Resistance of alfalfa to spotted alfalfa aphid in relation to environmental factors

Resistance in alfalfa to the spotted alfalfa aphid, Therioaphis maculata (Buckton), was studied in relation to several environmental factors. Two resistant parental clones of the variety Lahontan were studied in comparison with a susceptible variety, Caliverde. The two Lahontan clones showed a lower degree of resistance at low than at high temperatures. One clone, C-902, appeared to be completely susceptible at 50° and 60° F, but became progressively more resistant at higher temperatures. The other clone, C-84, maintained at least an intermediate degree of resistance even at 50° F, and was usually immune to aphid infestation at temperatures of 68° F or higher. The decrease in expression of resistance at low temperatures was shown to be due mainly to a change in the resistant plant, which in turn affected aphid survival and reproduction. Only a few days’ time was required for resistance to become reduced at low temperatures or regained at high temperatures. Under controlled conditions of alternating high and low temperatures, the mean rather than the high or the low temperature appeared to be the most important in controlling the degree of resistance expressed. Differences in photoperiod apparently had no effect on aphid survival and reproduction. C-84 and C-902 plants watered with phosphorus- or potassium-deficient nutrient solutions were significantly different in resistance from plants watered with complete nutrient solutions. Phosphorus-deficient plants became more resistant, and potassium-deficient plants became less resistant. Susceptibility of Caliverde plants was not affected.