Why you may see black bags on trees across forests and towns in France

Wandering around French towns and forests you may have noticed several trees covered in large plastic bags. 

The number of trees cladded in the bags increases greatly during the spring and summer season, leading to queries about their presence.

These sacks, which look similar to bin bags, come in colours including black, green, and even transparent with the hue of the bag giving a vital clue to the reason it is there. 

Below, we look at the meaning behind each of the bags. 

Avoid trees with black bags

If you see a black or opaque bag on a tree, stay well away. 

These are in fact eco-traps for processionary caterpillars, a native pest in France that has been dubbed a ‘harmful species’ since 2022. 

These ‘hairy’ caterpillars can cause irritation to humans and can be potentially fatal if they come into contact with dogs, come in the form of two species, of the ‘oak’ and ‘pine’ varieties.

Between them, they are seen from January - August, with peaks in March and June.

The caterpillars are in ‘nests’ that look like balls of cotton wall high up in trees, before climbing down the trunk to ground level where they present the most danger to humans and pets

These black bags or ‘tree bands’ prevent caterpillars from crawling onto the ground, instead catching them in a layer of soil filling up the bag.

Alternative traps include a circular plastic band that goes around the tree’s trunk that collects descending caterpillars and siphons them into a plastic bag.

Caterpillar-catching traps are usually higher up on the tree and fully encircle the trunk, and are found in both towns and rural areas to prevent their spread.

Even if the tree seems free of caterpillars or it is outside the usual season for the species, only professionals should touch these bags. 

France’s Office national des forêts says the ‘hairs’ from caterpillars can cause injury to humans and pets for up to three years after being dislodged from the caterpillars, meaning those present in the soil can still cause harm.

The next most common type of bags are green or brown. These are often seen around trees found in town centres, and are placed further down near the base of the tree. 

The bags are used to help younger trees that may suffer during hot periods, as their roots have not had time to expand (or are restricted by concrete, etc).

Alternatively, summer drought restrictions which limit free watering of city trees may be in place.

The bags contain around 50 litres of water which is slowly released across five to nine hours to water a tree. 

Finally, transparent plastic bags may also be present. Again most common in town centres, these bags can have several purposes, including trapping other insects. 

In parts of south-west France, bags of this kind wrapped around pine trees may have resin-trapping mechanisms.