Pesticide susceptibility of potato tuberworm in the Pacific Northwest

Chad H. Dobie

The purpose of this study was to determine the baseline susceptibility of potato tuberworm, Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae), in the Pacific Northwest to insecticide exposure. Potato tuberworm (PTW) is a major economic pest in all major potato production areas of the world. Control through the use of insecticides is an effective, inexpensive, and therefore popular pest management method utilized by growers. In this study, the lethal concentrations and lethal times (LC and LT, respectively) were determined using potato leaf bioassays. The LC studies evaluated mortality levels after subjecting the neonate larvae to a range of concentrations of the insecticides methamidophos (Monitor), indoxacarb (Avaunt), spinetoram (Delegate), and esfenvalerate (Asana). Bioassays were evaluated at three and 24 hours after larvae were placed on treated leaf disk surfaces. The dose-response results for each insecticide were transformed to units of mass active ingredient (a.i.) per acre so that bioactivity could be compared to recommended product label application rates. LC50 values of 29.2 x 10-3 lb a.i./acre and 23.1 x 10-3 lb a.i./acre were estimated using probit analysis for 3- and 24-h exposures, respectively, of PTW larvae to methamidophos on treated leaf disks. PTW larvae responded to 3- and 24-h exposures of indoxacarb with estimated LC50 values of 1.43 x 10-3 lb a.i./acre and 0.026 x 10-3 lb a.i./acre, respectively. LC50 values of 2.84 x 10- 3 lb a.i./acre and 0.148 x 10-3 lb a.i./acre were estimated for 3- and 24-h exposures, respectively, of PTW larvae to spinetoram. LC50 response of PTW larvae to esfenvalerate following 3- and 24-h exposures was estimated to be 0.013 x 10-3 lb a.i./acre and 0.006 x 10-3 lb a.i./acre, respectively. The LC50 estimates for exposure of PTW larvae to all insecticides ranged from at least one order of magnitude to three orders of magnitude less than product label application rates. Larval mortality was observed at increasing time intervals of exposure to leaf disks treated with insecticides at rates equivalent to their estimated LC95. Larvae died within two hours after exposure to all insecticides, but onset of lethality varied somewhat among the tested compounds. Time to larval death was slowest for methamidophos exposure with estimated LT50 and LT95 of 75 min and 146 min, respectively. Indoxacarb results were similar to methamidophos with LT50 and LT95 values of 53 min and 159 min, respectively. Spinetoram caused mortality much faster than both indoxacarb and methamidophos. The LT50 and LT95 values were 26 min and 58 min, respectively. PTW larvae died most quickly following esfenvalerate exposures with estimated LT50 and LT95 values of 11 min and 25 min, respectively. Based on this study, low concentrations of esfenvalerate and spinetoram caused high levels of mortality in the shortest amount of time. Indoxacarb and methamidophos caused sufficient morality but were comparatively slower acting. To predict the persistence of bioactivity under field conditions, literaturederived half-lives for the four insecticides were used to model decline in mortality based on the dose-response data from the probit analyses. Methamidophos, indoxacarb, and esfenvalerate were predicted to cause 95-100% mortality for about five days after a theoretical application of the maximum recommended product label rate. In contrast, spinetoram associated mortality was predicted to decline to about 80% within two days after application.

Pesticide susceptibility of potato tuberworm in the Pacific Northwest

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