LIFE Oak Processionary

Limiting oak processionary infestations with common ivy

Reducing the nuisance of oak processionary with ivy in the oaks

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Common ivy – an unexpected sidestep

Our most common oak species, the pedunculate oak (Quercus robur), is a pocket-sized ecosystem. An estimated 450 animal species can live in the tree, along with a host of mushrooms and other fungi.

For dozens of butterfly species, such as the common hairstreak, its rare cousin the brown hairstreak, and of course the oak processionary, the pedunculate oak is the sole host plant. This means their caterpillars can only survive if they are fed oak leaves. The caterpillars betray their presence by chewed leaves and hundreds of wads of droppings under the tree.

From roadside surveys conducted in recent years, we know that oaks covered with common ivy have significantly fewer oak processionary nests than trees without. The reason for this is not yet entirely clear. The vine may hinder the caterpillars’ movements, or the dense ivy canopy may create a microclimate less suitable for the caterpillars. What we also don’t know yet is whether other butterfly species are affected by the climbing plant.

The ivy experiment

That’s why we set up a new large-scale field trial in 2024 to determine the impact of ivy on caterpillar populations in pedunculate oaks. Caterpillar droppings were collected from trees with and without ivy at ten locations in the Antwerp Kempen region. The droppings were collected in cardboard boxes—the familiar pizza boxes—under the tree, then weighed and compared. We left the boxes for at least 24 hours, and repeated this several times throughout the caterpillar season. In this way, we hoped to determine whether there was a difference between the number of caterpillars in oaks with and without the vine. The research was conducted in collaboration with the University of Antwerp.

So, if you happened to spot a few pizza boxes under a tree during a walk in the Antwerp Kempen region in 2024, know that science was at work!

What did we learn from the ivy research?

What does ivy do to the oak processionary caterpillar? 

  • In trees with common ivy, we find on average 50% fewer nests of oak processionary caterpillars than in trees without.
  • Moreover, the nests are much smaller and therefore contain significantly fewer caterpillars.
  • So, we can conclude that the presence of common ivy on the trunks of oak trees reduces the oak processionary population in the tree.
  • The reason is not clear. Perhaps the plant forms a physical barrier to the caterpillar’s movements, or the microclimate beneath the ivy’s canopy is less suitable for creating nests.

Does the presence of common ivy impact other butterfly species?

  • This doesn’t seem to be the case—there was no significant difference in caterpillar droppings between trees with and without ivy. We can conclude that ivy on oak trunks has no measurable impact on other butterfly species.