Monitoring the mission blue butterfly using immature stages

Monitoring of the Mission blue butterfly has been restricted to periodic counts of adults sighted during the flight season during transect walks. However, adult count data is a reliable index of population size only if the probability of detection is high and stable across varying ecological conditions. Determining these probabilities requires marking and capturing of a large number of individuals, which is harmful and difficult to implement for protected species. Therefore, we explored the potential of using immature stages (eggs, late-instar larvae and the feeding damage inflicted by larvae) as potential measures of population status for monitoring purposes. For each of these measures we examined its correlation with adult counts, its detection probability, and its repeatability. We found that immature stages and feeding damage are highly correlated with adult counts, are highly repeatable, and have detection probabilities that exceed estimates of the probability of detecting Mission blue adults. We conclude that late instar larval counts and foliar feeding damage are superior to adults as measures of population status for population monitoring. Furthermore, if sighting-resighting based counts of immatures or feeding damage are made on the same set of host plants on successive days, then estimates of abundance and its standard error adjusted for detection probabilities can be obtained.