Exploration and assessment of the oviposition substrate by the cabbage root fly, Delia radicum (Diptera : Anthomyiidae)

Oviposition behaviour of Delia radicum is not only influenced by host plant duality but also by the duality of the substrate in which the plant grows. Direct behavioural observations showed that the females partition their vi sits to a host plant (cauliflower) into ovipositional bouts separated by ex ploration of the host plant surface. Ovipositional bouts were further parti tioned into acts of egg deposition separated by exploration of the substrat e. While the mean number of ovipositional bouts per visit (2.6), and eggs l aid per egg deposition event (1.4) were stable, the mean number of egg depo sition events per ovipositional bout significantly varied (from 2.1 to 7.3) with the duality of the substrate and the physiological state of the femal e (egg load). Ovipositing females adjusted the final number of eggs laid ar ound the plant during the behavioural stage of substrate exploration. Addit ional experiments using plant surrogates treated with methanolic extract of Brassica leaves mounted in different substrates showed that: (a) the prese nce of living Brassica, Hordeum or Allium roots in a substrate enhances the number of eggs laid into this substrate, but females do not discriminate b etween the different plants; (b) females avoid both wet and dry substrates and prefer the substrates with a dry surface and moist particles directly a ccessible at a depth of about 5 mm; (c) substrates rich in organic matter a re preferred to sand; (d) olfactory perception of volatile chemicals from t he substrate must at least partially be responsible for the differences in oviposition in various substrates.