Does the repellence of an insecticide affect its efficacy (measured in insect mortality)?
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.