Toxicological studies on a beetle, Alphitobius laevigatus (F.)

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Abstract
SUMMARY: The influence on the toxicity of insecticides of the site of topical application on the insect's body has been studied, with Alphitobius laevigatus beetles as the test insects. Four toxicants, n‐valone (2‐n‐valeryl 1,3‐indandione), DDT, dieldrin, and allethrin were used in solution in a non‐volatile mineral oil, Shell Risella 17 (R17).To facilitate the interpretation of the results of the toxicity tests, the spread of oil after application to different sites was followed. R 17 oil was marked with a fluorescent material and its spread over beetles was observed under ultra‐violet light. Doses of 0·05 μl. spread over considerable areas of cuticle from most of the sites of application, though doses of 0·02 μl. spread rather less. The greatest spread occurred after dosage between the front coxae, and the least after that on the pronotum. Although the areas of wetted cuticle overlapped following application to different sites, there were possibly quantitative differences in distribution not detectable with the method used. For experimental work applications to the mesoscutellum and hind coxae were the easiest.Toxicity tests for nine sites of application were done, using fixed doses. During and after treatment on the pronotum, elytron, or tarsi, beetles were held in wire loops to prevent their shedding any of the dose. After treatment on the other sites, the beetles were kept in groups running freely within containers. The site of application influenced slightly but significantly the toxicity of n‐valone DDT, and allethrin, but scarcely at all that of dieldrin. Generally the first three toxicants were most toxic when applied on or near the prothorax.The toxicities of the applications made to the mesoscutellum and hind coxae were examined in more detail, varying the doses. Pairs of parallel probit‐log‐dose lines fitted satisfactorily responses for the two sites, concentration of toxicant and volume of solution being varied in respective experiments with each toxicant. Combining these results with those from the first paper of this series indicates that the respective probit planes in log‐concentration and log‐volume for the two sites were parallel.From the toxicity experiments as a whole, it appeared that the toxicant was at most about 1·9 times as effective on the ‘most sensitive’ site as on the ‘least sensitive’.