Stick bug eggs can survive being eaten, pooped out by birds: study

The stick bug has used nature to its advantage by looking like a stick to avoid being eaten. And researchers have discovered that this cleverly disguised insect may be using another bit of nature to its advantage: getting eaten by birds.

These crafty insects may be using birds to their advantage

On the left is an image of stick insect eggs collected from the excrement of brown-eared bulbul. On the right, a stick insect born from the eggs. (Kobe University)

The stick bug has used nature to its advantage by looking like a stick to avoid being eaten. And researchers have discovered that this cleverly disguised insect may be using another bit of nature to its advantage: getting eaten by birds.

Stick bugs, or phasmids, have developed their stick-like characteristics to conceal them from predators, but their camouflage isn't always successful: they are preyed upon by spiders, rodents, reptiles and even birds.

Some phasmids also have another fascinating ability. They are parthenogenic, meaning that they are able to produce offspring from an unfertilized egg. As well, the eggs of most stick bugs are hard and durable, more so than those of other insects.

Kenji Suetsugu, an associate professor from Kobe University Graduate School of Science in Japan, was curious: Could the eggs of these stick bugs survive passing through the digestive tract of a bird? Could they hatch?

The seeds of some plants use this method to extend their habitat​, so Suetsugu, along with researchers from Kochi University and the Tokyo University of Agriculture and Technology, conducted two experiments with three different types of stick bugs — Ramulus irregulariterdentatus, Neohirasea japonica, and Micadina phluctaenoides — and one of their main bird predators, the brown-eared bulbul (Hypsipetes amaurotis).

In the first experiment, they fed more than 40 eggs of each species to the birds and found that, after they were defecated out three hours later, between five and eight per cent of the eggs didn't suffer any damage. However, though intact, none of the eggs hatched within two years.

In the second experiment, however, conducted in 2017, 70 eggs from R. irregulariterdentatus were fed to the bird, and 20 per cent remained intact; two of the 14 that survived hatched.

This is an image of the mechanism for stick insect habitat expansion suggested by these new findings. For insects with very low mobility, such as stick insects, bird predators could be helping them to extend their territory. (Kobe University)

The findings are significant because stick bugs don't travel very far and may be able to expand their territory if their eggs are carried farther by birds.

"[The] successful dispersal of stick insect eggs via the predation o[n] adult females would be an infrequent occurrence and would not be the usual dispersal strategy," Suetsugu told CBC News. "However, considering that stick insects are slow-moving and often flightless, with a limited capacity for dispersal, the benefits of long-distance dispersal via bird predation should not be underestimated."

It's unknown whether the durability of the eggs has evolved in order to utilize the birds this way or for other reasons that just ended up being good luck for the stick insects. The researchers note that egg production in these stick insects happens to coincide with the migration of these bulbul birds.

Suetsugu said that the findings may be good news for other types of insects. 

"Our results overturn the dogma that insects eaten by natural enemies will end their descendants," he said.

About the Author

Nicole Mortillaro

Senior Reporter, Science

Nicole has an avid interest in all things science. As an amateur astronomer, Nicole can be found looking up at the night sky appreciating the marvels of our universe. She is the editor of the Journal of the Royal Astronomical Society of Canada and the author of several books.