Oviposition Site Resource Quantity and Larval Establishment for Orellia occidentalis (Diptera: Tephritidae) on Cirsium wheeleri

We investigated factors associated with larval establishment of the tephritid fly Orellia occidentalis (Snow) on the thistle Cirsium wheeleri (Grey) Petrak. Adult tephritid appearance was temporally synchronized with the peak flowering of its host plant. During the 14-d period adult flies were active, 47% of the capitula were in a phenological state appropriate for female oviposition. Plant attributes such as plant height or capitulum size were good predictors of potential resource availability. The larval establishment pattern appeared nonrandom. We found a positive correlation between larval establishment and capitulum size; the largest capitula are also the rarest in the population. We propose that adult phenology is adaptive, enabling O. occidentalis access to the largest percentage of resources in the environment for egg laying and larval development.