Combined effects of two arthropod herbivores and water stress on growth of Hypericum species.

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

Is the increased vigour of invasive weeds explained by a trade-off between growth and herbivore resistance?

Blossey and Nötzold (1995) recently hypothesised that the increased vigour of certain invasive plant species has been at the expense of defences against natural enemies. A prediction of their evolution of increased competitive ability (EICA) hypothesis is that invasive genotypes are relatively poorly defended. We tested this prediction with herbivore bioassays and with direct quantification of plant secondary metabolites comparing non-indigenous genotypes of Lythrum salicaria L. (purple loosestrife) with indigenous forms. The herbivore bioassays revealed no significant intra-specific variation in herbivore resistance between indigenous and non-indigenous hosts. The phenolic content of L. salicaria leaves was significantly higher in indigenous genotypes, as predicted by the EICA hypothesis. The average phenolic content of leaves (regardless of their origin) was, however, low, implying that the role of plant phenolics in purple loosestrife anti-herbivore defence is probably limited. It is suggested that the EICA hypothesis, as tested in the current study, does not explain the increased vigour of L. salicaria in non-indigenous habitats.