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 age nt for the weed scentless chamomile, Tripleurospermum perforatum (=Matricar ia perforata), in Canada since spring 1999. A series of three experiments w as carried out in 1996 and 1997 to inves tigate the effect of three nutrien t and two water levels on T. perforatum, on the performance of Rhopalomyia n. sp., and on the interaction between scentless chamomile and the gall mid ge. The hypotheses were that (1) the performance of Rhopalomyia n. sp. is f avored 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 an d 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 nit rogen concentration of plants in one of the experiments, but decreased plan t vigor. The potential fecundity of gall midge females was reduced by water shortage in one of the experiments. The results of this study generally su pport 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 treatm ent. We recommend the release of Rhopalomyia n. sp. preferentially onto sce ntless chamomile plants at nutrient-rich, moist sites. (C) 2000 Academic Pr ess.