COMBINED EFFECTS OF 2 ARTHROPOD HERBIVORES AND WATER-STRESS ON GROWTHOF HYPERICUM SPECIES

The interaction between physiological stress and arthropod herbivory w as studied using two perennial species of Hypericum, Seedlings of H. p erforatum, a herb introduced to Australia and weedy in places, and H. gramineum, an indigenous species, were subjected to water stress and/o r herbivory by a mite, Aculus hyperici and an aphid, Aphis chloris. Bo th arthropods have recently been released in Australia for biological control of H. perforatum. Individually, stresses reduced measures of p lant growth. Combinations of the three stresses decreased plant growth by slightly more than the product of their separate effects, suggesti ng that there is a weak positive interaction exacerbating the damage c aused by each stress. Seedlings of the target weed and the 'non-target ' indigenous species were equally affected by the arthropods. The impl ications for weed biological control are discussed.