Distribution of the Black Pecan Aphid on Pecan Leaf Surfaces: an Overview

Three species of aphids regularly attack pecan, Carya illinoinensis, foliage. Two of them, i.e., the blackmargined aphid and the yellow pecan aphid, are predominantly distributed on the lower leaf surface, as are adults and nymphs of most tree-feeding aphid species. This surface provides aphids protection from rain, solar radiation, honeydew dropped by aphids on leaves above and the protruding leaf veins of the lower leaf surface protect from dislodgement by the movement and abrasion of other leaves. However, nymphs of the third species, i.e., black pecan aphid (BPA), do not follow this distribution pattern. Although adult BPA are mostly found on the lower surface, nymphs are distributed about equally between both leaf surfaces. BPA nymphs need to remain stationary to elicit chlorotic feeding lesions and this may explain their movement to the upper leaf surface. Natural enemies spending more time searching the lower surface coupled with BPA nymphs staying in one location to feed provides the best explanation for the observed distribution of BPA nymphs and adults. BPA nymphs that do not remain stationary to elicit chlorotic lesions face higher mortality (in the absence of predators), take longer to develop and result in smaller adults. Thus, BPA nymphs likely move to the upper leaf surface as a predator avoidance strategy.