1Ecologie Animale et Entomologie, Institut de Zoologie, Université de Neuchâtel, rue Emile Argand 11, 2007 Neuchâtel, Switzerland.
2Institut für Pharmazeutische Biologie der Technischen Universität Braunschweig, 38106 Braunschweig, Germany.
Several leaf beetles in the genus Oreina sequester pyrrolizidine alkaloid N-oxides (PAs) from their host plants (e.g. Adenostyles spp., Asteraceae). We studied the efficiency of these compounds as defense against a harvestman (Mitopus morio, Phalangidae) which is common in habitats where Oreina spp. occur. M. morio was observed to feed on larvae of O. speciosissima and O. cacaliae both in the field and in a laboratory experiment. Two populations of M. morio were compared with respect to the deterrent effect of PAs and the efficiency of detoxification. One of the populations comes from a site dominated by Adenostyles alliariae. These individuals are likely to have come into contact with prey containing PAs. The other population comes from a site where Adenostyles spp. are absent and is therefore likely to have little experience of PAs. For the test of deterrence, the individuals were offered artificial food (minced meat) containing the PA senecionine N-oxide in different concentrations. The efficiency of PA detoxification was studied in a tracer feeding experiment. The two populations differed in behavior. Individuals from the population lacking Adenostyles plants were more deterred by the alkaloid compared to those from the other site. Both populations were able to detoxify PAs. They had efficient ways to eliminate the alkaloid and did not sequester these compounds for their own defense. We suggest that the efficiency of PAs as defense for sequestering herbivores may differ between populations.