The invisible world
Oak avenues are a typical feature of the Dutch landscape. However, since the oak processionary caterpillar found its way to the Netherlands, we look with new eyes to these endless rows of green itch-cannons. The local governments try to find ways to minimize the nuiscence of the oak processionary with ribbons, burners, vacuum cleaners and… Microbes.
The oak processionary caterpillar is the larval stage of the oak processionary moth, a small moth. This moth lays its eggs on a specific host plant: the oak tree. In spring, these egg-packages hatch. When the first leaves start to grow, the caterpillars march in long lines (procession, hence the name) to the top of the tree in a search for food. Starting early May, the fully grown caterpillars will form their recognizable nests, and start to develop long white hairs and defensive hairs. These defensive hairs are spears of microscopic scale. The caterpillars can actively shoot them towards predators to defend themselves. And that’s exactly the problem: the stinging hairs are causing irritation at the human skin as well, resulting in a severe itch.
Oak trees are virtually everywhere in the Netherlands: alongside roads, at swimming lakes, near schoolyards and in nature areas. And because of the long, hot summers of the past two years, the oak processionary decided that he liked our country. On top of that, not only living caterpillars cause problems. Also dead caterpillars have defensive hairs that can be spread by the wind. These hairs remain active for years. Time for action, so to say. Local governments have tried al sorts of methods to stop the advance of the oak processionary. And three opponents of microscopic size turned out to be very effective.
The nematode Steinernema feltiae normally lives underground, but is now used as a biological pesticide for the oak processionary. The nematodes are sprayed into caterpillar-inhabited oak trees. This happens when the oak processionary caterpillars are still young, before they develop the defensive hairs. Inside the Steinernema lives the bacteria Xenorhabdus nematophilus. These two organisms collaborate intensively, which results in advantages for both species. This is called symbiosis. The dynamic duo has an efficient method. The nematode invade the caterpillar either through the mouth or the anus. Once inside, the bacteria hop of their nematode-taxi and spread in the intestines of the caterpillar. The bacteria infect the digestive system and weaken the immune system, which eventually kills the caterpillar.
The bacteria Bacillus thuringiensis can tackle the task all by himself. This bacteria forms spores, which are basically survival capsules. These spores stick to the oak tree leaves and make them toxic for caterpillars, but not for other organisms. When a caterpillar eats a leave, the spores enter its digestive system. The toxins that are in the spores, are activated in this highly acidic environment. This shuts down the digestive system of the caterpillar. It stops eating and will starve eventually. This pesticide is used in early spring as well, before caterpillar develops the defensive hairs and when the trees doesn’t have many leaves. Because of this, the bacterial spores reach even the tip of the oak tree. The toxins stay active for days, what makes this method very effective.
Both kinds of micro-opponents are very effective for killing oak processionary caterpillars. However, not everybody is enthousiastic about these remedies. The reason for this, is that they not only work against the oak processionary. In other words, they are not species-specific. When these remedies are used, every caterpillar that lives in the tree will die. And that is bad for biodiversity. The oak tree is not only host to the oak processionary, but also to the oak-tree pug, the purple hairstreak and and the brindled green. Since these butterflies are innocent, it is unnecessary to kill them.
Why are the microbes Steinernema, X. nematophilus and B. thuringiensis still used against the oak processionary when it is bad for biodiversity? The answer to that is simple: we don’t have a better solution at hand. Burning or vacuuming the caterpillars nests is labour intensive, and therefor too expensive to do on a large scale. Great tits and starlings, on the other hand, quite fancy a juicy caterpillar. By hanging nest boxes, local governments hope that, in time, more and more natural predators of the oak processionary can do the job.
Maybe a definitive solution for the problem of the oak processionary can be found, again, in the microworld. There are some micro-organisms that have a more species-specific approach. Beauveria-fungi are already used in the Netherlands against insects such as the white fly and some bug species that eat crops. This fungus normally lives in soil, but in large numbers it is lethal to insects. The fluffy fungal threads just grow inside the insects body, eventually killing it. It might be that the microworld has such a fascinating mechanism against the oak processionary in reserve.
