Tri-Trophic Interactions of Cabbage Cultivars, a Generalist Aphid, Myzus persicae (Sulzer) and the Braconid Diaeretiella rapae McIntosh

The tri-trophic interactions among five cabbage (Brassica oleracea var. capitata) cultivars—Copenhagen Market (CM), Red Acre (RA), Vertus Savoy (VS), Sugarloaf (SL) and Danish Ballhead (DBH)—a generalist aphid, Myzus persicae (Sulzer) and its parasitoid, Diaeretiella rapae McIntosh, were assessed. Copenhagen Market, known for its reported aphid resistance, recorded the fewest aphids (9.11 per plant; F = 6.50; df = 4, p < 0.01), followed closely by Red Acre (11.83 per plant), which was statistically similar. Upon aphid attack, all cultivars produced plant volatiles that influenced the attraction of the parasitoid, D. rapae. Copenhagen Market consistently showed a higher response across cultivar combinations, attracting 62.59%–67.50% of parasitoids. It also recorded a higher parasitisation rate (39.9%; F = 5.94; df = 4/20, p < 0.01) in a multiple-choice test. The results indicate that cabbage cultivars employ both constitutive and induced resistance, as aphid-resistant cultivars were also found to be more attractive to parasitoids. In addition to plant odour cues, factors such as plant colour, texture and host aphid kairomones can influence the orientation and parasitisation by D. rapae.