Effects of variation in plant quality on densities and sizes of herbivores, primary and secondary parasitoids

To investigate the effects of variation in host quality on densities and sizes of three levels of consumers, a cabbage-aphid-parasitoid foodweb was analyzed. Three close related cabbage populations were studied: Brassica oleracea var. gemnifera cv. Cyrus, a feral Brassica oleracea and a wild cabbage type Old Harry as source of variation in host plant quality. A field experiment were established in Wageningen, The Netherlands to measure the effects on densities of herbivores. Aphids and lepidopteran species were monitored weekly during eight weeks on fifteen randomized plants within each plot. Parasitism in aphids and numbers of predators (coccinelids and syrphids) were also estimated. Repeated Measures Analysis of Variance using a Mixed Model was used to compare insect densities on different cabbage populations in each spatial arrangement. Mummies were also collected from the field during the whole season. Size of mummies of B. brassicae and hind-tibia length of the emerged parasitoids were determined to investigate the effects on sizes. Moreover, the parasitoid was identified and sex determined. To analyze mummy sizes and parasitoid sizes a General Linear Model (ANCOVA) was used, with fixed factors (cabbage population and spatial arrangement) and mummy size as a covariate.Groups of treatments were made based on LSM and Tukey’s post hoc correction. Effects of cabbage population were found to be significant on densities of the aphid M. persicae and the lepidopteran species M. brassicae, P. rapae, P. xylostella and coccinelidae. No effects of cabbage population were found for densities of the aphid B. brassicae, the lepidopteran P.brassicae, syrphids and parasitism in B. brassicae and in M. persicae. Effects of spatial arrangement were not significant for all the species of aphids, lepidopteran, predators and parasitoids analyzed. Effects of time (week) were significant for all the species analyzed.Effects of cabbage population on sizes were significant for overall mummy size and for sizes of mummies hosting D. rapae, A. suspensus and Alloxysta spp. For sizes of mummies hosting A.vulgaris the effects were not significant. Sizes of mummies hosting males and females of D.rapae and A. vulgaris were comparable but significant differences were found for mummies hosting males and females of A. suspensus and Alloxysta spp. Effects on overall hind-tibia length (parasitoid size) were significant for cabbage population and for sex of the parasitoid. Sizes of males and females of D. rapae and A. vulgaris were comparable. However, females of A.suspensus and Alloxysta spp. were larger than females.These results indicate that effects of plant quality on aphid densities were different to those on lepidopteran densities. Aphids performed better in feral Brassica and lepidopteran better in Cyrus. Differences in plant chemistry between cabbage populations, effects mediated by content of glucosinolates very probably are playing a role in lepidopteran-cabbagge interactions.Possible effects of phenological variation in host plant-food quality between the three cabbage populations can be occurring for the lepidopteran M. brassicae, P. xylostella or P. rapae.Differences for parasitism were not significant and for predators differences in characteristics of the physical environment possibly play a role in its abundance. The effects of host plant quality were transmitted to the second, third and fourth trophic levels as variation in body sizes.Mummies were the largest in feral Brassica plants, and in turn parasitoids and hyperparasitoids were also the largest. Mummies, parasitoids and hyperparasitoids in Cyrus and Old Harry were smaller.