Comparative Susceptibility of Two Strains of California Red Scale to HCN, with Special Reference to the Inheritance of Resistance

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Abstract
A study of strains of Calif. red scale, Aonidiella aurantii differing widely in susceptibility to HCN which were obtained from 2 sources and reared on potted lemon trees in adjacent compartments of the insectary was made. For > 6 yrs. there was a considerably higher kill of the nonresistant than the resistant strain in each developmental stage tested and in both sexes. There was no evidence that susceptibility between strains had changed during this period. Difference between the strains was not due to permeability of the wax covering although its removal before fumigation increased the kill. 5 generations of hybrid stocks obtained from reciprocal crosses of the resistant and nonresistant strains were fumigated and the mortality was compared with that resulting from fumigation of the corresponding generation of resistant and nonresistant strains. In the F1 of the reciprocal crosses the [male] [male] were like the mother parent with regard to susceptibility, suggesting that the gene for resistance is sex-linked, but the [female] [female] were more like the resistant than the nonresistant strain, suggesting that resistance is incompletely dominant. Further crosses, recrosses, and back-crosses verified these findings. The data indicated scales nonresistant to hydrocyanic acid fumigation were present in the resistant strain and probably insects resistant to fumigation were present in the nonresistant strain. Repeated fumigations of the resistant strain increased its relative resistance. Similarly, the susceptibility of the non-resistant strain was somewhat decreased by one fumigation.