Toxicity of Hydrocyanic Acid to Eggs of Black Scale (Saissetia oleae (Bern.))1

Abstract

HCN vapor proved highly toxic to the eggs of black scale at short exposures and in concs. suitable for fumigation of citrus trees. Winter eggs are less susceptible than summer eggs, but much higher concs. of HCN can be used in winter fumigation than in summer. The minimum normal dosage for winter fumigation is 20 cc. per unit, which is equivalent to the maximum conc. used in these expts.; the normal summer dosages are 16 cc. and 18 cc. per unit, which are equivalent to the 0.08 and 0.09% HCN concs. used. At these concs. no eggs hatched in any of the expts. after June 4, which was 6 or 7 weeks earlier than commercial fumigation started in any of the localities from which eggs for these expts. were obtained. The fact that the difference in susceptibility to HCN of black scale from different localities is not carried over into the eggs and the further evidence of the high toxicity to the eggs of HCN at concs. usable in field fumigation indicate that satisfactory control of "resistant" black scale might be secured by earlier season fumigation.