Distribution of the black pecan aphid, Melanocallis caryaefoliae, on the upper and lower surface of pecan foliage

Three aphid species regularly feed on pecan [Carya illinoinensis (Wangenh.) K. Koch (Juglandaceae)] foliage: the black pecan aphid, Melanocallis caryaefoliae (Davis), the yellow pecan aphid, Monelliopsis pecanis Bissell, and the blackmargined aphid, Monellia caryella (Fitch) (all Hemiptera: Aphididae). Adults of M. caryaefoliae and both the nymphs and adults of M. pecanis and M. caryella mainly feed on the lower surface of leaves. Nymphs of M. caryaefoliae appear unique by frequently feeding on the upper surface of pecan leaves. This is risky behavior given the environmental hazards (e.g., rain, solar radiation, and dislodgement) associated with the upper surface. Thus, we determined the leaf surface distribution of M. caryaefoliae on trees in an orchard and on pecan seedlings in the laboratory. A pecan orchard survey found all three aphid species and stages predominantly on the lower leaf surface, except for the nymphs of M. caryaefoliae, which were evenly distributed between upper and lower leaf surfaces. This survey also found aphidophagous lacewing (Neuroptera) larvae predominantly on the lower leaf surface, whereas ladybird beetle (Coleoptera: Coccinellidae) larvae were more evenly distributed between upper and lower surfaces. Laboratory experiments using single or multiple pecan aphid species revealed M. caryaefoliae distribution on pecan seedlings similar to orchard data. Nymphal M. caryaefoliae require nearly 2 days to elicit chlorotic feeding lesions on leaves; without these lesions, nymphal development is hindered. The similar distribution of nymphs of M. caryaefoliae on both leaf surfaces likely reflects a strategy of predator avoidance allowing a proportion of the population to survive.