Item

Effects of insecticides on Sitona discoideus (Gyllenhal) (Coleoptera: Curculionidae) and its parasitoid Microctonus aethiopoides (Loan) (Hymenoptera: Braconidae)

McNeill, M. R.
Date
1991
Type
Thesis
Fields of Research
ANZSRC::070308 Crop and Pasture Protection (Pests, Diseases and Weeds) , ANZSRC::079902 Fertilisers and Agrochemicals (incl. Application) , ANZSRC::060808 Invertebrate Biology , ANZSRC::060307 Host-Parasite Interactions , ANZSRC::060207 Population Ecology
Abstract
Laboratory and field experiments were conducted to evaluate the effect of insecticides on the egg and adult stages of Sitona discoideus (Gyllenhal), and its natural enemy Microctonus aethiopoides (Loan). A range of insecticides were applied to S. discoideus eggs with the aim of assessing the ovicidal properties of the insecticides and to provide additional control to that already achieved against the adult weevil. The Potter tower direct spraying method was used to apply the insecticide at near field rates. Chlorpyrifos and fenitrothion exhibited the greatest activity followed by deltamethrin, diflubenzuron and ɤ -HCH. Summer oil and fenvalerate were ineffective as ovicides. Following treatment, chilling of eggs for 30 days below the threshold temperature for development, reduced the ovicidal activity of both chlorpyrifos and fenitrothion with the former showing an increase in LC₅₀ and change in the log dose-probit slope, while the latter produced a shift in LC₅₀ only. The effect of chlorpyrifos and fenitrothion on the parasitoid M. aethiopoides (Loan) was investigated and compared to its host S. discoideus. Topical dosing with chlorpyrifos and fenitrothion showed that the parasitoid was more susceptible than the weevil. The selectivity ratio of LD₅₀ parasitoid/LD₅₀ weevil confirmed that both insecticides were selective towards the parasitoid, with fenitrothion relatively more selective than chlorpyrifos. Fenitrothion was more toxic to aestivating than reproductive weevils, although the slopes of the log probit-dose line were similar. Possible reasons for the susceptibility of aestivating weevils relate to biochemical and physiological changes associated with aestivation, including greater sensitivity of cholinesterase or lack of food in the gut predisposing the weevil to the insecticide. The tolerance of the pupal stage of M. aethiopoides to both chlorpyrifos and fenitrothion was demonstrated using the Potter tower and topical application. Direct application using the Potter tower, with concentrations in excess of field rate did not affect pupal survival. However, longevity and the ability to parasitise weevil hosts were impaired in females surviving chlorpyrifos at 2.4 g/l. The effect of chlorpyrifos on parasitoid longevity was greater in males. The high tolerance displayed by the pupae was repeated when cocoons were topically dosed with methylethyl-ketone solutions of chlorpyrifos and fenitrothion. An LD₅₀ for pupae could not be determined for either insecticide at the range of concentrations tested. A field experiment investigated the impact of chlorpyrifos applied at 0.3 kg a.i./ha on M. aethiopoides. Application in mid-May 1988 resulted in a 96.2% control of the weevil population leaving a post-spray density of 0.8 weevils m⁻². The parasitoids occurring as larvae inside the weevil were particularly susceptible to the insecticide at this time. Surviving weevils were 87.5% parasitised, and it is speculated that the resulting parasitoids would form the nucleus, albeit at low density, to attack any surviving weevils in the following spring. In conjunction with the field study, laboratory experiments were conducted to assess the toxicity of field weathered chlorpyrifos residues on both the adult parasitoid and weevil. At 0.3 kg/ha, chlorpyrifos residues caused over 60% mortality of M. aethiopoides for three days after spraying, but activity declined rapidly thereafter. A more prolonged level of activity was recorded against S. discoideus, with c. 60% mortality one day following application declining slowly through to fifteen days post-spray. Chlorpyrifos at 1.0 kg/ha produced between 78-100% mortality in both weevil and parasitoid for ten days following application. Twenty-two days after spraying, mortality the 1.0 kg/ha rate still remained at ≥ 40% for both insects. There appeared to be no obvious interaction between weevil mortality and the presence of larval parasitoids. However 4th instar larva emerged and successfully pupated from dead weevils up to 24 hours after weevil mortality had been noted. Based on the field experiment, timing of insecticide application is important for the survival of the parasitoid. Delaying application till June as opposed to May, would allow a portion of the parasitoid population to move from the vulnerable larval stage to the highly tolerant pupal stage. This would provide a nucleus of parasitoids to attack surviving weevils in spring. However, despite virtual elimination of the parasitoid when lucerne is treated in mid-May, the management practice of applying a single spray in autumn combined with the limited areas being treated, ensures that reservoirs are maintained outside sprayed paddocks. The parasitoid is then reintroduced in the following autumn during the post-aestivatory reinfestation flights by the weevil.
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