The augmentation of Pristhesancus plagipennis as a biological control agent of Helicoverpa spp., Creontiades dilutus and Nezara viridula in summer oil seed crops

Abstract

Augmented natural enemies have proven to be an economical pest management option for some agricultural pests. However, the majority of commercially available natural enemies in Australia consist of predators and parasitoids for scale insects, mealy bugs or the eggs of lepidopterous pests. This leaves few management alternatives for different pest insects, other than spraying, which poses a problem for growers who are implementing biological control. The Australian assassin bug, Pristhesancus plagipennis (Walker) (Hemiptera: Reduviidae), is a predator of insect larvae and nymphs in a range of cropping systems and presents a potential solution to this problem. However, P. plagipennis has been previously discounted as a useful natural enemy due to its infrequent occurrence, generalist feeding behaviour and difficulty to mass-rear and release.This thesis deals with the potential for augmenting P. plagipennis in field crops as a biological control against key insect pests. It covers a number of aspects including the ability of P. plagipennis to;(a) be mass reared,(b) be integrated with insecticides,(c) be retained during winter in vegetative refuges,(d) prey on specific pest insect instars,(e) show preference between prey types,(f) disperse and establish after field release,(g) prey on key pests in pulse crops, and(h) prey on key pests in cotton.Previous studies had concluded that reduviids were difficult to mass-rear due to cannibalism, excessive handling and a requirement for live prey. However, I developed a containerised mass-rearing method for P. plagipennis that minimised cannibalism, avoided the need for live insect prey and was space and labour efficient. Experiments on the effects of diet concluded that hot water-killed larvae of the yellow mealworm, Tenebrio molitor L. were the most suitable prey for minimising nymphal development time and mortality while producing insects with the highest body weight. Further experiments demonstrated that when reared on a diet of T. molitor, the optimum rearing density was 20-27 nymphs per container. When nymphs of the same age were placed together, this rearing density minimised mortality to 16-22%. The optimum density for oviposition was 16 adults per container which provided the best compromise between egg production and space utilisation. The cost of rearing nymphs to third instar for release using these methods (including labour) was approximately $A3.50 per hundred which compares favourably with retail prices charged for other natural enemies in Australia.A problem for growers attempting to implement Integrated Pest Management (IPM) programs is the lack of information regarding the compatibility of insecticides with natural enemies. Therefore, I evaluated the acute and residual effects of 11 commonly used insecticides on nymphs of P. plagipennis under both laboratory and field conditions. For each insecticide, the length of time that residues caused g50% mortality was evaluated and compared against the LC50 to create plots that combined the acute and residual toxicity of each insecticide. Results suggested that carbaryl, esfenvalerate, endosulfan and deltamethrin had low residual and acute toxicity to P. plagipennis whereas chlorpyrifos, methomyl and monocrotophos were highly toxic at low concentrations and left persistent harmful residues. Cypermethrin, methidathion, malathion and dimethoate were moderately toxic. This research demonstrated the resilience of P. plagipennis to survive exposure to a number of insecticide products which gives it an advantage over many other natural enemies that are highly susceptible to these insecticides and difficult to integrate within IPM programs.The effects of Bacillus thuringiensis (cv Kurstaki) on P. plagipennis was tested as other studies had suggested the possibility of these products having detrimental effects on predators when ingested through infected larvae. However, predation of B. thuringiensis infected H. armigera larvae had no significant effects on nymph mortality, suggesting that these products would be suitable for integrating with P. plagipennis within IPM programs.n n n