The Influence of Pyrrolizidine Alkaloids of Senecio Jacobaea On Tyria Jacobaeae, Brachycaudus Cardii and Haplothrips Senecionis

THE INFLUENCE OF PYRROLIZIDINE ALKALOIDS OF SENECIO JACOBAEA ON TYRIA JACOBAEAE, BRACHYCAUDUS CARDII AND HAPLOTHRIPS SENECIONIS* by KLAAS VRIELING, LEO L. SOLDAAT and WOUTER SMIT (Department of Population Biology, Research Group Ecology of Plants and Herbivores, University of Leiden, P. O. Box 9516, 2300 RA Leiden, The Netherlands) SUMMARY The effect of natural differences in pyrrolizidine alkaloid (Pa) concentration in Senecio jacobaea on attack of performance of three herbivores was studied. Differences in larval performance and development of Tyria jacobaeae were not correlated with total Pa- concentration or the concentration of the different Pa's. The population growth rate of the aphid Brachycaudus cardii was negatively correlated with total Pa-concentration of the S. jacobaea plants on which they were reared. In our study area S. jacobaea plants with and without Haplothrips senecionis did not differ in total Pa-concentration, in- dicating that food plant choice of the thrips was not affected by Pa-concentration. Thus Pa's in S. jacobaea act as a chemical defence against the polyphagous aphid B. cardii, whereas the monophagous T. jacobaeae and H. senecionis are not influenced by Pa's in their diet. These results are in agreement with the predictions of the plant apparency theory which states that specialist herbivores (monophagous) are adapted to a qualitative chemical defence (Pa's) and generalists (polyphagous) herbivores are not. KEY WORDS: Senecio jacobaea, Tyria jacobaeae, Brachycaudus cardii, Haplothrips senecionis, pyrrolizidine alkaloids, chemical defence. INTRODUCTION Although Senecio jacobaea L. plants are known to possess pyrrolizidine alkaloids (Pa's), they are frequently defoliated by the larvae of Tyria jacobaea L. (DEMPSTER & LAKHANI, 1979; VAN DER MEIJDEN, 1979; MYERS, 1980; CRAWLEY & GILLMAN, 1989) and they are attacked by at least fifty other (insect) herbivores (HARPER & WooD, 1957). Pa's are reported to be not only toxic to vertebrates (BULL et al., 1968; MATTOCKS, 1986) but also to insects (BERNAYS & CHAPMAN, 1977; BENTLEY et al., 1984; DREYER et al., 1985; BOPPRE, 1990; VRIELING et al., 1991). We were interested to know whether the Pa's of S. jacobaea function as a defence against its numerous herbivores. S. jacobaea contains at least seven different Pa's (APLIN et al., 1968; SEGALL, 1978; PIETERS et al., 1989), which can be present as free bases