Effects of reduced water availability and insecticide on damage caused by cabbage root fly larvae

The incidence of horticultural pests in combination with increased frequency of extreme weather events, like summer drought, can compromise crop production in Nordic agricultural systems during the already short summer growing season. In particular, rootfeeding insects can severely damage plants when combined with water deficits because root herbivores reduce plant water and nutrient uptake (Zvereva & Kozlov 2012). The cabbage root fly Delia radicum (L.) (Diptera: Anthomyiidae) is a major pest of cauliflower Brassica oleracea (L.) (Brassicales: Brassicaceae) crops in Iceland (Halldorsson 1989). Its larvae feed on roots of brassica plants, initially on root hairs and then by tunnelling into the taproot. Cabbage root flies overwinter as pupae in diapause in the soil around brassica plant roots (Bažok et al. 2012). In Iceland, adults emerge in June and females lay eggs between late June and early July (Halldorsson 1989). Eggs are laid on the soil surface around the base of the stems of brassica plants and are relatively resistant to desiccation (Lepage et al. 2012). In contrast, survival of first-instar larvae is compromised at low levels of soil moisture (Lepage et al. 2012). The aim of this study was to investigate the potential effects of reduced water supply on damage by cabbage root fly on cauliflower plants using a field experiment in Iceland. Specifically, we assessed the effect of reduced water supply on larval densities and on early-season growth of cauliflower plants. We expected reduced biomass production in plants that were not treated with insecticides; we hypothesized that this effect would be aggravated during periods of reduced water availability because the water deficit imposed by root damage would be stronger when combined with reduced water supply.