LIFE Oak Processionary

Attracting parasitoid wasps and flies

Oak processionary caterpillar control by attracting parasitoid wasps and flies through adapted road verge management.

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Attracting parasitoid wasps and flies by means of adapted road verge management

Hairy caterpillars can be significantly influenced by natural enemies, such as parasitoid wasps and flies. This type of natural enemy can be particularly effective in more natural environments. In some situations, they can reduce oak processionary populations by up to 90%. They do this by laying their eggs inside the caterpillars; the larvae of the wasps and flies then eat the caterpillars from the inside out.

In this study, we wanted to verify whether natural verge management leads to an increase in parasitoids and, in turn, a reduction in the numbers of the oak processionary. 

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  • Sluipvlieg (Carcelia iliaca) bij eikenprocessierups
  • Carcelia iliaca
  • Carcelia iliaca

Comparing various types of road verges

To investigate which type of road verge is most attractive to parasitoid wasps and flies, the project compared various forms of road verges with one another, at trial locations in Flanders and the Netherlands. In concrete terms, this means that each type of verge will be assessed to understand how many and which species of parasitoid wasps and flies are found in the nests of the oak processionary and the corresponding impact on the quantities of caterpillars.

  • Type 1. Frequent gemaaid grasachtig - Grassy roadsides without flowers or blackberries
  • Type 2. Bloemenrijk zonder braam - Flowery without wood or blackberries
  • Type 3. Bloemenrijk met braam - flowery with wood or blackberries
  • Type 4. Grasachtig 2x gemaaid - Rough roadsides - grassy roadsides with nettles
  • Type 5. Gefaseerd maaien - Phased mowing
  • Type 6. Ingezaaide mix eikenprocessierups - OPC flower seed mixture min

The project partners at the trial locations ensured that the road verges with infected oaks are managed in various ways:

  • Grassy verges, with few or no herbaceous plants
  • Naturally floral verges, with delayed grass-cutting (1x per year), without woody vegetation or brambles
  • Naturally floral verges, with delayed grass-cutting (1x per year), with woody vegetation or brambles
  • Rough verges (stinging nettles, etc.); cut back 2x per year
  • Phased mowing (only some of the vegetation is cut)
  • Verges sown with oak processionary flower seed mix

Monitoring the impact of road verge management

Nectar index

In the first instance, we examined the effect of verge management on vegetation. The habitat quality for natural enemies (and pollinators) was evaluated by establishing a nectar index. This index is based on the number of flowering plants. The measurements for this were taken annually, in June and July.

Parasite levels in the oak processionary nests

We compared the degree of parasitic activity in the oak processionary between the various methods of verge management. To this end, three oak processionary nests were gathered each year during the summer months. All parasitoids were then collected from the nests, counted and the species identified.  

  • 3 ecologische technieken - Sluipwesp uit EPR-nest
  • Sicus ferrugineus in EPR-nest

Population size – oak processionary

In order to establish the impact of verge management and the number of parasitoid wasps and flies on the oak processionary, we also measured the number and size of the nests each year.

What did we learn from the roadside survey? 

Do parasites really play a role in controlling caterpillar populations? 

  • Besides the caterpillars, two species of parasitic flies and two species of parasitic wasps were found in the caterpillar nests. Parasitic flies are much more numerous – an average of 60 flies compared to two wasps per nest. 
  • The average parasitisation rate across all roadsides is around 70%. Therefore, without these parasites, three times as many oak processionary moths would be flying around and laying new eggs for the following season. 

What is the parasitisation rate on the different types of roadside verges? 

  • We see a high level of parasitisation, particularly in flower-rich, diverse roadsides with lots of composites such as tansy and yarrow, and with a high Nectar Index. In the most natural roadsides and under high pest pressure, this level even rises to 90%. This leads to significantly less oak processionary nuisance in and around natural roadsides. 
  • This effect is strongest in verges in open landscapes with abundant grasslands and diminishes with increasing forest density in the surroundings. 
  • Regular mowing management, including clearing of the grass clippings, contributes to the parasitisation. 

Further reading