Influence of Host Condition on the Performance of Rhopalomyia n. sp. (Diptera: Cecidomyiidae), a Biological Control Agent for Scentless Chamomile, Tripleurospermum perforatum

The gall midge Rhopalomyia n. sp. has been used as a biological control agent for the weed scentless chamomile, Tripleurospermum perforatum (=Matricaria perforata), in Canada since spring 1999. A series of three experiments was carried out in 1996 and 1997 to investigate the effect of three nutrient and two water levels on T. perforatum, on the performance of Rhopalomyia n. sp., and on the interaction between scentless chamomile and the gall midge. The hypotheses were that (1) the performance of Rhopalomyia n. sp. is favored when larvae develop on fertilized, well-watered plants of scentless chamomile (vigor hypothesis) and that (2) fertilized plants compensate for gall midge attack. Two different seed origins of tetraploid T. perforatum, Canadian or European, which resulted in two different phenostages, rosettes or bolting/flowering plants, respectively, were used. Weekly application of nutrients resulted in a 64–85% increase in the nitrogen level of plants and in increased plant vigor. On rosettes, the addition of nutrients did not influence gall midge performance, whereas on bolting/flowering plants, more galls were induced, and potential fecundity of females was increased by 17–19%. Within the low nutrient level, reduced water supply increased the nitrogen concentration of plants in one of the experiments, but decreased plant vigor. The potential fecundity of gall midge females was reduced by water shortage in one of the experiments. The results of this study generally support the vigor hypothesis. The height of the main shoot was always reduced in midge-attacked compared to unattacked plants, irrespective of nutrient treatment, whereas the impact of Rhopalomyia attack on average shoot height and on the number of shoots produced per plant varied with nutrient treatment. We recommend the release of Rhopalomyia n. sp. preferentially onto scentless chamomile plants at nutrient-rich, moist sites.