LIFE Oak Processionary

Breading and releasing the forest caterpillar hunter

Oak processionary caterpillar control by breeding and releasing the forest caterpillar hunter (Calosoma sycophanta).

The forest caterpillar hunter

The forest caterpillar hunter is a ground beetle which is indigenous to Europe. The species is currently under serious threat in Belgium and died out completely in the Netherlands some decades ago. Because the beetle loves to eat a range of primarily hairy caterpillars, which tend to be avoided by other animals, the forest caterpillar hunter is a welcome species in areas where certain types of moth/butterfly are multiplying out of control.

A beetle pair consumes around 200 caterpillars per season. A larva will eat up to 40 caterpillars and 13 pupae per season. The beetle has therefore been employed as a biological control agent by means of introduction into areas where they do not usually occur (e.g. USA and Canada). The beetle has also been released in areas where the species is already found but where their quantities are too small to tackle the caterpillar plague.

We wanted to test this last option on a small scale in Belgium and, if successful, potentially expand it to the Netherlands. In the long term, we planned to collaborate with pest control companies on this topic.

Calosoma sycophanta (CS) expert group

In order to effectively manage and monitor the breeding and release of the Calosoma Sycophanta or forest caterpillar hunter in Belgium, a CS expert group was established, comprising specialists and entomologists from Belgian and foreign research institutions. They met regularly and monitored the project from breeding to releasing the species.

Habitat model – forest caterpillar hunter

If you wish to release or reintroduce a species, you are advised to do so at a location (habitat) that is suitable for this species. That is why we first developed a habitat model which helped us identify, find and select suitable locations for the forest caterpillar hunter. The habitat model is a GIS exercise, which makes use of the most recent habitat map of Flanders. On the basis of the habitat requirements that we know for beetle populations from neighbouring countries (Germany/France), the Research Institute for Nature and Forest (INBO) drew up a habitat model for the forest caterpillar hunter which can also be verified in the field. This model can be used to select the most promising locations for reintroduction.

Habitat map for the forest caterpillar hunter in Flanders

Breeding the forest caterpillar hunter

Various institutions in Turkey have developed expertise in breeding the forest caterpillar hunter. There, they use this beetle to (biologically) tackle the pine processionary; a caterpillar that is closely related to the oak processionary. Project partners from Limburg, Antwerp and the INBO went for a week to Turkey to learn about breeding the forest caterpillar hunter and then start up breeding centres in Limburg and Antwerp. The aim was to breed at least 10,000 beetles, 6,000 of which would be used for field experiments.

Larve grote poppenrover - Calosoma sycophanta larva — Larva forest caterpillar hunter – *Calosoma sycophanta* larva

Calosoma sycophanta pupa — Forest caterpillar hunter – *Calosoma sycophanta* – pupa

Releasing the forest caterpillar hunter

In the end, at 2 locations in the province of Antwerp and 2 locations in the province of Limburg, which would be selected on the basis of the habitat model, we wanted to reintroduce the forest caterpillar hunter to the soil in the spring, during the last larval stage or as a pupa. This would, of course, take place near oak trees that are colonized by the oak processionary. After a few weeks, the beetles would emerge and, hopefully, eat the hairy processionary caterpillars. We wanted to keep a close eye on exactly when the beetles crawl out of the ground and climb into the trees.

Tracking the forest caterpillar hunter in the field

At each location, we wanted to equip 10 beetles with a lightweight transmitter (0.2g) and then release them into suitable environments, with a few weeks between each release. We would do this in order to monitor the comings and goings of the beetles and learn more about their preferred habitat. Light traps would also be used to catch the beetles and monitor their populations. The work with transmitters would be conducted with the University of Hasselt.

The transmitter deployment was ultimately carried out on a small scale within the breeding greenhouses of the lab in Genk and went as planned.

Demonstratie eikenprocessierups — Demonstration event oak processionary

Demonstration moments on site

During various demonstrations, various stakeholders were invited to the breeding labs to present the results of the breeding experiment.

The ultimate goal was to convince pest control organizations and government agencies to incorporate the large pupae predator into their control plans.

Unfortunately, this project didn’t achieve that.

What did we learn from 3 years of breeding forest caterpillar hunters?

Due to a delay at the start of the experiment because of the Coronavirus-restrictions, and again because of a false start of the breeding experiment with an unsuitable beetle population, we could not achieve all our objectives during the project lifetime. However, we did learn a lot about this beetle:

After three years of intensive beetle breeding, we know that beetles can be induced to lay eggs after being fed oak processionary, and the larvae can be reared with more diverse food sources, after which they can successfully pupate into adult beetles.
With some practical improvements, we are convinced our approach can also be applied on a larger scale.
However, before we can consider commercialization, there are still several significant hurdles to overcome:
The beetles lay eggs only once a year, and must have been fed on oak processionary to produce fertile eggs;
The newly hatched larvae are highly cannibalistic and must therefore be raised individually.
A key point to consider is that local processionary pest pressure fluctuates from year to year, making local small-scale commercialization challenging. Breeding the forest caterpillar hunter is a long-term, continuous process, while local demand for oak processionary management will fluctuate.

The release of the beetle was not possible. Besides the delay in the breeding experiment, the unexpected resurgence of the beetle in Flanders and the Netherlands – with almost 20 sightings in 2023 – forced us to abandon our plans to introduce these Turkish beetles here.