Susceptibility of pecan to black pecan aphids

The host-parasite interaction between the black pecan aphid (BPA) [Melanocallis 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 damage, 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' exhibited the least damage, whereas 'Choctaw','Oconee', and 'Sumner' exhibited the greatest, with 'Sioux' and 'Choctaw' exhibiting the greatest extremes in susceptibility. Subsequent evaluation indicated that the foliage of pecan genotypes can exhibit an antibiotic-like effect, resulting in the suppression of resident BPA populations, However, the relationship 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 population growth, this population was able to inflict relatively severe damage to leaves. An evaluation of feeding preference indicated that BPA alate viviparae (winged females) preferentially feed upon host cultivars on which they have been previously feeding. This feeding preference was eliminated by rinsing leaves with distilled water; hence, a water soluble factor(s) appears to be involved in host preference.