Effects of plant micro-environment on movement of <Emphasis Type="Italic">Helicoverpa armigera</Emphasis> (Hübner) larvae and the relationship to a hierarchy of stimuli

Locomotory behaviour of 1st instar Helicoverpa armigera is influenced by a complex of micro-attributes, the leaf “environment”, comprising odours, wax chemistry, trichomes and grip texture. Larval movement speeds on leaves of different types varied more than eight fold. On garden pea, Pisum sativum, there is a hierarchy of stimuli perceived by larvae resulting in differing behavioural responses. Light and angle are paramount in responses to micro-environment. These influence responses to local stimuli. Experiments in darkness produce different responses from those under laboratory light. In darkness, on horizontal surfaces as found for most leaves, preference for leaf surface is driven by micro-environment associated with leaf waxes. Larvae prefer the abaxial surface. In light, on horizontal surfaces, larvae seek enclosed spaces and foray under leaf surfaces. They wander more openly in the dark. Such information is important in building a model of larval behaviour and predicting behaviour on differing plant architectures.     

SEX-RATIO VARIATION IN THE GYNODIOECIOUS SHRUB HEBE STRICTISSIMA (SCROPHULARIACEAE)

The frequency of females was determined for eight populations of the gynodioecious shrub, Hebe strictissima (Scrophulariaceae) and related to plant vigor among populations, as indicated by the average number of leaves per shoot. The purpose was to test the idea that females should be more prevalent in relatively poor sites where plant vigor is low. This hypothesis was based on sex-ratio theory, coupled with the idea that fruit-set in the polleniferous morph (i.e., in “males”) is more dependent on vigor than it is in females. I found that, within populations, females produced significantly more fruit than males and that plant vigor did not differ significantly between the sexes. Fruit-set on males was positively and significantly correlated with the number of leaves per shoot within plants, among plants within populations, and among populations. No such correlations were found for females. The greater plasticity of the males altered the relative seed fitnesses of the two morphs among the eight populations, resulting in a negative correlation between female frequency and average plant vigor. I suggest, in general, that such plasticity may be an important factor in sex-ratio variation among populations and that it should be incorporated into models of sex-ratio evolution.