Bugging You From San Juan Island

Check out my story about praying mantids on San Juan Island!  They’ve been a victim of hyper-sensationalism.  Seriously.  Read my article before you squish one.

Revenge of the mantids!

Lots of folks are commenting about seeing praying mantises on the island lately. Rumor has it they’re highly invasive and they’re eating hummingbirds! While they are non-native, I’m hoping to dispel some of the sensationalist hysteria about their presence on the island by offering information for consideration.

The mantids we are seeing right now are a species called Mantis religiosa, introduced into the southern interior of British Columbia to control grasshoppers in 1937-1938 and have expanded their range since the late 1990’s. They are identifiable by a black-ringed white spot on the inner base of their foreleg.

This taxonomic feature differentiates them from the exotic Chinese Mantis, Tenodera aridifolia sinensis and native Ground Mantis, Litaneutria minor found in Washington, but not on San Juan Island. Egg sacks for European and other mantis species are available online and at garden centers for purchase. Aside from releasing them to control agricultural pests, lots of people keep mantids for pets!

As to the mantids eating hummingbirds…

While worldwide reports exist about various mantids capturing and eating hummingbirds, these are sparse, highly sensationalized accounts, most of them not occurring in the U.S. at all. A 2017 article in Wilson Journal of Ornithology reports hummingbird predation by Mantis religiosa (the ones purportedly “invading” the San Juan Islands). However, for the ENTIRE U.S., there are only 3 substantiated reports, none of which occurred in Washington, much less on San Juan Island. If you’re worried about birds, what about outdoor kitty cats with sharp teeth and claws?

The mantids are killing all our pollinators…

We’ve read about pollinator die-off due to habitat degradation, pesticides, parasites, and pathogens but are mantids a menace too? Let’s look at mantis development, and consider the impact.

The mate-seeking adult mantids will die off as temperatures drop, but need to reproduce before the end of their life cycle. A mated female’s remaining energy will be spent producing a frothy egg mass of 100-400 offspring. The mass hardens into a styrofoam-like ootheca, cemented in some cryptic spot where it will remain until spring.

In spring, mantid nymphs emerge simultaneously from the egg sack. The highly competitive and cannibalistic hatchlings eat many of their siblings. It is estimated that approximately 10% survive this first stage (Hurd and Rathet, 1986).

The hatchling Mantis religiosa nymph, a tiny replica of the adult, must complete 8 or 9 developmental stages, shedding its exoskeleton (molting) each time. During a molt, the mantis must hang upside down to shed its skin, leaving it extremely vulnerable to predation and to accidental injury. The mantis cannot fall from where it hangs or it risks becoming stuck in the old skin. With each stage of development, the mantis must find food and shelter and manage to evade predation by birds, reptiles, spiders, or even other insects like robber flies or dragonflies.

During spring and into early summer, the mantis is pretty small. Although hundreds emerged from the egg sack, very few become adults. They will prey on pollinators, but so do birds, reptiles, spiders, dragonflies, robber flies, and even certain wasps. One fact you may not know is mantis nymphs often survive in spring on pollen! 

I kept two European mantids as pets for about ten months and found they preferred to dine on flies. They would eat yellowjackets, but typically only if they were really hungry and only if I cracked open the hard-shell for them first. Their appetites increased as fall approached. One night a swarm of termites emerged from a tree stump by my door. Lucky for the mantids, I had my bug net handy. One ate 30 or 40 termites that night. A few nights later, I had an egg sack cemented to the window screen. I found “Merlina” on the floor the next morning, a victim of feline curiosity.

Most islanders didn’t notice these mantids until late summer and early fall. If you saw one eating a pollinator (unless it’s a bumble bee queen or honey bee), that insect was nearing the end of its natural life cycle and doesn’t overwinter. The pollinators have done their job. The trees, garden plants, and flowers aren’t in need of pollination anymore. Winter is around the corner. If the mantis catches and eats one of these, I believe it is nature’s way of cleaning up.

About the author

Cynthia Brast has a M.S. in Entomology and Nematology from the University of Florida, Gainesville. She welcomes your bug photos and questions.

References

Cannings, R.A. 2007. Recent range expansion of the Praying Mantis, Mantis religiosa Linnaeus (Mantodea: Mantidae), in British Columbia. J. Entomol. Soc. Brit. Columbia, 104: 73-80.

Hurd, L.E., and I. H. Rathet. 1986. Functional response and success in juvenile mantids. Ecology, 67(1):163-167

Kral, K., 2014. Orientation behavior with and without visual cues in newly hatched and adult praying mantis. Journal of Insect Behavior, 27: 192. https://doi.org/10.1007/s10905-013-9415-3

Nyffeler, M., M.R. Maxwell, and J.V. Remsen. 2017. Bird predation by praying mantises: A global perspective. Wilson Journal of Ornithology, 129(2), 331-344. doi:10.1676/16-100.1