VECTOR BEHAVIOR AND THE TRANSMISSION OF ANTHER-SMUT INFECTION IN SILENE ALBA

Anther-smut disease caused by Ustilago violacea is transmitted by inse cts that visit diseased flowers and then deposit spores on healthy Sil ene alba plants. The transmission rate of this disease therefore depen ds on spore deposition by the vector species and preferences for infec ted vs. healthy hosts. We observed natural populations of S. alba to d ocument the most abundant insect visitors, which include syrphid flies , andrenid bees, bumblebees and a variety of nocturnal moths. Using bu mblebees and moths in flower preference studies, we showed that bumble bees preferentially visited healthy flowers, but the strength of this preference declined if the bees had prior exposure to diseased flowers . Nocturnal moths showed less discrimination with respect to disease s tatus when plants were arranged in a field population of S. alba, and preferentially visited plants with more flowers. A laboratory analysis of spore deposition by bumblebees showed that whereas most spores wer e deposited on the first several flowers visited, flowers beyond the 1 5th visit may still receive enough spores to produce a new infection. Spore deposition was also influenced by changes in vector behavior ass ociated with spacing in artificial arrays of S. alba plants. Models we re fitted to deposition data to compare estimates for spore dispersal rates and the initial number of spores acquired by vectors. This study demonstrated several ways that vector behavior can influence rates of disease spread in natural populations. Preferences for healthy vs. in fected hosts can cause disease dispersal to vary from that expected by randomly foraging insect vectors. Changes in visitation behavior asso ciated with host density can affect the magnitude and distance of spor e deposition in host populations.