2024 - Looking back on a difficult research year - LIFE Oak Processionary Project

As we leave another year of research and experimentation behind us it’s time to take a look back. And we do so with mixed feelings. It was certainly not a top year for research into the oak processionary caterpillar. The species is clearly in a deep trough, at a number of project locations they have even completely disappeared. The predators that specialize in the caterpillar – such as certain parasitic flies, parasitic wasps and the forest caterpillar hunter – were also difficult to find in 2024. We do not yet have any figures on health complaints, but we assume that these will also be much lower than in the peak years of 2020 to 2022. A very good year for public health thus.

It was a difficult year for our experiments, but nevertheless we made some important steps forward in 2024.

Tits experiment: No news from the tits front

As expected, providing nest boxes helps attract great and blue tits, the natural enemies of our oak processionary caterpillar. What we cannot yet demonstrate is that the tits actually feed processionary caterpillars to their young. We wanted to demonstrate this in 2024 by using nest cameras to closely observe the feeding behaviour of tit parents.

However, the lack of oak processionary caterpillars in the project area is putting a spanner in the works. The planned experiments in the tits research – such as camera observations – have been put on hold for the time being. It remains to be seen whether this can be restarted in 2025.

Parasitoid flies and wasps: Waiting for the results

Parasitoid flies and wasps are valuable allies in controlling oak processionary moth plagues. To learn more about the impact of these parasites, dozens of caterpillar nests were collected on the site in 2023 and ‘bred’ in boxes in the lab in Limburg. To ensure that all parasites present had the opportunity to develop, the nests were left untouched for more than a year. The hatched moths and parasites were collected in jars, identified and counted..

In November we carefully opened the boxes and counted the remaining caterpillars, butterflies, parasitoid flies and wasps and other guests. The results of this entire operation are still being analysed, but it is already clear that, even with a very low population pressure of the oak processionary caterpillar, parasitoids still play an important role in controlling caterpillar plagues.

However, it is not clear how we can attract more parasitoid flies and wasps. Can this be done by providing the right type of food – nectar and/or pollen?

To answer that question, in 2024 we went in the field to look for parasitoid flies on and around the processionary moth nests. The limited presence of oak processionary moths certainly did not make that easy, but in the end we were able to catch a few dozen parasitoids.

The flies are currently being examined by experts from the Botanic Garden of Meise. Using DNA sequencing techniques, they want to find out which pollen the flies carry on their mouthparts, and therefore which plant species they have visited. We can expect the results in early 2025. So we’ll just have to exercise a little patience.

More about parasitoid fly and wasp research can be found here.

Common Ivy experiment: Impact of ivy on other butterfly species limited?

Contrary to the other research themes, we were able to continue with the ivy experiment without oak processionary caterpillars. We had already noticed that oaks covered with common ivy have considerably fewer nests of the caterpillar than trees without. What we did not know until now was whether other butterfly species were affected by the climbing plant.

To investigate this, we organised a large field experiment this year, in collaboration with internship students from the University of Antwerp. At eight different locations in the Antwerp Kempen, from Schilde to Geel, we selected two oaks with and two oaks without ivy on the trunk. Using pizza boxes, we collected caterpillar droppings under each tree for 24 hours. The test was carried out twice, in May and in July. In total, we took 256 samples.

The droppings were sorted in the lab and weighed. The results still need to be analyzed further, but a first look at the results indicates that, if there is an impact of ivy on other caterpillar species, that impact is rather limited.

If this impression is confirmed, it means that we can possibly add common ivy to our list of allies in the management of processionary moth plagues.

The common ivy experiment is an offspring of the experiment with parasitoid flies and wasps. More about this research can be found here.

Forest Caterpillar Hunter experiment: Major breakthrough in 2024

We have already reported extensively about the forest caterpillar hunter this year, as recently as September. For the first time, we have succeeded in rearing in the lab a new generation of this iconic species to adult beetles, a first-ever for Western Europe.

We can add that at this moment, the entire beetle population, both the original Turkish specimens and the home-bred offspring, enjoy a well-deserved winter sleep. We expect to see them again in the spring, and hope for another successful breeding year!