The efficacy of a fumigant is determined by the gas concentration and exposure time. Typically, the gas concentration and exposure time are equally important in fumigation efficacy and their relationship is expressed as Ct = k, where C is the concentration, t is the time and k is a constant (Haber’s rule). However, deviation from Haber’s rule, i.e., the relative importance of time over concentration, has been reported for several stored-product pests, and a modified form (Cn t = k) is used to express the relationship between C and t. This study aimed to determine if this modified form could be applied to phosphine fumigation of the cigarette beetle, Lasioderma serricorne (F.). Adult beetles from six cultures with different phosphine-resistance levels were exposed to phosphine (1–2000 ppm) at 25 °C for either 24, 72, 120, or 168 h, and the phosphine concentration necessary to achieve 50% lethality (LC50) was calculated. The phosphine concentration (C) and the exposure time (t) relationship at the LC50 for all six cultures were determined by regression analyses. The equation Cn t = k was a good fit for all the cultures tested, irrespective of phosphine resistance. The results showed the exponent n was less than 1 (0.51 ≤ n ≤ 0.71, coefficient of determination 0.90–0.99), suggesting that time is more important than concentration in toxicity expression of phosphine against L. serricorne. Therefore, extending the exposure time rather than increasing the concentration should be emphasized to manage the future threat of resistance problems with phosphine.
