The host-parasite interaction between the black pecan aphid (BPA) [Mel
anocallis caryaefoliae (Davis)] and pecan [Carya illinoensis (Wangenh.
) K. Koch] was investigated. Three years of field observations of the
ability of BPA populations to induce chlorotic blotches, or visual dam
age, on 32 pecan cultivars revealed considerable variation in cultivar
susceptibility to BPA damage. Among the most commonly grown cultivars
, 'Sioux','Cape Fear', 'Farley', 'Cowley', 'Grabohls', and 'Barton' ex
hibited the least damage, whereas 'Choctaw','Oconee', and 'Sumner' exh
ibited the greatest, with 'Sioux' and 'Choctaw' exhibiting the greates
t extremes in susceptibility. Subsequent evaluation indicated that the
foliage of pecan genotypes can exhibit an antibiotic-like effect, res
ulting in the suppression of resident BPA populations, However, the re
lationship between the degree of this antibiotic effect and the degree
of damage exhibited by trees, or field tolerance, was negligible (r =
-0,10), For example, while 'Choctaw' foliage greatly suppressed BPA p
opulation growth, this population was able to inflict relatively sever
e damage to leaves. An evaluation of feeding preference indicated that
BPA alate viviparae (winged females) preferentially feed upon host cu
ltivars on which they have been previously feeding. This feeding prefe
rence was eliminated by rinsing leaves with distilled water; hence, a
water soluble factor(s) appears to be involved in host preference.