Conservation biological control in brassica crops using Australian native plants

The practice of planting companion plants to enhance the efficacy of natural enemies as a pest management tool is a form of habitat management. This has an important role in conservation biological control. Recent research into this practice has focused on the provision of multiple ecosystem services that contribute to creating sustainable agro ecosystems. The use of native plant species in habitat management is advantageous as they are well adapted to their environment and may have reduced supplementary needs for water and nutrients compare to introduce species. Their use is important from the perspective of biodiversity conservation. In such a context, this thesis investigates the use of Australian native plants in habitat management to support conservation biological control in brassica crop systems in temperate Australia. The study was not limited to pest management, observations on complementary ecosystem services are included. To reduce the establishment costs of habitat management, an assessment of established flowering weeds as a habitat management tool was conducted.
The first objective was to assess the potential of Australian native flowering plants for use in habitat management. Laboratory assessments determined their relative benefits to longevity of parasitoids of brassica pests. The second objective was to investigate the ecosystem services that could be provided by some Australian native plants when established adjacent to a brassicas that were identified as having potential in first objective. These ecosystem services included biological pest control services, pollinator enhancement, encouraging biodiversity such as native butterflies and biological activity in the soil. The third objective was to assess the spatial scale of benefits from adjacent perennial woody Australian native vegetation for conservation biological control when it is growing adjacent to conventional commercial brassica crop fields. The fourth objective was to assess if established flowering plant species normally considered to be weeds provide a biological pest control service in a brassica crop fields.
The longevity assessment study identified several native plant species that have potential to benefit parasitoids of brassica pests. The native plants Leptospermum sp. and Callistemon citrinus enhanced the longevity of the aphid parasitoid Diaeretiella rapae as much as the exotic, frequently used, habitat management plant Fagopyrum esculentum. Out of 11 Australian native plant species assessed, longevity of one or more parasitoids was enhanced by seven, Westringia fruticosa, Mentha satureioides, Callistemon citrinus, Leptospermum cv. ‘Rudolph’, Grevillea cv. ‘Bronze Rambler’, Myoporum parvifolium and Lotus australis. However, those plants also enhanced the longevity of brassica pest Plutella xylostella. The benefits provided by the plant flowers were not selective to the parasitoid Diadegma semiclausum compared with its host P. xylostella; however, the effect of M. parvifolium and Grevillea sp. flowers on the longevity of D. semiclausum was relatively higher compared with the effect on pest longevity.
In the field study with brassica plants, M. satureioides had the most comprehensive positive effect on pest management. Compared with the grassy control, M. satureiodides significantly enhanced parasitism of P. xylostella larvae and reduced the aphid, Brevicoryne brassicae, densities at a rate that was comparable to F. esculentum. Although the flowers of M. satureiodides appeared attractive to adults of the pest Pieris rapae, the resulting increase of eggs on the adjacent cabbage foliage did not translate to higher numbers of larvae which reflected the higher densities of parasitoids and predators, especially spiders that were present. Other ecosystem services, such as pollinator abundance was enhanced by F. esculentum, L. australis and M. satureioides. The native plants L. australis and M. satureioides were found to be attractive to native butterflies. Biological activity in the soil was lower around the exotic F. esculentum compared to all native plants.
Woody perennial native vegetation adjacent to brassica fields was found to be donor habitat for natural enemies but the effect was overridden by pesticide use on heavily sprayed sites. Natural enemies especially ground active predators were uniformly in low abundance in crops with intense insecticide regimes. The effect of adjacent perennial vegetation on natural enemies, except canopy dwelling predators and large parasitoids, was only observed where pesticide use was low. The reduced impact of pesticides on the canopy dwelling predators including canopy dwelling Araneae and large parasitoids reflected their ability to rapidly recolonize a field.
The flowering weeds, Sisymbrium officinale and Foeniculum vulgare were attractive to parasitoids when compared to common weedy grasses, whereas the grasses were more attractive for Coccinellidae. Higher P. xylostella parasitism in the brassica crop was observed adjacent to S. officinale. The abundance of ground dwelling predators such as Formicidae, Forficula sp., Araneae and Carabidae were not affected by adjacent weeds. However, the flowering weeds also attracted adult brassica insect pests and S. officinale hosted their larvae. Most natural enemy populations were low within the brassica crop when compared to the weedy margins.
These results have implications for designing conservation biological control and other complementary ecosystem services either in brassica or other cropping systems. This thesis identified several native plants that benefit parasitoids, although these were not selective. These results also are important to systems where the parasitoids D. rapae, Cotesia spp. and Diadegma spp. can be used. Native plants that provide resources to natural enemies and complementary ecosystem services such as enhancement of pollinators and wild butterflies were identified. This shows scope for farmers to take advantage of potentially multiple ecosystem services by incorporating native flowering plants into farming systems. This work highlights the importance of plant selection and the risk of trade-offs among ecosystem services. This research also highlights the need for reductions in pesticide use to assist conservation biological control.