oa Journal of the Entomological Society of Southern Africa - The toxicity of methidathion and citrus spray oil to mature and immature stages of OP-resistant and susceptible red scale, Aonidiella aurantii (Mask.) (Hemiptera: Diaspididae)
The toxicity of methidathion and narrow range citrus spray oil to three stages of red scale of two different field strains was investigated. Scales of the Letsitele strain were found to be highly resistant to methidathion compared with those of the Karino strain, and each stage was more resistant than the one preceding it, the resistance factor ranging from 75 for the first stage to 183 for the unfertilized adult female. Unfertilized females of the same strain were about six times more susceptible to methidathion alone when they were on leaves than when they were on fruits of lemon. The susceptibility of three developmental stages to spray oil did not differ. The addition of 1% spray oil to methidathion increased the toxicity to Letsitele scales by about 40 times. Fertilized females surviving treatment with 1,6% oil produced live crawlers (most of which later died as a result of the long residual effect of oil in the laboratory) but those surviving treatment with a mixture of methidathion plus oil did not; all the crawlers died under the scale of the female. Since unfertilized females are more susceptible to insecticides than fertilized females, sprays in orchards will be more effective if they are applied when the former are at their peak. This period can readily be determined by monitoring the males, which are present simultaneously.
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