Implications of Chaoborus pupation and ecdysis in cold water

1. Cold water acted differently to delay and lengthen the pupation period for the larvae of two species of the zooplankton predator Chaoborus (Diptera: Chaoboridae). During Chaoborus pupation, the zooplankton community is released from predation, while the dark-coloured Chaoborus pupae are more susceptible to their own predators. 2. Fourth instar larvae of Chaoborus americanus and C. trivittatus, collected from an oligotrophic lake, were reared individually at 5 degreesC in the dark. Chaoborus americanus was also reared at 9 and 12 degreesC under spring photoperiod conditions (L : D, 16 : 8 h). Individuals were observed through pupation to emergence (ecdysis) or death. 3. Chaoborus americanus pupated at 5, 9 and 12 degreesC with substantial emergence only at 12 degreesC. In comparison, C. trivittatus emerged at 5 degreesC. Light was not a necessary cue for pupation and ecdysis, contrary to previous reports. Cold water delayed the onset and lengthened and increased the variability of the duration of pupation. 4. In Shirley Lake, C. americanus pupated in late June-early July while C. trivittatus pupated first in April and again in June-July. 5. Chaoborus americanus pupae needed a temperature cue to complete ecdysis. The ecdysis temperature threshold helps to explain the difference in pupation timing, and the geographical distribution, of C. americanus and its relatively inflexible life history contrasted with C. trivittatus. Delayed predator pupation in years with low spring temperature can affect the community dynamics of the prey.