Floral resources for the enhancement of the efficacy of Diadegma semiclausum (Helen) (Hymenoptera: Ichneumonidae), a parasitoid of Plutella xylostella (L.) (Lepidoptera: Hyponomeutidae) in broccoli (Brassica oleracea L. var. italica)

Abstract

In this thesis, conservation biological control experiments have been conducted in the broccoli agro-ecosystem in New Zealand, to target the diamondback moth (Plutella xylostella (L.) (Lepidoptera: Hyponomeutidae)) and its key parasitoid Diadegma semiclausum (Helen) (Hymenoptera: Ichneumonidae).

From a suite of flowering plants chosen for their ability in other studies to enhance parasitoid longevity and fecundity, a series of experiments in the laboratory was conducted to determine the plant's effects on the parasitoid and its host. Two plant species were chosen on the basis of their selectivity in favour of the parasitoid's fitness, rather than that of the host. These plants were buckwheat (Fagopyrum esculentum) and phacelia (Phacelia tanacetifolia). Results were compared with other parasitoid-pest systems.

Buckwheat was then studied as a resource subsidy in the field. The effect of the deployment of buckwheat on parasitism rates, physiological state and abundance of the parasitoid at distances from the buckwheat was studied. Parasitism rates were doubled when buckwheat was present in the field, compared with plots with no flowers. Nevertheless, the percentage of parasitoids which had fed on nectar did not differ between flower and non-flower plots.

The need for studying parasitoid movement and the associated use of markers was apparent from the field studies. Therefore, the need for effective marking and tracking techniques for studying predator and parasitoid movement was reviewed. The usefulness and limitations of marking and tracking techniques for predator and parasitoid studies was analysed. Aspects of predator and parasitoid movement are relevant to the role refugia could play in influencing, via natural enemy movements, a reduction in pest populations. Following this review, a more detailed one was conducted to determine the most promising techniques to study movement for habitat manipulation and conservation biological control studies. From the possible techniques, rubidium, a trace element was chosen to study movement after feeding on nectar of buckwheat. The marker proved useful in the field to confirm nectar feeding and movement of D. semiclausum after feeding on nectar of rubidium-sprayed buckwheat plants. Although results suggested that the parasitoids moved up to 80 m in a short period of time, parasitism rates were lower in a non-flower plot separated from a flower plot by just 60 m. It seemed clear that mobility studies alone would not give an indication of the spatial arrangement to follow in order to produce enhanced parasitism rates.

Finally, the behaviour of fed and unfed parasitoids in the laboratory was studied to determine if searching behaviour could be enhanced after patch encounter. D. semiclausum spent a considerable amount of time displaying characteristic behaviours related to active searching, irrespective of the treatment. This suggests that well-fed parasitoids do not search more actively for hosts once they encounter a host-patch. However, whether the parasitoid in the field would search for more host patches and therefore parasitise more hosts when fed, as has been suggested in the literature, needs further consideration.

It is clear from the results of this study and others that careful laboratory screening can increase the chances of success in conservation biological control. Further studies should not only consider the use of selective resources, but the role of these in the pattern of movement and parasitism of the parasitoid. The use of selective resource subsidies in the broccoli system can increase parasitism rates, therefore constituting an important tool to help manage P. xylostella in these conditions.