Methods of Calculating and Correcting the Mortality of Insects1

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Abstract
As a result of studying the application of rotenone dusts against pea aphids, Macrosiphum pisi, it was found that wide differences in the mortality of aphids can be obtained depending on the methods of calculation. For example, by treating pea aphids with a 0.25% rotenone dust the mortalities of adults, nymphs, new-born aphids, all aphids, dusted aphids, and apparent mortality (on the basis of population counts made before and after treatment) were 18.2, 77.6, 37.1, 62.7, 75, and 56.3%, respectively. Such differences in the susceptibility and changes of population of aphids would also complicate field expts. The use of Abbott''s formula in the correction of natural mortality is a common practice, but no serious consideration has been given to the correction of results when an increase in population is observed. From theoretical populations on treated plants, a general equation was derived so that the results can be corrected for increases or decreases in population. The corrected percentage re-[image] where Pt is the % of apparent mortality in treated plots and Pck is the % change of population in the check. The sign in the above equation will be positive when the population in the check is increased and negative when it is decreased. The derivation of the above equation supports the application of Abbott''s formula not only for correcting for natural mortality but also where increases in population are involved. Abbott''s formula becomes the same as the suggested equation if the % of living insects is converted into the % of dead ones or vice versa, although they were obtained independently by different means. Examples indicate that in the case of an increase in the check population the apparent mortality is generally lower than when the results are calculated by the above equation. Only when the population of insects is uniformly distributed is the % reduction of population calculated on the basis of the population of the check about the same as that obtained by the suggested equation. A population not uniformly distributed will give misleading results, even if the % of change in population remains the same. The suggested equation eliminates large errors from this source. In one instance, when the population in one treatment is much smaller than that in the check, the mortality of aphids expressed by comparing with the check population is 40.2% higher than the corrected reduction of population.

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