an associate professor in the School of Agricultural Science at Kobe University in Japan, was well aware of this physiological feat when he conceived of an experiment to pit praying mantises against bombardier beetles, insect versus insect. As he wrote in the paper describing his experiment, "Bombardier beetles of the tribe Brachinini store hydroquinone and hydrogen peroxide separately in two reservoirs in the abdomen. When the aqueous solutions of hydroquinones and hydrogen peroxide reach the reaction chamber from each reservoir, enzymes (catalysts) facilitate oxidation of the hydroquinones and decomposition of the hydrogen peroxide. An explosive reaction ejects the reactants and boiling water."

But can this burning chemical cocktail deter vastly larger, armored, ravenous praying mantises?

To answer that question, Sugiura collected sixty adult bomardier beetles (P. jessoensis) and sixty adult mantises of three different species (T. sinensis, T. angustipennis, and H. patellifera) from grasslands and forest edges in the Kinki region of Japan. After starving the mantises for 24 hours so they'd be hungry, he placed an individual mantis and an individual beetle in a clear plastic case and observed the outcome. Invariably, over thirty repetitions with different insects, the mantis would lunge at the beetle, get 'bombed', then pull back and groom the body parts sprayed with hot chemicals, as if nursing its wounds. Not a single mantis predation attempt was successful.

Next, Sugiura simulated "attacks" against beetles with forceps to cause them to exhaust their chemical defense, then, one by one, he faced these "treated" beetles off with a praying mantis. This time, the results were brutal. Not a single beetle survived without its 'bomb'. The mantises gobbled up their meals, leaving behind only a few broken parts.

Shinji Sugiura
Leftover antennae, legs, elytra, and hindwings of the beetle.

The results clearly demonstrate the effectiveness of beetle 'bombs' against praying mantises. Without them, bombardier beetles are walking feasts. As the two insects share a natural habitat in Japan, it's possible that these predator-prey interactions occur every day in the wild. Then again, it's also possible that mantises avoid bombardier beetles, not wanting to experience their explosive defense.

Source: Sugiura S. 2021. Beetle bombing always deters praying mantises. PeerJ 9:e11657

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