Ecological role of the pine processionary

The processionary (Thaumetopoea pityocampa) is a nocturnal butterfly native to the Mediterranean basin whose nutritious plant for its caterpillars is pine, cedar and fir (we already talked about it in This other post which we also recommend to you).

The cycle of Pino processionary begins with the laying of eggs during the summer months, when butterflies mate and lay their eggs in the tops of pine trees. After a couple of months, at the beginning of autumn, they hatch, and the larvae develop a gregarious behavior as a defense mechanism. The caterpillars have abundant stinging hairs loaded with thaumatopine, a substance that irritates the mucous membranes. These hairs release the active ingredient through friction when handling caterpillars, or when ingested by animals.

Through secretory silk glands, they build the famous processional bags, where they take refuge during the night, when there is bad weather and when they have to shed their skin.

This does not prevent a wide range of birds from feeding on processionaries during the larval stage, always removing stinging hairs by rubbing them against the branches.

There is a false legend that bags can fall on people and cause alopecia, but the stinging effect of caterpillars does not have that property, and bags only fall when they decay, months after the caterpillars have left them.

These caterpillars are the main consumer of pine needles, transforming tons of fibrous and difficult to decompose leaves into debris rich in nitrogen and phosphorus that can be easily assimilated by plants in every hectare of pine trees. Without the processional system, the process of degradation of these waters is extremely slow in arid areas, where the lack of moisture hinders the formation of humus and soil development.

Conifers and processionary trees have co-evolved for millions of years, and although under particular circumstances caterpillars can completely defoliate trees, they sprout when the larvae burrow to metamorphose.

It is at this time that the caterpillars form rows that descend from the trees, the famous “processions”, maintaining their gregarious character to avoid being preyed upon.

Underground, they form a cocoon, causing metamorphosis, which culminates in the month of June with the emergence of the nocturnal moth that will restart the cycle with the laying of eggs.

These moths are an essential food source for a multitude of species such as bats, nightmares and small nocturnal birds of prey such as the little owl or the owl.

Given the possible harm to pets or young children that caterpillars may have when they appear in urban parks, it is advisable to avoid planting conifers in these places, instead using species not affected by the species such as false plantain, oak or poplars.

A mitigating measure to minimize the presence of the species is the placement of nest boxes for the reproduction of chickadees and blackheads, birds specialized in the consumption of caterpillars as well as the creation of bat shelters to promote the predation of adult moths.