Do itchy caterpillars have an advantage in cities?

This is a guest post by PhD student Kristi Backe, who is currently studying the pine processionary moth in France.

The caterpillars of the pine processionary moth (a.k.a. “PPM”) shoot itchy, projectile hairs that cause allergic reactions in humans, pets, and other animals. They also work in groups to build big, ugly silk nests in pines and other coniferous trees. Obviously, this doesn’t make PPM a fan favorite, but that hasn’t stopped entomologists from studying it intensively for decades (in fact, it’s mostly why).

Pine processionary moth (a.k.a. PPM; Thaumetopoea pityocampa) caterpillars. Photo:Kristi Backe

The PPM is native to the Mediterranean Basin, and as winters have become warmer over the last several decades, it’s been expanding its range farther north. For many years, scientists have used forest maps to focus their predictions about where the PPM might spread. The underlying idea, and it’s a good one, is that the PPM isn’t going to survive for too long in areas with no food. But, it’s becoming increasingly clear that the PPM doesn’t actually need forests. It just needs trees, and it’s willing to accept the ones in the front yard, thank you very much.

In 2012, a group of French scientists led by Christelle Robinet found evidence that the PPM might actually be doing better on trees in cities than on trees in forests because cities are warmer (a phenomenon scientists have named “the urban heat island effect”). If you’ve been following this blog for awhile, this idea probably sounds familiar. (And if you haven’t, check out this, this, and/or this.) Because I study the distribution of insects in urban areas and I want to spend eight months of my life eating cheese and baguettes, I jumped at the chance to head to France to study how the PPM is doing in cities.*

At the end of August, I started a stint with Alain Roques in the Forest Zoology lab at the French National Institute for Agronomic Research (INRA) in Orléans, France, to try to figure out whether the warmer temperatures in cities are giving urban-dwelling PPM caterpillars an advantage. The end of the summer might seem like a strange time to start an entomology field season, but the PPM caterpillars buck the usual trend and are active in the winter. They also feed at night, when the urban heat island effect is the strongest, which (fingers crossed) makes them a good study subject for this type of research.

A black pine (Pinus nigra) nursery infested with PPM. The big white “ornaments” on the two middle trees are PPM nests. Each nest contains hundreds of caterpillars. Photo: Kristi Backe

Field work in France is equal parts caterpillars and castles. Photo: Kristi Backe

In general, there are two main ways that warmer temperatures in cities could help the PPM survive during winter at the northern part of its range: 1) if caterpillars grow faster in the city’s warmer temperatures, they may be bigger and hardier than the forest caterpillars when cold temperatures finally arrive, and 2) if the minimum temperatures during a cold snap aren’t quite as cold in the city as in the forest, it’s less likely that caterpillars will freeze to death. Now all that’s left is figuring out whether either of these is making a difference for the PPM in urban areas.

I’ll be following up with a couple more posts about how we’re trying to answer our research questions and what it’s like to spend my days with millions of allergenic insect hairs.** In the meantime, enjoy this video of young PPM caterpillars doing their thing:

* If, like me, you’re a PhD student interested in overindulging in cheese and baguettes during your dissertation work, check out the Chateaubriand Fellowship Program from the Embassy of France in the U.S. Students with NSF graduate research fellowships (MS and PhD) can also apply for supplemental funding through NSF GROW. Thanks to both of these programs for supporting my trip!

** Remember when you were little and you dreamed of having a big-kid job where you’d be pelted in the face by barbed, projectile caterpillar hairs? Me either.

2018-01-24T09:14:21+00:00 January 17th, 2018|Categories: Urban Ecology|Tags: , , , , |

About the Author:

Kristi Backe
I am interested in urban ecology and am studying relationships between street tree diversity and the makeup of arthropod communities in trees.