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The first oak processionary caterpillars hatch from their eggs!

Aart Huibers

Accountmanager Public Green & Export
Aart Huibers

At the oak processionary caterpillar testing station in Drenthe on 5 April the first oak processionary caterpillars hatched from their eggs. In the testing station in Noord-Brabant they hatch on 6 April. For the first time, they hatch earlier in the north than in the south of the Netherlands. As the oaks are already leafy, the growing conditions for the caterpillars are favourable.

 The oak processional testing stations in Dieverbrug (Drenthe) and Mill (Noord-Brabant) closely monitor the development of the oak processionary caterpillars. In the stations, eggs from various regions of the Netherlands are placed. As the development of the caterpillars in both stations runs parallel to the development of the caterpillars in nature, we can conclude that the first caterpillars are present in the oak trees throughout the country. The date of release was only a few days earlier than last year, despite the fact that March 2019 was much warmer than March 2018. However, at the beginning of April last year the maximum temperature also rose to well above 20 degrees, which caused an accelerated outcome.

Sufficient food

The development of the young caterpillars depends on the availability of food. If there are no oak leaves yet, they eat from the buds of the oak. They will only molt to the second larval stage after the first leaf development. Caterpillars in the second larval stage need chlorophyll for their development. Throughout the country, the first oaks already appear in foliage. This is significantly earlier than last year. Then the caterpillars had to wait much longer for the first oak leaf. The current food situation is therefore more favourable than a year ago.

 The expectation is that after a number of very hot days the temperature will drop again in the coming days. This means that the young caterpillars will remain in the first larval stage in many places. After 15 April the temperatures will rise again to above normal levels. We expect that a large part of the caterpillars will reach the second larval stage by then.

Burn hairs at the beginning of May

In recent years we have seen that during a warm period with available oak leaves, the young caterpillars only stayed in the second larval stage for one week and then moulted to the third larval stage. This is the stage where the burn hairs appear. If the temperature suddenly rises considerably, there is a good chance that the first caterpillars with hairs will be present at the beginning of May. Given the large number of butterflies caught in 2018, the good condition of the eggs and the current good food situation, there is a real chance that there will be many more nests than in 2018. However, everything depends on the weather conditions in the coming months.

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