We're talking processionary caterpillars again

Why do we only talk about the pine processionary in winter?

There are two species of processionary caterpillars in Europe:

• that of pine, Thaumetopoea pityocampa, which winters in silky nests at the end of the branches of pines, Himalayan cedars or more rarely firs.
• The caterpillar of the oak processionary moth Thaumetopea processionaewhich have a staggered cycle of about 6 months compared to the pine processionary moth.

The pine processionary

The adults fly and lay their eggs in the summer, but it is the caterpillars that are dangerous when they migrate in long processions to bury themselves to transform into butterflies. They have stinging hairs, called mirrors, which they project in case of danger. These hairs can persist in the environment and remain prickly for several years.
It is therefore necessary and even essential to prevent these larvae from reaching the ground.
There are several control methods:

• Treatment of young caterpillars with Bacillus Thuringia, sometimes difficult to perform on large trees, to be carried out in early September. Well managed, this type of fighting can show good efficiency after two or three years of practice (80% or more)
• Weeding which consists in cutting the tips of the branches that carry the nests during the winter. It is advisable to carry out this operation in cold and wet weather so that there are a maximum of caterpillars in the nests, but this operation is not easy or even dangerous for arborists. It is also an operation that damages trees.
• The descent traps (Ecopièges) which, if well positioned, recover almost 100% of the insects. These traps need to be set up on logs in winter before the big caterpillar migration begins. This is the most interesting type of trap for isolated trees or in small groves, especially in urban areas.
• Pheromone traps. These traps attract and capture the males of these butterflies who, in this way, will not be able to fertilize the females. This type of entrapment does not have an efficiency higher than 50%. Interesting in the woods to limit the importance of attacks but very insufficient in inhabited centres. Please note that the efficiency increases over the years. But they require the use and renewal of pheromone capsules at least once a season, which are currently still expensive, which makes this fight quite expensive in relation to its yield.

Eco-trap installed on a pine trunk
– The great tit

Oak Processionaries:

The adults, which also overwinter in the ground, emerge and fly in the spring. The caterpillars develop during the spring and summer and bury themselves in the fall. The caterpillars also take refuge in nests which are generally built along trunks and large branches, at fork level, during the summer. In terms of control, other than the dates that need to be adapted to the species, they are about the same as those of oak. It is also possible to destroy these nests in summer with a strong jet of water (fire hose) because the thick branches cannot be cut. Be careful, mirrors of this species can be stored for up to 5 or 7 years in the forest litter.

For our region it is the pine processionary that poses the greatest problems.
Furthermore, a ministerial decree requires the fight against this parasite.

Natural enemies of processionaries

Processionary caterpillars are insects that are defined as invasive, meaning that they have arrived too recently in a territory to have natural enemies capable of controlling their development. However, some organisms are already starting to parasitize or prey on these two species:

• Some species of entomophagous fungi that have very little action on caterpillars or butterflies in the wild.
• A soil bacterium, the Thuringian bacillus, Bacillus thuringiensis, but most Lepidoptera species have already developed defense mechanisms against this bacterium, and that’s good, because there wouldn’t be many butterflies in our regions anymore. For this reason, laboratories breed specific strains of this microorganism and in fact we are not dealing with the bacterium, which would be dangerous for biodiversity, but with a crystal that it secretes and which has a very low persistence, from about three to five days at depending on the intensity of the light.
• The nightjar, Caprimulgus europaeus, a very shy migratory bird that nests on the ground in the forest and hunts insects at dusk, when the processionaries fly. But this auxiliary operates only in particular areas, far from inhabited centres. It cannot be relied upon to control these parasites.
• Microbats or bats that feed on nocturnal insects can be good auxiliaries but are rarely sufficiently present in agglomerations.
• Athena’s Owl, Athene noctua, which is the only owl that eats insects here. But moths aren’t their favorite food. Also, owls and bats don’t mix well, the former being able to feed on the latter.
• The boobs. Tits are sometimes seen attacking and emptying caterpillar nests. They are voracious birds, but not numerous enough to control outbreaks of these moths.

great tit
– Jacques Ginet

In France, therefore, there are currently no sufficiently effective enemies to effectively control the proliferation of processionary caterpillars.

Ask all your questions during the Sunday morning gardening program on France bleu Isère from 9:00 to 10:00 by calling 04 76 46 45 45.

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