Pine Processionary Moth in Cities 2018-05-18T11:09:58-04:00

Pine Processionary Moth in Cities

​Many terrestrial animals are shifting or expanding their ranges with anthropogenic climate change. This is important to understand because species of conservation importance could move north due to climate but arrive in an otherwise unlivable place. Plant pests may also expand their ranges and enter new geographic regions. In some cases, pests that are dangerous to humans could move into urban areas where their harm is magnified due to high human populations. Given that urbanization is expected to increase in concert with climate change, it is important to understand how cities affect climate change-induced range expansions.

The pine processionary moth (Thaumetopoea pityocampa; PPM) is a pine defoliator native to the Mediterranean Basin. It is one of the best documented examples of an organism expanding its range with climate warming. PPM caterpillars are covered with irritating hairs that cause allergic skin reactions in humans, with highest human exposure when the caterpillars make “processions” down their host trees to pupate in the soil. During PPM processions, hundreds of caterpillars follow each other across the ground, crossing roads, sidewalks, school yards, and other populated areas.

In addition to the human health risks posed by PPM, defoliation from caterpillar feeding reduces tree growth, which can diminish productivity of plantations, the conservation value of forests, and the ecosystem services city trees provide to residents.

Feeding damage from pine processionary moth caterpillars. Football-shaped caterpillar nests can also be seen near the tops of these trees. Photos: Kristi Backe

Researchers at the Institut National de la Recherche Agronomique (INRA) in France are the world’s experts on this species, and in recent years they have considered how cities might contribute to PPM range expansion. Still, very little is known about PPM biology in urban areas, though several lines of reasoning suggest that PPM could benefit from city life. First, in central France, urban areas contain ample PPM host trees, with most host trees located outside forests and in close proximity to human dwellings. In addition, cities are warmer than surrounding natural areas and may protect PPM from winter temperatures that normally restrict its northern range.

The warmer temperature in cities may affect PPM survival directly by buffering cold winter temperatures, or indirectly by increasing caterpillar growth rates.

Research Approach

Our goal is to determine how PPM responds to urban heat and how this response will affect its range expansion. To achieve this, our PhD student Kristi Backe spent 8 months in France conducting research on this rather unpleasant critter.

Kristi counted A LOT of caterpillars. Photo: Kristi Backe

Objective 1

Determine how urban heat islands affect PPM growth and phenology, and thus indirectly affect survival.


Because temperatures are warmer in cities, urban PPM colonies may have more time for feeding than colonies in colder rural areas. Caterpillars with faster growth will take less time to reach later instars, which are more resistant to cold due to their size and thicker nests.

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

Objective 2

Determine whether urban heat islands buffer against extreme cold temperatures and provide thermal refuge for the PPM at the northern end of its range.


PPM has been accidentally transported from the main range to northern urban areas on ornamental plants. These northern colonies might be able to survive because the urban heat island effect buffers against low winter temperatures. Because the PPM is experiencing a climate change-induced range shift at its northern range edge, this pattern would have important implications for predicting the species’ range expansion.

PPM caterpillars live inside nests, which provide protection from predators and the elements, especially winter cold. Photo: Kristi Backe

Objective 3

Incorporate the effects of urban heat islands into PPM range expansion models.


Previous research has documented specific day and nighttime temperature thresholds for PPM feeding and survival, and these data have been used to predict the PPM range expansion but do not capture urban heat islands. These temperature data will be used to improve models of range expansion to account for the thermal refuge that cities may provide at otherwise inhospitable latitudes.

Kristi is still hard at work getting her fieldwork in France wrapped up, and will be taking a look at the data next. Read more about her study system here and here, and stay tuned for what her research reveals about PPM in cities.