Striking varietal differences in susceptibility to attack by the lettuce root aphid, Pemphigus bursarius L., were first found in lettuces grown at Wellesbourne in 1955. Subsequent work has confirmed that White Favourite and Imperials E-4 and 1955 I-M are highly resistant; Imperial 45634-M, Continuity and Iceberg are markedly resistant. Midas, Salad Bowl and Imperial 4164 each appear to be a mixture of susceptible and resistant plants. Immigrant winged forms of P. bursarius showed no preference for colonizing any particular variety of lettuce, and it seems that resistance to attack results from antibiosis. It is suggested that varietal differences in the composition of root sap may account for the differences in susceptibility to lettuce root aphid, but analyses of water and alcohol extracts of root sap from resistant and susceptible lettuce varieties have not shown consistent differences.
