HOST-PLANT MANIPULATION BY GALL-INSECTS - A TEST OF THE NUTRITION HYPOTHESIS

1. Nitrogen is generally regarded as a crucial, limiting nutrient for phytophagous insects. Nitrogen levels in two species of cynipid galls (Neuroterus quercus-baccarum L. and Andricus lignicola Hartig) on oak (Quercus robur L.) were monitored after experimental enhancement of host-plant nitrogen levels over 2 years. 2. The nitrogen content of gall-tissue was unaffected by fertilization of the host-plant, although the nitrogen content of leaf-tissue was elevated. Furthermore, fertilization did not improve gall-insect performance. These results superficially run counter to the predictions of the nutrition theory for the adaptive nature of plant galls. 3. However, in one of the two species (Neuroterus) survivorship was highly variable across individual oak trees, and was negatively correlated with gall nitrogen content. Between-tree differences persisted across years, irrespective of fertilization treatment. 4. Consistent with these results, experimental reduction in number of Neuroterus galls per leaf did not lead to an increase in nitrogen levels in the remaining galls, or to an improvement in insect performance. Thus, there was no relationship between gall density and gall nitrogen content, nor between density and insect performance. 5. We conclude that cynipid gall-formers may be able to manipulate host nitrogen levels to their own advantage, by preventing an increase in gall nitrogen levels on fertilized trees; such an increase would reduce insect survival rates. Our results are therefore consistent with the nutrition hypothesis to explain the evolution of insect galls.