Although the Silver Y moth, Autographa gamma, is recognised as one of Britain's commonest migrant insects, little is known of its exact habitat requirements. This study attempts to define these requirements more clearly by assessing foodplant preferences and norms of reaction to environmental parameters under controlled conditions. The high level of polyphagy found in this species is produced by a female oviposition strategy which utilises many hostplants, high larval mobility and an ability to grow to maturity on a wide range of plant species. Selective pressures leading to polyphagy are discussed. Reduced growth rates and adult size were produced by different foodplants, larval density and temperature changes. The size changes are due to a subtle interaction between growth rates and the hormonal control of moulting. The susceptibility to size variation is the price paid for short generation times. Factors affecting adult fecundity were assessed adult food intake was found to be a major influence, providing a mechanical stimulus initiating rapid reproductive maturation, as well as energy which increases egg production. The most suitable habitat for a Silver Y would be a mesic enviromrent with a temperature of about 17.50C and abundant nectar. No Palaearctic or Mediterranean region provides these conditions all year round. As the Silver Y lacks a well-defined developmental arrest and trials showed a poor ability to overwinter in Britain, the moth must continuously track a shifting patchwork of favourable habitats. The spatia-temporal pattern of habi tat changes is discussed in relation to whether true migration (sensu Lack) or dispersal is an appropriate life history tactic. Flight records purporting to demons.trate true migration are shown to be compatible with a random flight direction model and it is concluded that insect species such as the Silver Y are best regarded as nomadic generalists fundamentally different from classic migrants like the Monarch butterfly.
Permanent link to this resource: https://doi.org/10.24384/tv5r-0w43
Duthie, David J.
Supervisors: Owen, Denis
Department of Biological and Medical Sciences
Year: 1983
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