The genus Megalagrion (Odonata: Coenagrionidae) includes about 25 damselfly species, all endemic to Hawaiʻi. The endangered orangeblack Hawaiian damselfly (Megalagrion xanthomelas) was formerly one of the most widespread Hawaiian damselflies, breeding in lowland wetlands across the state. The species suffered rapid declines following the introduction of mosquito fish (Gambusia spp.) in 1905, and is now restricted to scattered populations on several islands. It was thought to be extinct on Oʻahu until 1994 when a small population was discovered in a short stretch of stream at Tripler Army Medical Center (TAMC).
Since the rediscovery of M. xanthomelas on Oʻahu, multiple translocations have been attempted through direct transport of adults, naiads (aquatic larvae), and eggs from TAMC to other sites, but none have succeeded. In Nov 2018, we began a captive rearing and reintroduction program for M. xanthomelas. Our strategy is a two-month process of collecting eggs from TAMC, hatching and rearing naiads to the final instar, then releasing them into fish-free habitat. The predaceous naiads must be reared in individual containers and fed daily with live brine shrimp and freshwater zooplankton. Survival in captivity is high (>90%), and with current resources and space, we can produce about 300 naiads in each rearing cycle.
As of Oct 2019, we have released over 1,000 captive-reared naiads at two sites: artificial ponds at the University of Hawaiʻi Lyon Arboretum and a natural stream at Waiʻanae Kai Forest Reserve. At Lyon Arboretum, adult damselflies were consistently observed mating and laying eggs for about a month after releases, but reproduction was confined to taro patches that were heavily infested with mosquito fish. No reproduction was observed in the fish-free ponds where naiads had been released, and the site is therefore not considered viable.
At Waiʻanae Kai, preliminary results are more promising. We continue to observe damselflies laying eggs in the fish-free stream over 40 days after the last naiad release. Although monitoring is ongoing, and it remains to be seen whether damselflies will persist at Waiʻanae Kai over the long term, our captive rearing strategy has been more successful than previous efforts relying on direct translocation of wild-caught individuals. The limiting factor is not whether large numbers of individuals can be reared, but whether we can identify suitable reintroduction sites that provide predator-free habitat for naiads as well as attractive breeding sites for adult damselflies.