LIFE Oak Processionary

Attracting tits as predators

Oak processionary caterpillars as food for tits

Oak processionary caterpillars as food for tits

Great tits eat the oak processionary at all larval stages, as well as the pupa and moths. So, in effect, they eat the oak processionary at every stage of development. During observations of the blue tit, it was only demonstrated that they eat the 1st and 2nd larval stage.

The hairy caterpillars are ‘peeled’ by the tits and then shared out between 8 to 14 chicks. In two weeks, a young tit can eat around 800 caterpillars. Fruit growers realised the support that tits could offer long ago; they hang nesting boxes in order to reduce the damage caused by caterpillars by a quarter.

Small-scale studies have shown that attracting insect-eating birds by means of nesting boxes can help to drastically reduce the population of the oak processionary. However, the percentage of reduction in the number of oak processionary caterpillars due to this predation was not yet known.

Great tit with oak processionary caterpillar

3 ecologische technieken - Nestkast voor mezen - Nestbox for tits — Nesting box for tits

In this LIFE project, we tested and measured the precise impact of installing nesting boxes on the population of the oak processionary (quantities and size of nests) on a large scale. This allowed us to provide statistical evidence of the method’s effectiveness. A robust experimental setup, comparing infected trees with nest boxes to trees without them, allowed us to clearly map the impact of the measures.

By taking stakeholders to the test sites, we showed them the effect of predation by tits on the number and size of caterpillar nests on the spot, and hopefully convinced them to use similar methods themselves.

Experimental set-up

In the provinces of Antwerp (Belgium), Limburg (Belgium), Gelderland (Netherlands), and North Brabant (Netherlands), 360 nest boxes were installed in an experimental trial. To determine the effect on the number and size of oak processionary moth nests, trial sites with nest boxes were compared with similar sites without nest boxes.

Twelve similar trial locations were selected per province, each comprising a row of 15 large, colonized oaks (at least two processionary caterpillar nests per oak). Six of these trial locations per province served as a control and did not have any nesting boxes installed while, at the other six locations, a nesting box was installed in each of the 15 oaks.

3 ecologische technieken - Koolmees eikenprocessierups boom - Great tit oak processionary tree — Great tit at a nest of oak processionary caterpillars

Measurements

In the period April-May, we counted the number of inhabited nesting boxes and the number of juvenile tits. In the summer months, both at the control and testing locations, we evaluated the size and number of oak processionary nests. This took place first prior to the experiment, to provide a baseline measurement.

This allowed us to investigate the effect of installing nest boxes on the number of oak processionary caterpillars and thus how effective our method is.

What did we learn from the bird research?

What was the effect of providing nest boxes on the birds?

As expected, we saw more birds at sites with nest boxes than at the control sites – about twice as many.
Five bird species visited our nest boxes, the majority great tits and blue tits.
On average, 50% of the nest boxes were used – less than expected, although occupancy reached 80% at some sites. Probably the nest boxes were placed too close together for the liking of these territorial birds.
Eggs were found in about half of the nest boxes, and young birds in about a third of the boxes.

Has the increase in bird numbers influenced the oak processionary population?

Over the course of the study, the number of caterpillar nests and the average size of the nests gradually decreased, both at sites with and without nest boxes. Natural population cycles and changing weather conditions play a significant role in this.
Regarding the number of caterpillar nests, we see no clear difference between the test and control sites.
However, there is a clear impact on nest size: caterpillar nests decreased in size year after year everywhere, but nest became smaller more quickly at the sites with nest boxes – approximately 30% faster per year.
Large nests contain exponentially more caterpillars than smaller ones, so we can conclude that additional predation by tits due to the installation of nest boxes indeed reduced the processionary moth pest pressure by approximately 70% over four years.

Read further:

How to attract Tits (Paridae) as a predator for oak processionary caterpillars
Increasing predation by birds – Final analysis (EN)
All outputs and reports about the bird research: Outputs & Reports – Tits as Predators.