Feeding of the coccinellid beetle, Curinus coeruleus, on extra-floral nectaries of Hibiscus brackenridgei as a mechanism for defense

The mutualistic relationship between the coccinellid beetle, Curinus coeruleus Mulsant, and the plant Hibiscus brackenridgei A. Gray, was investigated on Kauai, HI. The presence of extra floral nectar maintains the beetle presence on the plant. Because coccinellid beetles are predators on insects that are damaging to plants, beetle presence may increase plant fitness. Beetles were found feeding heavily on the extra floral nectaries of the Hibiscus. An examination of the beetle mouth parts with scanning electron microscopy revealed no structures specifically adapted for the consumption of nectar. The sensory ability of the coccinellids was tested to determine if they respond to visual or olfactory cues to detect the nectar. Studies with an eight-armed air-flow olfactometer concluded there was no olfactory cue. Tracing the pathways of beetles in laboratory experiments yielded results that suggest a visual cue. The extra floral nectaries are concluded to be plant defense mechanisms.