Seasonal variability of sex-ratio in Ips typographus (L.) pheromone traps in a multivoltine population in the Southern Alps

Abstract

The aim of the study was to monitor the seasonal variation in the sex-specific responses of Ips typographus to a commercial pheromone in relation to the two generations per year occurring in the Southern Alps. The study was carried out over 2 years (2000–2001). Each year, 15 Theysohn traps were baited with Pheroprax. Traps were emptied weekly and the number of beetles determined. In the same forest each year, three spruce logs were set up at the beginning of spring for natural colonisation by Ips typographus and then put in breeding boxes. All adults emerging from the logs or found in the traps were counted and a subsample sexed by genital extraction. Spring captures were always higher than summer captures (second generation). Similarly, the proportion of trapped males was always higher in spring than in summer. Except in the first and second week, the number of females was higher than males. During the first 3 weeks of monitoring, the number of trapped males dropped from above 50 to about 30%. Thereafter, the sex ratio was constant over time, about 33% males. In contrast to trap catches, about 50% of adults emerging from the breeding logs were males. The low proportion of males in traps depends both on discrimination (they avoid strong sources of male pheromone) and on differential mortality of the sex over time. As the pioneer beetles in I. typographus are males, their number is highest in the first part of the flight period when they search for suitable host trees. Such a search can be lethal for several males, decreasing the sex ratio.