Araschnia levana. Photo by Mia Valtonen.

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Anne Reichgelt 2007 : Density-Dependent Aggregation of Hyperparasitoid Mesochorus stigmaticus

Under supervision of Saskya van Nouhuys
August 23 2007

The taxonomy and biology of parasitoids is little known and likewise the conservation of parasitoids is undeveloped. The aim of this MSc thesis is to provide more knowledge on the behaviour of parasitoids and to raise awareness for the conservation of parasitoids. For that reason a narrative of both the conservation of parasitoids and past parasitoid behavioural research is included in this thesis in addition to a behavioural experiment. On the Åland Islands in Finland for more than a decade research has been carried out on the butterfly Melitaea cinxia and the insect community it is part of. The primary parasitoid Hyposoter horticola and the hyperparasitoid Mesochorus stigmaticus are parasitizing on M. cinxia. H. horticola parasitizes approximately 1/3 of a larval group and does this on all larval groups on the Åland Islands therefore creating a constant rate of mortality in both time and space. Why would H. horticola parasitize only 1/3 of a larval group? M. stigmaticus hyperparasitizes 25-60% of all H. horticola. Could it be that the hyperparasitoid influences the ovipositing behaviour of the primary parasitoid? In the experiment carried out for this MSc thesis the hypothesis was the following: M. stigmaticus reacts to the distribution of H. horticola in a density-dependent aggregative matter. In order to test this hypothesis the behaviour of M. stigmaticus in a lab experiment and in a large outdoor cage experiment was observed. In addition a field experiment was carried out to see if the rate of hyperparasitism by M. stigmaticus in different patches, i.e. small & isolated, small & connected and large & connected, is different for different rates of parasitism by H. horticola. The rate of parasitism by H. horticola was manipulated into 0%, 10%, 20% and 30%. In the lab and the outdoor cage experiment the giving up time and the number of larvae poked were measured. As a result, the data shows that the wasps do tend to stay longer on plants with larval groups that contained more larvae with H. horticola inside them. But due to circumstances the amount of data to make this statistically significant is not enough. An interesting observation was that the wasps seemed to learn from past experience as the wasps active in the lab were also active in the cage experiment. I recon that the field experiment will reveal that M. stigmaticus is a good indicator of habitat fragmentation. Hence more research into the value of parasitoids for nature conservation purposes is recommended.