Endangered Species
Ecological Services

Featured Species | Drosophila

by Karl Magnacca, Ph.D

Drosophila. Karl Magnacca, Ph.D (click to view more images)
Photo credit: Karl Magnacca, Ph.D


Far back in the deep gulches of the Wai'anae range, some of the rarest animals in Hawai'i survive in pockets of native forest. The picture-winged pomace flies, Drosophila, live quietly in the shaded valleys, almost always unseen; they can only be counted by tempting them out with fermented bananas and mushrooms. The flies are the product of 25 million years of evolution in the Hawaiian Islands, and hopefully the progenitors of millions more.

Conservation of insects has been late in coming to Hawai'i, and was long overdue. With more than 6,000 endemic species, including hundreds that are still undescribed, Hawaii's native insect diversity dwarfs that of better known groups. Preserved remains from caves show insects have suffered the same kind of mass extinction and range contraction as forest birds. Drosophila is the largest Hawaiian insect radiation, with an estimated 800 to 1,000 species—equal to or greater than the total number of endemic flowering plants or land snails. The 120 species in the charismatic picture-wing group of Drosophila include some of the largest and most colorful Drosophila in the world, dwarfing the laboratory genetics workhorse D. melanogaster.

All of the Hawaiian picture-wing flies breed in dead or dying native trees, where the larvae burrow into the rotting mushy bark and feed on bacteria and yeasts. Though often overlooked, saprophages (consumers of decaying material) like these play an important role in maintaining the balance of microbes and ultimately converting of dead plant matter into organic soil. Each species is particular to only one or two species of host plants. This dependence contributes to both their astonishing diversity and their current rarity, as many host plants have declined. Since the flies require a steady supply of mature trees that may die or have large branches broken off by other treefalls, maintaining a healthy forest with adequate numbers of all size classes is essential to their survival.

Hundreds of years of direct and indirect human impacts, including clearing for agriculture, fire and the effects of introduced animals such as rats, goats, pigs and ants have contracted the diverse moist forests of the Wai'anae range, and the native insects have retreated with them. Today, most of the native Drosophila are restricted to small patches of native-dominated forest in areas that are largely composed of alien vegetation. Despite being one of the smaller Hawaiian islands, O'ahu has the second-highest number of picture-wing Drosophila species with 33 (Maui has the most with 39). Six of the O'ahu species are listed as federally endangered (along with eight from other islands), and many more are equally rare. Three of the endangered species – D. Montgomeryi, D. obatai, and D. substenoptera – live on Schofield Barracks above the live-fire training areas.

Their confinement leaves the flies are vulnerable to random natural or human-caused disturbances, such as fires, landslides, drought, and flooding. A single event can wipe out what might be considered a thriving population of flies, particularly if the flies are confined to a small grove of trees in a single gulch. The arrival of a new invasive species is also a threat. The introduction of the western yellow jacket (Vespula pensylvanica) in 1978, a voracious and abundant general predator, has impacted Drosophila on Maui and Hawai'i islands in particular. Less conspicuous insects can have just as great an impact—an alien crane fly, still unidentified, that arrived in the late 1990s and feeds in the same decaying bark has dramatically reduced the number of native Drosophila that breed in many wet forest trees.

A lack of information is one of conservation management's biggest impediments. Until the Oahu Army Natural Resources Program (OANRP) began managing endangered Drosophila two years ago, relatively few surveys or collections had been done since the early 1970s. The flies are secretive creatures that are much harder to monitor than plants or birds; their attraction to bait varies and their numbers may fluctuate wildly through the year. However, in a year and a half of intensive surveys, OANRP and the Pacific Cooperative Studies Unit have discovered 11 new populations of the endangered species in OANRP management units, and rediscovered seven species that had not been seen in over 40 years. A new species was even found in the Ko'olau range, the first picture-wing Drosophila from O'ahu (and only the sixth statewide) to be newly discovered since 1973.

It is encouraging to find the flies surviving better than expected, but many challenges lie ahead with the transition from surveys to active management. Insect conservation in Hawai'i has mainly been directed at simply protecting habitat, such as caves or a specific host plant, and efforts elsewhere aimed at organisms such as butterflies and beetles are largely inapplicable to the unique island situation in Hawai'i. Successful management of Hawai'i's Drosophila will require different methods for expanding the area of habitat available, including control of invasive plants, outplanting both host plants and others that provide a sustainable forest environment and possibly releasing lab-reared flies at sites where they have been extirpated. As with any conservation program, the goal for restoration areas is to establish self-sustaining ecosystems with reproducing populations of both host plants and flies that will be able to maintain themselves in the long term. By ensuring the plants and flies survive, OANRP can perpetuate Hawai'i's natural history and continue their evolutionary legacy.

Dr. Karl Magnacca is an entomology specialist with the Pacific Cooperative Studies Unit working for the O'ahu Army Natural Resources Program. He has studied the evolution, ecology, and systematics of native Hawaiian insects for 20 years, working with the U.S. Geological Survey, National Park Service, and the University of Hawai'i-Hilo. He has described 64 new species, about one percent of the endemic insect fauna.

Last updated: October 8, 2015