In Australia, export grain, which constitutes 70% of total production, must meet a standard requiring total freedom from live insects. An ideal protectant should achieve complete prevention of infestation throughout a period of storage for 6-9 months. However storage for local consumption (the domestic market) can tolerate a low level of infestation.
Rhyzopertha dominica (F.) (Coleoptera, Bostrichidae), commonly called the "lesser grain borer", is a major, primary pest of stored grain both in Australia and throughout the world. Fumigants and synthetic pyrethroid insecticides are used to restrict infestation but with the development of resistance and a growing environmental lobby, alternative control measures are of interest. Botanical pesticides such as NEEM and one of its active components, azadirachtin (aza) offer an alternative. This botanical pesticide acts as an insect growth regulator (IGR) but many questions remain with respect to its efficacy. In this thesis I conducted a comprehensive investigation on the biological and behavioural activity of neem extracts of known aza content as grain protectants. Specifically, this project had two main objectives. The first was to determine the level of control of infestation attainable by aza either alone or in combination with synthetic insecticide or synergists. The second objective was to assess the effectiveness of aza as a primary and secondary behavioural modifier and the significance of these responses in complementing the physiological effects.
Aza is shown here to be ineffective in exerting a mortality effect on adult R. dominica. However, reproductive control in R. dominica is achieved through the use of aza. In stored grain, the main action of the aza residue on the beetles is reduction of new progeny through antiovipositional effects on adults and interference in development of immatures to adult. This control has both behavioural and physiological elements. The neem extracts used were shown to be effective at low aza rates that are equivalent to insecticides. A single neem treatment to wheat at a low aza application rate (e.g. 5-10 mg kg-1) at the commencement of storage provides a high degree of suppression (>95%).
I have found that differences in the effectiveness of neem extracts obtained from different geographical origins. I have also shown that the biological effects of other triterpenes present in the ethanolic neem extract provide only a slight improvement in suppressing F1 production as compared to purified aza. The neem kernel extracts used in this investigation appear to provide significant improvement over crude neem or purified aza, when looked at as a residual spray for long-term protection of stored wheat. However, my study showed occasional breakdown of activity of aza when applied at low rates (e.g. < 5-10 mg kg-1) which appears to be related to grain treatment.
Persistence of aza in application as a grain admixture is comparable to synthetic insecticides. However, to overcome the problem of multi-species infestations and occurrence of survivors, aza should be applied, at a reduced rate, with a sub-lethal concentration of synthetic insecticides rather than applied alone. Such combinations show great potential. The combination of aza with deltamethrin is shown to be highly potent. Addition of the commonly used synergist (piperonyl butoxide, PB) did not greatly improve the efficacy of aza: The synergistic effects of aza together with PB or sesamol were not apparent when the aged residues were assessed for long-term protection of the grain. Prior to my investigation the perception was that aza and other "natural" insecticides were less efficacious than synthetic insecticides and required considerably higher concentrations (therefore high costs of application) to be effective. I have shown that this is not the case if low levels of infestation can be tolerated or if aza is combined with sub-lethal concentrations of synthetic insecticides.
Uniformity of aza spray does improve its biological activity, but only to a small extent. Antifeedant activity requires that at least 50% of the grain be covered during treatment, which is achievable in commercial practice. Effective insect control appears to be more dependent on the concentration of aza used rather than on diluent volume applied and fraction of the targeted grain treated. Consequently, aza can be applied in the same manner as synthetic insecticides.
My findings on the feeding behaviour of adult beetles provide an insight into the low potency of aza, as reflected by occurrence of F1 emergence. Significant antifeedant effects were only seen once a concentration of aza of 10-50 mg kg-1 was applied. However, even at these concentrations, variability in the beetle behaviour was observed. At higher rates of application (e.g. 50 mg kg-1) feeding is temporarily suppressed to very low levels. But over time, feeding rates were comparable with control grain. Reduced feeding, as a result of toxicity is reversible: it occurs only for a short period prior to attainment of tolerance. In terms of control though, the feeding behaviour of the survivors does not result in substantial grain loss because R. dominica exhibit a decline in food intake over time.
The mechanism of neuroreception of aza has been investigated. I have demonstrated a critical role for contact chemoreception and feeding in the behavioural response to aza and grain. The poor olfactory sensitivity of the extracts used was confirmed by electrophysiological and bioactivity analyses.
Until the realisation of a commercial synthetic analogue or of an aza mimic, with a broad spectrum efficacy, possessing both desired modes of action (e.g. antifeedant and or IGR), the immediate approach for protection of export grain is for natural aza to be applied as an anti-ovipositant and IGR, in combination with a sub-lethal concentration of insecticides.