Integrating biological control and botanical pesticides for management of Plutella xylostella

A large number of different plant species have natural pesticidal properties, and man has made use of this since early times. By applying plant extracts to other susceptible plant species the defence of the susceptible plant is improved. This thesisfocuseson the possibility of integrating botanical pesticides with biological control for management of the diamondback moth, Plutella xylostella , inSouth Africa Twenty-one species of primary parasitoids have been collected from P. xylostella in the field inSouth Africa. Biological control therefore provides a natural control technique. However, biological control alone is insufficient to provide adequate protection and requires integration with other control techniques. Plant products from the Meliaceae family have been widely used to control insect pests, particularly products from the neem tree, Azadirachta indica . The neem tree does not grow inSouth Africabut the closely related syringa tree Melia azedarach is a widespread invasive plant found throughout the country. In this thesis I make use of a commercial neem product, Neemix 4.5 ® , and aqueous leaf extracts derived from the syringa tree.The neem- and syringa- derived botanical pesticides had adverse effects on the development, reproduction and survival of P. xylostella . These botanical pesticides also reduced feeding and oviposition, which are important factors in pest control. However, if a botanical pesticide is to be combined with biological control it must not hamper natural enemies. The neem- and syringa- derived botanical pesticides did not have a directly negative impact on the survival of Cotesia plutellae or Diadromus collaris two of the most abundant natural enemies found in South Africa. In a glasshouse, a significantly higher proportion of P. xylostella larvae were parasitised by C. plutellae on plants that been treated with the syringa extract than on control plants. Results from a choice test in a windtunnel showed that C. plutellae was attracted significantly more often to cabbage plants treated with the syringa extract than to the control plants. Headspace analysis revealed that treatment of cabbage with syringa extracts caused an increased emission of volatiles by the cabbage plants. This may explain the increased attraction of C. plutellae to plants that had been treated with the syringa extract. It was important to verify results from the laboratory under more realistic conditions in the field. We did not find a difference in P. xylostella infestation levels between the treated and the control plants in the field. However, the damage on plants treated with the botanical pesticides was significantly lower. Therefore, it seems that reduced feeding by P. xylostella larvae was a more important factor in the reduction of damage than the actual population density. The proportion of P. xylostella larvae that had been parasitised was significantly higher on the treated plants than on the control plants. Direct observations showed that plants that had been treated with neem- and syringa- derived pesticides were still visited by parasitoids. Therefore these botanical pesticides do not appear to interfere with parasitoid foraging. I assessed the possibility for introducing this control method to the rural farming community inSouth Africa. Syringa trees are invasive plants found throughoutSouth Africaand therefore provide a free local resource for the botanical pesticide. Results indicated that the use of syringa extracts could be introduced. Water is the main factor limiting the introduction of this technique in more arid environments. Results presented in this thesis indicate that biological control and the use of botanical pesticides derived from the neem and syringa trees can be integrated for the management ofP. xylostella. However, mammalian toxicity and residual effects still require extensive investigation before any further recommendations can be made.