The effects of insect visitation on floral colour change

Abstract

This study investigated the effects of flower visitation on floral colour change and the subsequent influence of such change on insect foraging behaviour. Colour change was examined in six plant species; Myosotis sylvatica, Echium vulgare and Lonicera periclymenum were studied locally to St. Andrews, Fife, Scotland and Echium judaeum, Lupirtus pilosus and Alkanna orientalis were studied in various locations in the eastern Mediterranean region. Patterns of colour change were recorded both with natural insect visitation allowed and excluded, to establish whether the rate of colour change could be altered through visitation per se or rate of visitation. Detailed observation of the flower handling characteristics of all visiting insects allowed artificial floral manipulations to be devised that simulated the different aspects of visitor behaviour. This enabled the effects of simple mechanical handling on colour change to be separated from those of pollen deposition and post pollination events. Floral reward was measured in relation to flower colour phases to assess whether the change in colour was acting as a functional signal to flower visitors; insect choice of flower colour was noted, to determine whether reward status affected foraging behaviour. One or more factors significantly altered the characteristics of colour change in all species except Lonicera periclymenum. The triggering factor could be the exclusion of visitors, rate of natural visitation, floral manipulation, or aspects of the pollination process. In Lupinus pilosus pollen deposition and/or pollen tube growth was the trigger for colour change. Pollen deposition was also the most likely trigger in both Alkanna orientalis and Myosotis sylvatica, although the varied patterns of colour change in these species could be related to wound responses and/or senescence. Pollination processes were not involved in colour change in either species of Echium. The first recorded example of a 'reverse' colour change is reported fox Echiumjudaeum. Floral reward varied between colour phases in all plants except Echium vulgare, and visiting insects did not show any bias towards particular flower colour phases in this plant. In all other species a variety of flies and bees visited the most rewarding colour phase preferentially. A model is presented that incorporates all influences on floral colour change in a single framework, potentially unifying the concepts of age-related' and 'inducible' change which have previously been thought to be distinct.