THE EFFECT OF FLORAL HERBIVORY ON MALE AND FEMALE REPRODUCTIVE SUCCESS INISOMERIS ARBOREA

Abstract

Flower-feeding herbivores can directly reduce plant reproduction by consuming gametes. They may have additional indirect effects if their damage disrupts pollinator service and causes uneaten gametes to go unused. In a two-year study we investigated direct and indirect effects of florivory by a pollen beetle, Meligethes rufimanus, on the male and female reproductive success of the andromonoecious shrub, Isomeris arborea. We measured pollen export, pollen receipt, fruit set, and outcrossing rates on plants exposed to herbivores (most flowers damaged) and protected from herbivores (few flowers damaged) by systemic insecticide. Pollen export per undamaged flower was reduced by one-half in exposed plants, as estimated by pollinator transfer of florescent dye, which demonstrated indirect negative effects on male reproductive success. Damaged flowers on exposed plants had a lower stigmatic pollen load than undamaged flowers on either exposed or protected plants. Furthermore, exposed plants produced fewer fruits and seeds during the year than protected plants. Although damage reduces pollen receipt, hand pollination experiments showed that neither exposed nor protected plants were pollen limited, which suggests that floral herbivores primarily affect female function through their direct destruction of gametes. Outcrossing rates did not differ between exposed plants (ts = 0.920) and protected plants (ts = 0.806), suggesting that herbivory does not indirectly reduce plant reproductive success by inducing autogamy and subsequent inbreeding depression. Overall, floral herbivory has direct consequences for both male and female reproductive success, but indirect effects are limited to male function.