An academic article published by Larry J. Zettler and Dennis W. Keever as early as June 1994 in the Journal of Economic Entomology (vol.87 issue 3) indeed reported that out of 21 field strains of adult cigarette beetles collected from tobacco storages in the southeastern United States between July 1990 and October 1991, 11 strains showed resistance to phosphine fumigation at a concentration of 125 ppm, their survival rates ranging from 2 to 97%.
Although the report conceded that “no control failures have been attributed yet to phosphine resistance in the cigarette beetle,” it also noted that “the frequency and intensity of resistance suggest that normal fumigation techniques will become increasingly less effective."
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A more recent study published in 2005 in the Japanese journal Applied Entomology and Zoology (vol.40 issue 1) and authored by Hori Masatoshi and Kasaishi Yoshihiro related how six strains of the adult beetle were exposed to phosphine in a controlled lab setting. According to that study, one strain showed high resistance with almost all beetles surviving at even 1.000 ppm phosphine exposure. Nevertheless, the remaining strains demonstrated high susceptibility to medium resistance under treatment of up to 300 ppm. The report noted: “The probit regression line for the middle resistant strain lay between that of the (most) susceptible strain and that of the highly resistant strain, although it was nearer to that of the susceptible strain.”
The newest study of this phenomena was conducted by the United States Department of Agriculture’s Agricultural Research Service (USDA ARS). During this as of yet unpublished study on highly-tolerant insects, it was determined that control can still be achieved when phosphine gas concentrations are maintained between 500 and 1,000ppm (with 750ppm being the so-called “sweet spot” for optimal efficacy) over the course of at least 3 days at 21°C (70°F) or above.
It is important to note in this context that the “sweet spot” for a given species is stable and does not change for relatively tolerant populations. The study’s findings also clearly indicate that the key to successful phosphine fumigations is maintaining the concentration as close to the “sweet spot” as possible over the duration of the fumigation. If the presence of tolerant insects is suspected, the duration of the fumigation should be extended, with an effort made to increase temperature if possible.
Based on all these reports, one could probably argue that both sides of our article contributors were right in their assessments and that it can be chalked down to semantics whether to use the term “resistance” or “tolerance”. One thing seems to be clear, though: More and more strains of the beetle appear to become decreasingly susceptible to conventional phosphine treatment. This fact even prompted CORESTA in its Guideline No. 2 (“Phosphine Fumigation Parameters for the Control of Cigarette Beetle and Tobacco Moth”, revised version of October 2013) to advise that “…all possible efforts must be made to enforce good fumigation practices to control infestation and minimize the development and spread of phosphine resistance.”