Conservation biological control of key pests of Brussels sprouts (Brassica oleracea L. gemmifera)

Brussels sprouts are attacked by a variety of herbivore pests from several insect families. The most important leaf feeding ones are the cabbage aphid, Brevicoryne brassicae (L.), the cabbage moth, Mamestra brassicae (L.) and the diamondback moth, Plutella xylostella (L.). As Brussels sprouts are a high yielding crop with an almost zero damage threshold, growers want to keep the sprouts free from damage by pest insects. Several measures are available to control insect pests, of which chemical control is most commonly applied. However, because of the negative side-effects of pesticides and in respect to the forthcoming implementation of the European directive 2009/128/EC, growers have to resort to Integrated Pest Management strategies. The first principle of this strategy focusses on the prevention of pest outbreaks, which can be achieved by enhancing the populations of naturally occurring beneficial organisms (Conservation Biological Control). In Brussels sprouts, pest insects are attacked by several natural enemies which may all contribute to biological control. In a first part of the research, the phenology of the above mentioned pest insects and their key natural enemies was examined at three locations in Flanders (Beitem, Kruishoutem and Sint-Katelijne-Waver). The results revealed that the abundance of natural enemies was strongly related with pest abundance. Despite their high abundance, however, the natural enemies were unable to keep the key pests under control. Further, a substantial variability in the phenology of both pest insects and their natural enemies was found among locations and seasons, suggesting that for effective pest control monitoring should be done at each location of interest. In order to strengthen the biological control function of natural enemies, measures can be taken to increase their abundance in agro-ecosystems. Therefore, the impact of an annual flower strip on the pest complex in Brussels sprouts and the associated natural enemies was assessed. The results showed that the impact of a flower strip on the insect complex in and around Brussels sprout fields is not unambiguous. Despite the presence of an annual flower strip to enhance pest control by natural enemies, the sprouts could not be kept free from economic damage. Besides enhancing natural enemies by habitat management measures, conservation biological control also focusses on insecticide selectivity. Therefore, a set of laboratory experiments was carried out to assess the lethal and sub-lethal effects of selected insecticides (flonicamid, pirimicarb, pymetrozine, spinosad, spirotetramat and thiacloprid) on the hoverfly, Episyrphus balteatus (Degeer), as a key natural enemy of the cabbage aphid. The susceptibility of the hoverfly differed among the selected insecticides and the experimental methods. Further, susceptibility to the tested compounds also varied among the life stages. It was concluded that flonicamid and pymetrozine were compatible in an IPM approach with this hoverfly. In a last part of this study a worst-case laboratory test was performed in which the selectivity of the above mentioned insecticides was examined on the hymenopteran parasitoid, Microplitis mediator (Haliday), as a natural enemy of the cabbage moth. Similar to the results of the hoverfly, selectivity varied among the tested insecticides. Thiacloprid and spirotetramat were rated harmless for the adult wasp. The results further indicated that the cocooned pupae were more protected from the insecticides compared to the adult wasps.