Woglum, R.S. 1925 Observations on insects developing immunity to insecticides.. Journal of Economic Entomology 18: 593-597.

Notes: The development of increased immunity to hydrocyanic acid gas by the red scale and black scale in certain districts of California has become so pronounced that a single treatment even at a greatly increased dosage no longer can be depended upon to keep these pests in control. Species mentioned include Chrysomphalus aurantii and Saissetia oleae.