Treefall gaps versus forest understory as environments for a defoliating moth on a tropical forest shrub.

The moth Zunacetha annulata (Dioptidae) is a specialist on the understory shrub Hybanthus prunifolius (Violaceae) in the forest of Barro Colorado Island (BCI), Panama. The larvae, which are capable of defoliating entire shrubs, concentrate their attack upon the small minority of H. prunifolius individuals that grow in treefall gaps. Field experiments demonstrated that larval growth rates were 37% higher, and weights at pupation 25% higher, on shrubs in gaps than on shrubs in the understory. In a common environment in the laboratory, growth rates of larvae were 23% higher on foliage taken from shrubs in gaps than on foliage from shrubs in the understory.However, larvae grown in a temperature regime simulating that of gaps did not grow faster than larvae in an understory regime, when the two groups were reared in growth chambers on foliage taken from the same shrubs. In the field, predation appeared higher in gaps: experimental groups of larvae survived at rates of 65% per day in gaps and 78% per day in the understory. Quality of foliage, and not direct effects of the environment, appears to be responsible for the observed pattern of defoliation by this moth.