Department of Ecology, Swedish University of Agricultural Sciences, Skogsmarksgränd 17, S 901 83 Umeå, SWEDEN.
The diamondback moth Plutella xylostella L. (Lepidoptera: Plutellidae) is a serious pest of economically important crucifer crops such as cabbage. The moth has developed resistance to all tested insecticides and further studies on the potential role of factors affecting P. xylostella survival, including natural enemies, are urgently needed. One aim of this thesis was to identify the species that are natural enemies of P. xylostella and to evaluate their role in the natural biological control of this pest insect. Another aim was to gain knowledge that could be used to develop biological pest control methods with the potential for high efficacy against P. xylostella, thus avoiding the side effects of traditional chemical control while maintaining production and profits. This study was carried out in Estel., Nicaragua during five cropping seasons, from 2006 to 2008. The results indicate that there is a broad spectrum of predators present in habitats within and around cabbage fields, and that these have the capacity to feed on P. xylostella eggs and larvae under laboratory conditions. The predators with the highest consumption rates were insect larvae (Syrphidae) and spiders in the families Linyphiidae and Salticidae. The most abundant predators, which also had the highest consumption rate and consequently the highest potential for suppressing P. xylostella populations, were spiders (Lycosidae) and rove beetles (Staphylinidae), although sheet weaving spiders, jumping spiders, assassin bugs (Reduviidae) and damsel bugs (Nabidae) may also be important. It is concluded that these generalist predators should be considered for further study in the field as candidate species with a role in the management of the pest P. xylostella. An exclusion experiment in the field showed that flying and ground dwelling natural enemies of P. xylostella interact negatively with each other. In another study, leaf damage was found to be higher in insecticide treated fields than in untreated fields as a combined consequence of insecticide resistance in the pest and lower predation from natural enemies which are reduced in number by the insecticide applications. In the last study, the main focus was to identify whether a combination of bio-control agents, i.e. parasitoids and a biological insecticide (Bt), interact additively, negatively or positively, in affecting the mortality of P. xylostella. It is concluded that a combination of control measures, including the promotion of predators and parasitoids, is probably needed to achieve sustainable biological control of the diamondback moth. To succeed with such approaches we must, however, learn more about the particular roles of different predators.