RESPONSE TO METHYL-BROMIDE FUMIGATION OF CODLING MOTH (LEPIDOPTERA, TORTRICIDAE) EGGS ON CHERRIES

Fumigation experiments with methyl bromide (MeBr) were done in New Zealand over the summers at the end of 1987 and 1989. A total of 74,033 freshly laid codling moth, Cydia pomonella (L.), eggs on the cherry cultivars Dawson, Bing, Rainier, Sam, and Lambert were used. We found differences in sorption of methyl bromide between cultivars and between seasons. Large differences in results for 'Sam' between the two seasons suggest that the condition of the fruit or another factor associated with season may be more important than cultivar. The logit of mortality induced by treatment varied linearly with a power (almost-equal-to 2) of the concentration-time (c x t) sum. This relationship allowed us to predict accurately the LC50 and LC99 for the c x t sum. The sorption curves of the individual season-cultivar combinations were not relevant to this relationship, which was the same for all cultivars and seasons. We also considered other measures of concentration. A straight-line equation, with a constant term that depends on the cultivar and the season, allowed accurate calibration of c x t sums in relation to the dose levels injected. Lethal dose levels of 99%, although differing among season-cultivar combinations, were all at least 10 g/m3 below the injected dose of methyl bromide (64 g/m3, 2 h, 40% load, 12-degrees-C) used in commercial cherry fumigations. The commercial treatment thus affords a high level of security. Our results indicate that log (concentration) should not necessarily be used as the explanatory variable, as has been conventional when data from fumigation experiments are subjected to probit analysis.