Galls on thistles.

Plant galls - the more we learn, the more there is to know! Approximately 15000 insect species ( c. 2% of all known insects) form galls, and this habit is extremely common over a range of floras and latitudes. Yet, there is still no general consensus on why they exist. Thistle-galling insects are relatively well known and are used as biological control agents (of thistles) and model ecological systems.

Host-plant mediated effects of root herbivory on insect seed predators and their parasitoids.

The effects of root herbivory on a tephritid seed predator (Terellia ruficauda) and its parasitoids were investigated. Soil fauna were manipulated by insecticide treatment; host plant (Cirsium palustre) phenology and the oviposition behaviour of both tephritid and parasitoids (Pteromalus elevatus and Torymus chloromerus) recorded. Although insecticide-treated (and hence reduced root herbivory) plants had larger flowerheads, population abundances of both tephritids and parasitoids were greater on thistle plants subjected to root herbivory. Percentage parasitism was similar in both treatments. Root herbivory is thought to enhance the nutrient quality of plants and this may have resulted in the tephritid preferentially feeding on thistles whose roots had been attacked. Parasitoids on these plants were probably affected by a combination of increased plant attractivity (as for the tephritids), smaller flowerheads aiding ovipositor entry and more tephritid hosts being present. This is the first study to show that root herbivores, through plant-mediated interactions, can affect seed herbivores and also, albeit indirectly through the host, natural enemy trophic levels.

Direct and indirect effects of climate change on St John's wort, Hypericum perforatum L. (Hypericaceae).

We report results from a continuing, long-term field experiment addressing biotic responses to climatic change in grasslands. We focus on effects of summer precipitation (enhanced rainfall, drought, control) and winter ground temperatures (warming, control) on growth, reproduction and herbivory in St John's wort, Hypericum perforatum L. Both winter warming and summer rainfall regimes modified performance and interactions of H. perforatum, particularly those with herbivorous insects. Winter warming had positive effects, with earlier initiation of plant growth and reduced damage by gall-forming and sucking insects in spring, but also had strong negative effects on plant height, flowering, and reproduction. Summer drought reduced reproductive success, but even severe drought did not affect plant growth or flowering success directly. Rather, summer drought acted indirectly by modifying interactions with herbivorous insects via increased vulnerability of the plants to herbivory on flowers and capsules. Overall, the effects of summer precipitation were expressed mainly through interactions that altered the responses to increased winter temperatures, particularly as summer drought increased. The field site, in Oxfordshire, UK, is near the northern limit of distribution of the species, and the experiment tested probable responses of H. perforatum as climates shift towards those more typical of the current center of the distribution of the species. However, if climates do change according to the projected scenarios, then H. perforatum is unlikely to fare well near its northern boundary. Increased winter temperatures, particularly if accompanied by increased summer drought, will probably render this species even less abundant in England than at present.