Quantifying the effects of an invasive thief ant on the reproductive success of rare Hawaiian picture-winged flies

Threats to endangered insect species that act independently of those associated with habitat loss are often suspected, but are rarely confirmed or quantified. This may hinder the development of the most effective recovery strategies, which are increasingly needed for listed insects. Since 2006, 14 species of flies within the large, showy Hawaiian picture-winged Drosophila group have been added to the US threatened and endangered species list. Many of these species are thought to be limited by host plant rarity, but also by predation on immature stages by invasive ants. We tested the latter hypothesis with a field experiment involving Drosophila crucigera, a more common surrogate for sympatric endangered species, and the invasive ant Solenopsis papuana, on the island of Oʻahu. We established ant suppression and control plots across three forest sites. Within each plot we placed a host plant branch piece, into which lab-reared flies had oviposited, and subsequently tracked weekly emergence of adults. Numbers of flies that emerged were 2.4 times higher in ant-suppressed plots than in control plots; this 58% reduction in survival from egg to adult in the presence of ants was similar across all three sites. Among plots, numbers of emerged flies exhibited a pattern suggesting that the detrimental effect of ants is density dependent. These results confirm that S. papuana, and possibly other invasive ant species, can strongly impact the reproductive success of Hawaiian picture-winged Drosophila. They also point to several management actions, beyond habitat restoration, that may improve the recovery of these imperiled flies.