Oecologia (2003) 135:322–326 DOI 10.1007/s00442-002-1160-9 BEHAVIOURAL ECOLOGY Michle Roy · Jacques Brodeur · Conrad Cloutier Temperature and sex allocation in a spider mite Received: 5 September 2001 / Accepted: 27 November 2002 / Published online: 26 February 2003  Springer-Verlag 2003 Abstract Although temperature is the most important environmental factor regulating arthropod development and reproduction, its influence on sex allocation in haplodiploid arthropods remains largely unexplored. We investigated under laboratory conditions how maternal age and temperature mediate offspring sex ratio of the spider mite Tetranychus mcdanieli (Acari: Tetranychi- dae). Over nine temperature regimes, female-biased sex ratios were consistently observed, varying from 57 to 87% among progeny produced over lifetime. Spider mite sex ratio was affected by maternal age: more male progeny were produced at both the beginning and the end of the female lifespan, yielding a dome shaped curvilinear relationship. This pattern of variation with age probably results from constraints on using sperm at young ages and sperm depletion or viability at older ages. We found a significant curvilinear relationship between temperature and sex ratio. The proportion of female offspring was lowest at intermediate temperatures and highest at extreme temperatures. We suggest that increased fe- male-biased sex ratio at extreme temperatures is an evolutionary response of spider mites to deteriorating habitats as, in the Tetranychidae, females have better capacities than males to disperse and survive under harsh conditions. Keywords Haplodiploidy · Maternal age · Sex allocation · Temperature · Tetranychidae Introduction An important aspect of the study of sex allocation in haplodiploid arthropods is to explore selective factors leading to a biased sex ratio. Two main forms of selection have been generally identified. First, sex allocation is a typical case of frequency-dependent selection where the value of one sex changes with its frequency of occurrence in a population. The best documented case concerns the evolution of female-biased sex ratios in species where mating frequently occurs between siblings. In this instance, local mate competition models predict that females should overproduce daughters when their broth- ers compete for mates (Hamilton 1967; Charnov 1982). Second, sex allocation is affected by individual selection, which arises whenever the value of one sex varies under particular ecological conditions. For example, in para- sitoids, host quality models predict that female eggs should be laid in relatively large hosts when individual fitness increases with size more sharply in females than in males (Charnov 1979; Waage 1986). Sex ratio is a phenotypic trait that is likely to be affected by abiotic factors such as temperature, humidity, photoperiod, and light conditions (Pianka 1988; Wrensch 1993). Temperature is the most important environmental factor regulating arthropod development and reproduc- tion. Variation in temperature can also be a reliable indicator of habitat quality and trigger the production of individuals with differential abilities to reproduce, dis- perse, or enter dormant stages (Wellington et al. 1999). The influence of temperature on sex allocation in haplodiploid insects and mites remains largely unexplored (King 1987; Wrensch 1993) . Detailed studies are too few to reach conclusions and describe patterns about the relative importance and adaptive significance, if any, of temperature as a determinant of sex allocation. M. Roy Direction des services technologiques, Ministre de l’Agriculture, des PÞcheries et de l’Alimentation du QuØbec, 2700 rue Einstein, Sainte-Foy, Quebec, G1P 3W8, Canada J. Brodeur ( ) ) Centre de Recherche en Horticulture, DØpartement de Phytologie, UniversitØ Laval, Sainte-Foy, Quebec, G1K 7P4, Canada e-mail: jacques.brodeur@plg.ulaval.ca Tel.: +1-418-6562518 Fax: +1-418-6567856 C. Cloutier Centre de Recherche en Horticulture, DØpartement de Biologie, UniversitØ Laval, Sainte-Foy, Quebec, G1K 7P4, Canada