A Study of Fumigation Problems: “Protective Stupefaction”, Its Application and Limitations

In connection with the fumigation of citrus trees with hydrocyanic acid for the control of scale insect pests, it is shown that certain of these scale insects when first exposed to sublethal concentrations of HCN become stupefied and consequently more resistant to the action of HCN than do scale insects not so stupefied. Extensive experiments show that an exposure of more than one minute, but less than three minutes, to such sublethal concentrations is necessary, however, to cause this increased resistance. Studies of the concentration and distribution of HCN under fumigation tents show that under summer night fumigation conditions uniform distribution of HCN is accomplished in a short enough period of time to prevent any protection by stupefaction when HCN is applied either with an atomizer or vaporizer, but that under winter daylight conditions, the atomizing pump does not produce sufficiently rapid distribution of HCN to prevent this phenomenon. At any time when the temperature of the ground under the tent is lower than the temperature of the air above and consequently no convection currents are present, it is necessary to use the vaporizer to obtain quick and uniform distribution of HCN.