Felix L. Wäckers

Abstract:  Flower strips near crops may stimulate natural enemies by the provision of nectar and hibernation sites. However, these habitats may also be beneficial for potential pest species. We investigated the dynamics of the cabbage aphid Brevicoryne brassicae (L.) (Homo., Aphididae) and its primary parasitoid Diaeretiella rapae (McIntosh) (Hym., Braconidae) in brussels sprout fields and adjoining flower plots in winter. A wide variety of 14 plant species were included in the study, each established as monoculture plots. Brussels sprout fields and flower plots were established at two sites. One site was located in an open agricultural landscape, the other in a landscape dominated by mixed forest. Brevicoryne brassicae and D. rapae were found on brussels sprout plants but not in the flower plots. Brevicoryne brassicae was initially more abundant in the open landscape, but as their densities declined rapidly in time, no living aphids were recovered at both sites by February. The density of aphids parasitized by D. rapae showed a similar trend, but densities of eight mummies per brussels sprout plant were still present by the end of February. These findings suggest that (i) flower species under investigation do not function as sources of B. brassicae and (ii) brussels sprout plants that are not harvested may not only harbour D. rapae populations that may sustain biological control, but are also likely to act as sources of B. brassicae infestation.

Summary: Adult parasitoids depend on sugar‐rich foods such as nectar and honeydew to meet their energy requirements. Many laboratory studies have established fitness benefits of sugar feeding for parasitoids. Nevertheless, we know little about the nutritional ecology of parasitoids in the field, chiefly because of the limited specificity of methods applicable for field studies. Here we use high‐performance liquid chromatography (HPLC) analysis to study the sugar profile of Cotesia glomerata L. and Microplitis mediator Haliday relative to feeding‐treatment. We identify the overall sugar level and the ratio of glucose to fructose as two parameters that in combination unambiguously characterize an individual's nutritional state and feeding history. Unfed parasitoids contained low total sugar levels and glucose levels that typically exceeded fructose levels more than five‐fold. Parasitoids with constant access to sucrose had high overall sugar levels with a balanced glucose–fructose ratio. The marked shift of the glucose–fructose ratio after feeding was still evident in C. glomerata individuals whose total sugar content had decreased to unfed levels after 3 days of starvation. This makes it possible to distinguish between unfed individuals and those that did feed several days ago. In addition to revealing past feeding events, HPLC analysis also enables the identification and quantification of a wide spectrum of sugars present in each specimen. When food sources contain specific sugars this can provide additional information about the type of sugar source consumed. HPLC analyses revealed that trehalose, considered to be the ‘insect haemolymph sugar’, is not present in the two species investigated here. Overall, HPLC sugar analysis provides a powerful tool to investigate energy status, feeding history and nutritional physiology of field‐collected parasitoids.