Jan Kolsters & Pieter Wouters (2025) Supply of wasp nests and predation by Honey Buzzard Pernis apivorus in a Dutch brook valley in 2020-22. LIMOSA 98 (2): 49-58.

Wasp brood is an important food source for Honey Buzzards, but little is known about the availability of harvestable wasp nests. In this study we mapped wasp nests in a 320 ha brook valley in the province of Noord- Brabant in the southern part of The Netherlands. The area consists of small scale meadows, hedgerows, broadleaved woodlots and some plots of extensive farmland. We walked the area at a notably slow speed of 1.5 km/h keeping to a dense grid (Fig. 1), carefully watching a 7 m radius for low flying insects. Wasp nests that were found in this way were marked and checked every two weeks for activity and predation. We found 106 wasp nests in 2020, 1 in 2021 and 144 nests in 2022. Common Wasp Vespula vulgaris was the most numerous species in all years (Tab. 1). Most nests were found in the areas with small parcels. In the woodlots nests were often close to the edge, exposed to the sun. The study area was (a small) part of the foraging area of at least two pairs of Honey Buzzard, an occupied nest was found in 2020 and 2022. In 2020- 22 respectively 8, 0 and 18 wasp nests were predated by Honey Buzzard. In autumn, after wasp activity had stopped, 110 wasp nests were excavated to count the number of combs and number of cells (as a measure for size of the nest). Excavated nests consisted of, on average, 8.7 combs, with no significant difference between Common Wasp and German Wasp Vespula germanica (Fig. 3). The number of cells varied strongly between nests. Number of (the smaller) worker cells always outnumbered (the larger) queen/drone cells. The proportion of queen/drone cells increased with nest size (Fig. 5). For each wasp nest we also estimated how accessible the nest was for Honey Buzzards by looking at burying dept, physical obstacles like tree roots, blackberry bushes, and branches, and the presence of roads and buildings (disturbing factors). Most nests close to the surface, with a burying dept of 5-20 cm, but in some the distance between entrance and upper comb measured more than a meter. On the basis of depth, physical obstructions and chance for disturbance, we estimated that 45% of the nests was easy, 33% was possible and 22% was impossible to access for a Honey Buzzard. Of the nests harvested by Honey Buzzard, we regarded 76% easy and 26% possible to access. For 24 nests, wasp activity was quantified by counting the number of arriving and leaving wasps during one minute. On average about 50 wasp movements were counted per minute. Wasp activity was positively correlated with colony size (Fig. 7) (as determined in autumn when nests were excavated). Traffic rates for the Median Wasp Dolichovespula media and European hornet Vespa crabro, were lower than for Vespula species, often not more than 1-6 wasps per minute.

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