The bark beetle
The bark beetle is a small xylophagous insectthat’s to say feeding on wood, which belongs to the order Coleoptera like the ladybug or the beetle. Within this order of beetles, bark beetles are a subfamily of the family Curculionidae, a family of insect pests. With a size between 2 and 7 mm, the beetle has a cylindrical body of black or brown color, and covered with elytra which protect its wings. And of course, impossible to talk about an insect without talking about its larvae. Those of bark beetles are whitish, soft and arc-shaped.
Until then, the bark beetle was considered a useful insect insofar as its primary role was to promote forest regeneration. feeding on dead wood whose decomposition it accelerated. But here it is, the drought and the storms have been there! Faced with these, trees that suffer from dehydration or that have been mutilated by the wind emit stress hormones that attract bark beetles and other parasites.
The bark beetles therefore invade these trees when they are not yet dead. Another explanation why bark beetles now attack standing trees is the too large increase in their population. The dead wood is no longer enough to feed them all! Thus, bark beetles are increasingly feared, especially in forestry. For example, in the Landes, these insects causesignificant damage in pine forests. But this is not the only region affected, because this invasion now extends from the northern half of France to the Auvergne Rhône-Alpes region.
Different species
We have seen that bark beetles are a subfamily and this includes a large number of insect species that will attack different tree species. Thus, it can be coniferous or deciduous. In France, the most common species are:
- the stenographeror Ips sexdentatus, which attacks pines;
- I’Ips scolytus which attacks elms and is a vector of graphiosis, a disease which quickly kills these trees;
- I’Ips typographus which feeds on spruces;
- the Leperesinus varius which attacks the ash trees;
- the mali scolytus and the Scolytus rugulosus that destroy fruit trees.
But there are many others that attack very specific tree species, such as the almond bark beetle, the olive bark beetle, also called Neiroun, etc.
Life cycle of bark beetles
The bark beetle life cycle may have some differences, but overall it goes through the following stages:
- When the outside temperature reaches 18°C, swarming takes place.
- The adult bark beetles get under the bark of the tree either by drilling a hole or by infiltrating through a crevice to feed on the soft wood.
- Males and females then dig a nuptial chamber in which mating will take place.
- The female then undertakes to dig an egg-laying gallery in which she will be able to deposit her eggs. A female can lay up to 80 eggs.
- About 2 weeks later, these eggs give birth to very voracious larvae which, in turn, will undertake to drill other galleries perpendicular to the egg-laying gallery to feed. In doing so, they will cut off the upward flow of sap. These larvae will go through 4 stages before reaching the pupa stage, then the stage of emergence of the new adult.
- These young adults then leave the tree through exit holes that they create in the bark.
It takes approximately 6 weeks for the emergence of a new generation, but this cycle is reduced to 4 weeks when temperatures are higher. In addition, the reproduction of bark beetles is possible as soon as the temperature exceeds 15°C. This explains the reasons for the increase in their population.
How can bark beetles destroy forests?
Several elements favor the massive destruction of trees by bark beetles that we are currently witnessing:
- Normally, trees have the ability to defend themselves against this type of pest. Indeed, they release resin to which the insects adhere. However, due to the weather conditions, these proliferate so massively and rapidly that the trees can no longer fight. And for good reason, trees do not have the ability to produce as much resin.
- We saw that the females dig galleries under the bark, specifically in the cambium, to deposit their eggs. This act in itself condemns the attacked tree.
- Indeed, the larvae which will be born from these eggs will in turn dig galleries which will interrupt sap flow lines.
- Another phenomenon to consider is the climatological context : episodes of drought and storms weaken the trees which become more susceptible to attack and which emit stress hormones which attract bark beetles.
- The holes and galleries dug by bark beetles are so many doors open to fungi and parasites. The tree can therefore become ill.
Recognizing a bark beetle infestation
Fairly easy to recognize, bark beetle infestation is characterized by the following symptoms:
- The attacked tree has a crown that is first red, then gray.
- Small heaps of sawdust on the bark or at the foot of the tree due to drilling by bark beetles.
- In the case of thorns, drying out, then fall of the thorns.
- Holes present on the trunk, but also the thicker branches.
- Under the bark, a whole network of galleries is clearly visible. You can then see lines going in all directions.
Bark beetle control solution
No truly effective treatment exists. The only way to fight is prevention:
- As soon as signs of infection are observed on a tree, it is recommended to cut it down. This will prevent the spread to other trees. We then speak of sanitary cutting.
- Some bet on a diverse forest with different species to control the presence of bark beetles. This type of forest would be adapted to resist climate change.
- There was a time when we used traps filled with attractants to fight against the presence of bark beetles, but faced with their massive proliferation, they are no longer sufficient. However, it can be an additional measure that can be put in place or a measure to take if you only have a few trees.
- In summer, avoid water stress by watering your trees regularly and amending the soil around them to keep them healthy.
- Do not leave dead wood near your treesbecause do not forget that it is the favorite food of these pests