THE INFLUENCE OF PYRROLIZIDINE ALKALOIDS OF SENECIO-JACOBAEA ON TYRIA-JACOBAEAE, BRACHYCAUDUS-CARDII AND HAPLOTHRIPS-SENECIONIS

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, indicating 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.