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Toxicological studies were carried out to evaluate the susceptibility of codling moth, Cydia pomonella L. to different insecticides. Bioassay procedures for adults and larvae were standardized in terms of posttreatment holding and temperature, age and sex of insects, insecticide formulations and selection of a reference strain. This study detected a low level of resistance to azinphosmethyl in codling moth from Oregon. The LD50 value of the most susceptible strain was 0.038 ug AI/moth, while that of the most resistant population was approximately 4.3 times higher as determined by the topical application method on pheromone traps. The adult topical bioassay using moths reared in the laboratory showed that two of the seven strains tested (Crippens and Medford) had 4-5 fold higher LD50 values than reference (Saint Cloud) strain. In the topical and residual studies with first instar larvae, similar but slightly lower differences were observed among the different populations. There was a strong indication of low levels of resistance in populations to diazinon and fenvalerate in topical tests applied to both males and females. However, there was no significant difference for carbaryl or endosulfan. Larval topical bioassays showed a significant difference only to diazinon. Correlation analysis suggests that a degree of cross-resistance exists between azinphosmethyl and diazinon and to some extent to fenvalerate. Codling moths exposed to the residues of diflubenzuron showed a maximum of 2-fold difference between the susceptible and resistant populations. The differences were visible in terms of egg hatch and first instar larval entries. The pupal and adult stages emerging from the treated apples exposed to ovipositing females showed insignificant sublethal effects. A strong positive correlation between the resistance to azinphosmethyl and resistance to diflubenzuron was noticed. Selection of codling moths for resistance to diflubenzuron for six generations in the laboratory decreased susceptibility by 5-fold. Application of synergists such as PBO and DEF, combined with azinphosmethyl and diflubenzuron, showed slightly different effects on all strains of codling moth. Use of PBO (a P-450 inhibiting compound) with azinphosmethyl and diflubenzuron did not change toxicity of either of the compounds. However, the combination of DEF (an esterase inhibiting compound) with the insecticides significantly increased the toxicity of azinphosmethyl and to some extent diflubenzuron. The maximum synergistic ratios obtained were 8 and 2 for azinphosmethyl and diflubenzuron, respectively. These results indicate a possible involvement of esterases in development of resistance in codling moth to azinphosmethyl and maybe to diflubenzuron. The optimal synergistic ratio determined on a resistant strain of codling moth to various combinations of azinphosmethyl and DEF was 1:4 (50 ppm of azinphosmethyl 200 ppm of DEF). The phytotoxicity studies on young apple trees showed that DEF should be used at or below 500 ppm to avoid phytotoxicity while maintaining effective control of codling moth with a reduced dose of azinphosmethyl. / Graduation date: 1997

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Genetic and non-genetic effects of maternal; size were investigated in four introduced populations of Cinnabar moth Tyria jacobaeae (L.) (Arctiidae). Offspring of different female moths were reared to the pupal stage under greenhouse conditions with an overabundance of food. Mean offspring-maternal size correlations were highest for second generation laboratory offspring using pupal weight as the size criterion, maximum egg weight was not related to female size, although average egg weight tended to decrease, and fecundity increase with female wing length. Egg weight was not related to final pupal weight. Egg weight decreased over the oviposition period, but pupae reared from later batches were not consistently of lower weight when compared with pupae reared from batches laid on tie first day of oviposition. Under a less favorable temperature regime, hatching, success was reduced in later batches. It was concluded that maternal effects (genetic and non-genetic) on offspring size are unimportant under normal field conditions in comparison to the effects exerted by larval crowding and food availability. However under very poor conditions, selective mortality may be imposed on later batches, or on the lighter eggs of large moths. / Science, Faculty of / Zoology, Department of / Graduate

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Thesis (MScAgric)--University of Stellenbosch, 2008. / Bibliography. Also available via the Internet.