The Effect of Predation by Blue-Tits (Parus caeruleus) on the Sex-Ratio of Codling Moth (Cydia pomonella)

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Abstract
In an [apple] orchard where tits (Parus spp.) ate 95% of the overwintering codling moth larvae, only 35% of the surviving moths were females. In the absence of bird predation, equal numbers of each sex emerged as adult moths. Among larvae reared from eggs laid by wild moths, there were approximately equal numbers of each sex. In an aviary, blue tits took more female than male larvae from cocoons beneath the bark of logs. The greater susceptibility of female larvae was not associated with their greater average weight: when tits preyed on male and female larvae of similar weights the difference in predation remained highly significant. When they fed on heavy females and lightweight males, tits did not take significantly more females. Female larvae probably are eaten preferentially because they occupy more exposed positions. Male larvae were better able to squeeze into hiding places than females, because males have narrower head capsules: 4 times more male larvae were able to squeeze through holes of 1.6 mm diameter, but this ability was not related to larval weight. Since the number of male moths caught in pheromone traps is affected by the sex-ratio of moths in the orchard, it should be possible to tolerate higher catches in orchards where tits are important predators of overwintering larvae.