Influence of host density and host distribution on parasitism of Ceutorhynchus assimilis by Trichomalus perfectus.
The relationship between host density, host distribution and parasitism by T. perfectus, a larval ectoparasitoid of the cabbage seed weevil C. assimilis, was examined in field experiments on winter oilseed rape in 1993 and 1995 in Germany. Variation of host densities was established by releasing different numbers of adult seed weevils into cages. After oviposition, cages were removed and weevil larvae were exposed to natural parasitization. In 1993, only small differences of weevil larvae densities occurred between plots and the number of parasitised larvae was not affected by host density and host distribution analysed on a single-plant scale. On average 36% of larvae were parasitised. In 1995, the average proportion of infested pods varied between 15 and 57% in different plots. Correspondingly, levels of weevil larvae parasitism increased from 39 to 50%. However, parasitism was not related to the position of infested pods on the main and secondary racemes. As in 1993, a density-dependent relationship of parasitism of the seed weevil by its main parasitoid T. perfectus was not detected.