Methyl Bromide Residues and Desorption Rates from Unshelled Walnuts Fumigated with a Quarantine Treatment for Codling Moth (Lepidoptera: Tortricidae)

Abstract

California walnuts were fumigated unshelled with a quarantine treatment to control codling moth, Cydia pomonella (L.). The treatment was done with 56 g/m3 methyl bromide for 4 h at 15.6°C and a chamber pressure of 100 mm Hg. There were no significant differences in organic or inorganic bromide residues regardless of walnut cultivar or size. Inorganic residues were below the established tolerance level of 200 ppm. The ‘Eureka’ cultivar, although not significantly different in its desorption rate of residual methyl bromide, had higher organic residues than the other cultivars tested. Residue levels in treated nut meats showed no significant change in inorganic bromide content over a 25-d period. Accumulated inorganic bromide residues in nut meats fumigated once or twice with a domestic methyl bromide schedule (56 g/m3 for 24 h at 15.6°C) to control field infestation and stored-product insects followed by fumigation with the quarantine treatment did not exceed the established tolerance level. Residual methyl bromide in treated nut meats stored unshelled at 1.7 or 10°C was ≤10 ppb after 70 or 53 d, respectively, whereas those stored at 21 or 32°C had ≤10 ppb after 20 or 14 d, respectively.