Does the onset of sexual maturation terminate the expression of migratory behaviour in moths? A study of the oriental armyworm, Mythimna separata Xin Cheng Zhao a , Hong Qiang Feng b , Bo Wu a , Xian Fu Wu a , Zhong Fang Liu a , Kong Ming Wu a, *, Jeremy N. McNeil c a State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, West Yuanmingyuan Road, Beijing 100193, PR China b Institute of Plant Protection, Henan Academy of Agricultural Sciences, Zhengzhou 450002, PR China c Department of Biology, The University of Western Ontario, London, ON N6A 5B7, Canada 1. Introduction Insects have evolved two general strategies to cope with predictable or unpredictable changes in habitat quality, diapause or migration, which have been referred to as the ‘‘here later’’ and ‘‘there now’’ strategies (Southwood, 1977; Solbreck, 1978; Dingle, 1996). The observation that emigration is generally initiated by sexually immature individuals led to the formulation of ‘‘oogen- esis-flight syndrome’’ (Johnson, 1963), where reproduction and migration were considered alternate and ‘‘incompatible’’ physio- logical states. However, the actual relationship between repro- ductive status and the expression of migratory behaviour is not quite so clear-cut (Rankin et al., 1986; Sappington and Showers, 1992; Gatehouse and Zhang, 1995; McNeil et al., 1995). There is considerable support for the idea that in the Lepidoptera the onset of migratory behaviour is initiated by sexually immature adults (see Table 1 in Gatehouse and Zhang, 1995), However, this is not always the case for in the spruce budworm, Choristoneura fumiferana, the majority of females migrating out of heavily defoliated sites are mated and have already laid a certain proportion of their egg complement (Greenbank et al., 1980). This may be a bet hedging strategy against the possibility of failing to find more suitable oviposition sites elsewhere, but it may also be the result of physical limitations on flight as females with a full complement of eggs are very poor flyers (McNeil, personal observations). The southward fall migration of the monarch butterfly, Danaus plexippus, is undertaken by unmated individuals that remain in reproductive diapause at the overwintering sites in Mexico. However, while many of those emigrating northwards in the spring as still virgins, a certain proportion of the females are mated (Herman, 1985; Rankin, 1987). Similar patterns have been reported for a variety of migratory noctuid species, with a small proportion of migrants having some degree of ovarian maturation and, on occasion, being mated (see Table 2 in Gatehouse and Zhang, 1995). There are, however, several cases for noctuids where the incidence of mating was >25% in individuals presumed to be migrating (e.g. Callahan et al., 1972; Sparks et al., 1975; Sappington and Showers, 1992). Their trapping results for Agrotis ipsilon, together with additional tethered flight data of different aged Journal of Insect Physiology 55 (2009) 1039–1043 ARTICLE INFO Article history: Received 16 May 2009 Received in revised form 19 July 2009 Accepted 20 July 2009 Keywords: Mythimna separata Armyworm Searchlight trapping Migration Sexual maturation ABSTRACT It is generally accepted that in most insects adults are sexually immature when they initiate migration and that migratory behaviour terminates with the onset of sexual maturation. However, a few studies examining the mating status of field collected moths have suggested that sexually mature individuals may continue migrating, but in these cases it was impossible to completely eliminate the possibility that the mated females captured came from local, non-migrant populations. In this study we examined the ovarian development of Mythimna separata females captured using a vertical pointing searchlight trap on Beihuang Island in the Bohai Gulf, China, a site >40 km from land. Moths were collected from May to October from 2003 to 2008 in order to test the hypothesis that the onset of sexual maturation resulted in the termination of migratory behaviour. While females at the end of the summer had little ovarian development and were unmated, a significant proportion of those migrating northward in the early summer had developed ovaries and often had at least one spermatophore. Given that theses insects were captured while flying up to 500 m above sea level, at a site with no local populations, the findings would not support the hypothesis and suggest that both ovarian development and mating may occur during migration. ß 2009 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +86 13601076403; fax: +86 10 62896114. E-mail address: kmwu@ippcaas.cn (K.M. Wu). Contents lists available at ScienceDirect Journal of Insect Physiology journal homepage: www.elsevier.com/locate/jinsphys 0022-1910/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jinsphys.2009.07.007