Seasonal variation in recapture of mass-reared sterile codling moth, <i>Cydia pomonella</i> (L.) (Lepidoptera: Tortricidae): implications for control by sterile insect technique in British Columbia

Abstract

In 1992, the Okanagan-Kootenay Sterile Insect Release (SIR) Programme was initiated to eradicate codling moth, Cydia pomonella (L.), from montane, fruit-growing valleys in British Columbia (BC), Canada. Excessive damage in 1994, and failures to maintain sterile:wild (S:W) over- flooding moth ratios at 40:1, a target deemed necessary for eradication, led to concern about activity of sterile moths and recommendations to supplement control in spring. Using pheromone-baited wing traps and passive sticky pane traps we monitored operational S:W ratios to determine if they continued to fall below 40:1 post-1994. Seasonal flight activity and recapture of sterile moths was compared with that of wild moths from 7 May - 1 September, 1995 - 1999, in nine commercial orchards in Cawston, BC. Mean weekly catches of wild males in pheromone traps, reflected first- (May) and second-generation (August) peaks of flight activity in orchards supplemented with pheromone disruption, but only a single period of activity in insecticide supplemented orchards. Weekly catches of sterile moths in these same orchards were always at their lowest in spring, and activity was correlated with seasonal air temperatures. Yearly average S:W ratios in the insecticide-treated orchards ranged from 24:1 - 203:1 in 1995 - 1997. Examining S:W ratios using data from those weeks when wild moths were actually caught, indicates ratios were frequently (29 - 91%) less than 40:1 in spring but S:W ratios fell below 40:1 less often during summer than spring. Passive pane traps also revealed patterns of fewer sterile moth catches, and lower S:W ratios in spring, compared with summer. Our data suggest low overflooding ratios contributed to slower than predicted population reductions, and increased release of sterile moths, of improved quality, between 1995 and 1997 did not significantly increase mean weekly catches or S:W ratios in individual orchards in spring. Therefore, continued application of supplemental insecticides, or a pheromone disruption treatment that reduced catch of moths, but did not significantly affect S:W ratios in spring, is recommended. We conclude that similar analysis of trap data for the entire SIR Programme (1994 - 2004) and correlations with damage would provide recommendations for the best use of sterile insects as part of any future area-wide codling moth management programme.