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I am currently working on assessing the efficacy of a suspected insecticide on a particular species of insect. I happen to be looking into any repellence caused by the substance. I test for both effects at the same time by putting that insect in a no-choice bioassay. To be specific, my assay set-up consists of a petri dish, and the test article treated with the test substance is placed on one side of the petri dish. Insect species X is introduced into the petri dish, and every 24 hours, I get to count and remove the dead ones. All these are repeated for different concentrations of the test substance.
What I found is that mortality (proportion of insect X killed) initially increases with concentration, but then decreases after a particular concentration. I also found that repellence increases with the concentration of the test substance. So, what exactly is happening here? Is the repellence of the test substance having an effect on the mortality, such that repellence is causing a decrease in mortality?

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  • $\begingroup$ Edit: I forgot to mention that I am not qualitatively measuring the repellence of the test substance. The petri dish, that is the assay, is divided into two halves via an imaginary line, and the test article is placed on one side of the assay. An index, which I call the 'repellence index', is used to measure repellence by dividing the difference of the no. of termites between the two sides of the assay, by the total no. of termites observed. Hope that clarifies my question more. $\endgroup$ – user43591 Jul 3 '18 at 5:47
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You have a good hypothesis: the decrease in mortality is mediated by an increase in repellence. It sounds like you have a measurement of increased repellence, and it's a plausible mechanism (if an insect isn't near an insecticide, it can't be killed by that insecticide). Now you should test it.

If you are measuring repellence using a different assay (e.g., a choice assay, where the insect can enter a chamber with the insecticide vs. one without), than you could use as the denominator insects that enter the chamber with the insecticide. Compare mortality of insects that enter the chamber with insecticide changes with increasing concentrations in this assay with the mortality in your no choice assay.

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  • $\begingroup$ Edit: I forgot to mention that I am not qualitatively measuring the repellence of the test substance. The petri dish, that is the assay, is divided into two halves via an imaginary line, and the test article is placed on one side of the assay. An index, which I call the 'repellence index', is used to measure repellence by dividing the difference of the no. of termites between the two sides of the assay, by the total no. of termites observed. Hope that clarifies my question more. $\endgroup$ – user43591 Jul 3 '18 at 5:48
  • $\begingroup$ @user43591 since you are quantitatively measuring the repellence, than you can quantitatively measure the mortality of exposed (vs. non-exposed) as suggested in my answer. There is, of course, more to it, but your next step would be to use the data you already have to look at mortality in the termites on the side with the test article only and see what the curve looks like there. Depending on the result, this should give more or less support to your hypothesis that the decrease in mortality above a certain concentration is due to repellence. $\endgroup$ – De Novo Jul 3 '18 at 19:24
  • $\begingroup$ Whoa thanks man, never thought of that! Thank you very much! $\endgroup$ – user43591 Jul 4 '18 at 15:49
  • $\begingroup$ @user43591 you're welcome! If I answered your question, feel free to mark it the answer using the checkmark to the left of it see someone answers $\endgroup$ – De Novo Jul 4 '18 at 17:03

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