Pesticide applications are commonly used in New England to reduce either the first or second generation of the apple blotch leafminer, Phyllonorycter crataegella (Clemens), to hold the density of tentiform mines below one per leaf in the second generation. Pesticide use in apple, Malus domestica Borkhausen, orchards could be reduced if thresholds were revised to reflect levels of mortality from parasitism in the first generation. To achieve this will require training growers and scouts to recognize mines whose larvae have emerged as adult moths versus ones whose inhabitants have died from parasitism or other causes. Information on parasitism should be corrected for the obscuring effect of mortality other than parasitism through the calculation of marginal attack rates (Elkinton et al. 1992). The question “how much parasitism is enough?” can be answered by using population growth rates (R_o) which can be calculated from life tables. By using hypothetical life tables given here, we have calculated R_o values for a P. crataegella population both with and without parasitism.