Malathion tolerance of the rose-grain aphid Metopolophium dirhodum (Walker) reared on the ancient wheat variety Emmer (Triticum dicoccum L.) was tested by topical bioassay and found to be lower than that of aphids reared on the modern more susceptible variety Marts Kinsman (T. aestivum L.). On both varieties, tolerance was greater at stem elongation than at the tillering stage of plant growth. All statistically significant differences when plants were treated with Cycocel and MCPA showed that the two chemicals, respectively, reduced and increased aphid tolerance to malathion, in keeping with the effects of the chemicals on induced plant resistance to aphids reported in the literature. Probit mortality slopes were not progressively altered by changes in LC50, so that similar differences in aphid tolerance to malathion were found at both LC59 and LC90 levels.

The effects of established phenomena of plant resistance on the levels of tolerance to malathion of an organophosphate resistant strain of Myzus persicae were investigated using topical application insecticide bioassay. Tolerance was significantly decreased when aphids were reared on the moderately aphid-resistant Brussels sprout variety Early Half Tall or on sprout plants given reduced nitrogen fertilization. Differences in insecticide tolerance were found to occur only at older plant ages. It is argued that the differences (less than 50% in LC50 terms) reported are of practical importance, and attention is drawn to much greater differences (5-fold) obtained in experiments at different times of the year.