Moisture stimulates galler emergence from dry season dormancy in the leaf litter

In tropical regions, a great proportion of trees and shrubs shed their leaves over the dry season. Since gall-inducing insects are generally host species-specific, where do they go when their hosts have no leaves? There is practically no research into dormancy strategies of tropical galling insects. We hypothesised that the galling insect Eurytoma sp. (Chalcidoidea, Eurytomidae) undergo dormancy inside their galls on shed leaves of the host plant Caryocar brasiliense Camb. (Caryocaraceae). The insects would emerge from the galls in synchrony with the sprouting of new leaves, at the beginning of the wet season, stimulated by increased moisture after the first rain of the wet season. In order to test that, we collected leaves from the leaf litter in the last month of the dry season and gently showered half of them in a sink to simulate rain. In the two weeks before the first rainfall, the watered leaves presented an over trifold higher gall eclosion rate than the ones in the control group. There is no previous account of a tropical galling insect undergoing dormancy inside galls in the leaf litter. We contrast this evidence with the possibility that, at least in the tropics, galls may also have had an adaptive value as dormancy structures during their evolutionary history, and evolved in response to the deciduousness of plants growing in harsh environments.