|PAULSEN, C - University Of Georgia|
|RUBERSON, J - University Of Georgia|
Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2012
Publication Date: 12/7/2012
Citation: Palsen, C.M., Cottrell, T.E., Ruberson, J.R. 2012. Distribution of the black pecan aphid, Melanocallis caryaefoliae, on the upper and lower surface of pecan foliage. Entomologia Experimentalis et Applicata. 146:252-260.
Interpretive Summary: Black pecan aphids have been observed in large numbers on the tops of pecan leaves, which is unusual. The leaf top is exposed to more hazards than the underside, so other tree aphids, including the other pecan-feeding aphids, mainly settle on the underside. Field surveys and lab experiments were performed to see how often the black pecan aphids and other pecan aphids settle on the leaf top, and to see if predators or crowding can explain the pattern of black pecan aphid settling. In the field, black pecan aphid nymphs settled nearly equally on both leaf surfaces, but black pecan aphid adults settled mainly on the underside. Both adults and nymphs of other pecan aphids settled mainly on the underside. The same pattern was observed in the lab, with nymphs on potted pecan seedlings. In the field, lady beetle larvae were seen about equally on both leaf surfaces, and lacewing larvae were seen more often on the underside. It is possible that black pecan aphid nymphs settle on the upper leaf surface because fewer aphid predators search there. In the lab, black pecan aphid nymphs moved to the upper leaf surface more often when other pecan-feeding aphids were denser on the lower leaf surface, but black pecan aphids still fed on the upper surface when they were the only species present, regardless of their density. Crowding has an effect on black pecan aphid movement, but it is not the main reason why they feed on the upper leaf surface.
Technical Abstract: Three aphid species regularly feed on the foliage of pecan: the black pecan aphid Melanocallis caryaefoliae (Davis), the yellow pecan aphid Monelliopsis pecanis (Davis), and the blackmargined aphid Monellia caryella (Fitch). The black pecan aphid appears unique among these for frequently being observed feeding on the upper surface of pecan leaves. This is risky behavior, given the environmental hazards associated with the upper surface. Field surveys and laboratory experiments were performed to confirm the distribution of black pecan aphids, and to investigate density and predation as potential causes. An orchard survey found all three aphid species and stages predominantly on the lower leaf surface, except for the black pecan aphid nymphs, which were evenly distributed between upper and lower leaf surfaces. This survey also found aphidophagous lacewing larvae predominately on the lower leaf surface, while lady beetle larvae were evenly distributed between upper and lower surfaces. These distributions are consistent with the hypothesis that black pecan aphids’ settling on the upper leaf surface may be a strategy of reducing encounter rates with natural enemies. Observations of manipulated aphid nymphs on laboratory pecan seedlings revealed nymph distributions consistent with field observations. Black pecan aphid nymph movement to the upper leaf surface correlated with the density of other aphid species, but with other aphids absent the black pecan aphids still fed on the upper surface, regardless of conspecific density. Crowding-induced dispersal may influence nymph distribution, but it is not the primary cause of nymph movement to the upper leaf surface.