The discrepancies between extinction rates on islands may reflect recent collecting bias between islands rather than true differences in rates. It is highly unlikely that Oʻahu, with its relatively low remaining amount of intact, native forest, has suffered the lowest percentage of species extinction of Lepidoptera compared to all the other main islands. Instead, we suggest that Oʻahu has benefited from much more intensive entomological surveys over the past century and as a result its fauna is much more well-documented than the other islands. Oʻahu’s extinction rate is probably the most accurate as large numbers of surveys over decades often specifically targeting these “lost” species have so far failed to find them (Austin and Rubinoff 2023b). Additional surveys on islands with seemingly high extinction rates (e.g., Lānaʻi, Molokaʻi) may result in additional rediscoveries of “extinct” species and bring their extinction rate lower than currently recognized. However, ecological damage on Lānaʻi is more severe than Oʻahu, and it is possible that extinction rates on smaller, lower islands may be higher than that of Oʻahu. Even if lower extinction rates end up being verified for other islands, they are still among the highest recorded anywhere in the world.
Detailed understanding of the ecology of species that are apparently rare will be critical in understanding the broader patterns of extinction across Hawaiian insects. Some groups provide important examples of how or why some species are particularly vulnerable. The subgenus Euperissus, which primarily have larvae that feed naked (i.e., without cases), are generally faring much more poorly than subgenus Hyposmocoma, all of which make protective silk cases as larvae. This may be partially due to the close relationship some groups of Euperissus have with their host plants, many of which are now rare or endangered. It may also reflect the limited attention taxonomists have paid to Euperissus in the past century: no new species have been described in Euperissus since 1935, whereas 50 species in subgenus Hyposmocoma have been described since then, including 49 in just the past 20 years. Some groups of Euperissus can still be encountered fairly frequently, even abundantly. However, this may be a consequence of their protected feeding strategies: many of these more resilient Euperissus groups feed internally in bark, fruits, and seeds, and at least some are known to make cases. Many of the naked wood-feeding Euperissus species are now largely absent from otherwise suitable habitat and may have been outcompeted by termites and invasive moths (e.g., Opogona spp.) as well as preyed upon by ants, making the extinction estimates unfortunately accurate. Consideration of this ecological information is important in understanding the extinctions patterns and engaging in more effective conservation actions.
Extinction rates between groups of native moths appear to vary dramatically, with some extremely diverse genera having largely avoided extirpation while other genera are entirely extinct. The monotypic endemic genus Tritocleis (Geometridae) Meyrick is only known from the holotype collected in 1896. At least one other genus (Hypena, Erebidae; 3 presumed extinct / 2 possibly extinct / 0 extant or likely extant) appears to have become entirely extinct as well, with none of the five native species having been seen since 1941. This may be the result of rodent introductions on the Northwestern Hawaiian Islands (Gagné 1982); deliberately released, non-specific biological controls aimed at Lepidoptera or other insects (Funasaki et al. 1988); and the accidental arrivals of non-native parasitoids (Henneman and Memmott 2001). Other species may be declining because of the loss or severe decline of their host plants, such as Acrolepia (leaf-miners on Nothocestrum; 2 / 0 / 2), Mapsidius (stem- and leaf-feeders on Charpentiera and Chenopodium; 1 / 2 / 2), and Philodoria (specialist leaf-miners on various native plants; 5 / 5 / 35). Some may have been lost to hybridization with invasive congeners (Helicoverpa, 1 / 2 / 1; Mythimna, 0 / 0 / 3). Some genera that appear to be hard-hit (e.g., Batrachedrodes, Paralopostega) may actually be more common than currently known, but uncommon in collections due to their small size, specialized feeding habits, or difficult-to-rear larvae. Still others are declining despite no immediately obvious factor such as Merimnetra (leaf-miners, stem-gallers, and fruit-feeders in native Rubiaceae; 7 / 5 / 6) and Pararrhaptica (leaf-rollers on native Primulaceae; 3 / 5 / 13).
Coastal species have suffered the highest rates of extinction, with numbers and abundance of native species increasing with elevation and percent native forest coverage. Only a handful of native species can be encountered with any regularity at the lowest elevations (e.g., Tamsica, Plutella, some Hyposmocoma). Formerly abundant coastal and lowland species such as Agrotis crinigera were apparently driven to extinction by the arrival of invasive predators such as big-headed ants and the early introductions of poorly-guided attempts at biological control (mynah birds, cattle egrets, and various polyphagous parasitoid wasps and flies; Funasaki et al. 1988). Many of these formerly common native species have been largely replaced by a small set of polyphagous exotic species.
On the other hand, the few genera of native moths that appear to be surviving, even thriving, offer important management information as well. This is likely the result of their hosts remaining common even in highly disturbed and non-native forests, the ability to switch to feeding on non-native plants, and/or the predator protection derived from their specialized cases or behaviors of larvae. For example, several species of Eudonia (4 / 2 / 56) and Mestolobes (8 / 4 / 22) can often be seeing flying at dusk and night in the hundreds to thousands in highly degraded strawberry guava (Psidium cattleyanum) or ironwood (Casuarina equisetifolia) thickets where the larvae likely feed on the indigenous mosses that persist at the base of these trees.
Despite these glimmers of hope, the overall outlook for Hawaiian Lepidoptera is grim: though many species can still be found, exceedingly few are common or abundant. In many areas, non-native moths outnumber native species by an enormous margin. Extinction rates, high as they are, show no sign of abating. Hawaii’s rate of loss dramatically outpaces that of similar island ecosystems: New Zealand, with over 1,800 described native moth and butterfly species (roughly twice as many as Hawaiʻi), has zero species that can be “confidently considered extinct,” only 49 that are “threatened,” and 69 “at risk” (Stringer et al. 2012). The primary major threats to native Lepidoptera in Hawaiʻi likely mirror those of native insects more broadly, namely the loss of habitat and host plants due to displacement by invasive plants (Wagner and Van Driesche 2010, Medeiros et al. 2017), predation by non-native arthropods, especially ants and wasps (Gambino 1992, Krushelnycky and Gillespie, 2010), development (Polhemus 1993, Magnacca and King 2013), and climate change (Hobbelen et al. 2013, Hembry et al. 2021). The arrival of new invasive species in Hawaiʻi, both plants and animals, continues nearly unabated year after year (Matsunaga et al. 2019, Austin and Rubinoff 2022, 2023a, 2024). Whereas Hawaiian birds, plants, and charismatic marine life receive substantial conservation funding targeted to better understand species’ biology, distribution, and threats, only a tiny fraction goes towards insects (Medeiros et al. 2013), despite Hawaiian insects being more than twice as diverse as the aforementioned groups combined (even excluding undescribed insect species). Compounding this is the difficulty in accessing type material (most of which is stored in NHMUK in London), only a fraction of which is digitized and available for study. Researchers wishing to study and protect the Hawaiian fauna must fly halfway around the world to see the only identified specimens of a species that may now be extinct. Repatriation of this material to Hawaiʻi would not only benefit taxonomists working in Hawaiʻi, but also greatly accelerate the conservation of all Hawaiian insects.
Our approach is not an overestimate of the true number of extinctions in the Hawaiian Lepidoptera fauna, because in the past 50 years commensurate extinctions have been rampant in better known groups like birds and plants. Regardless, our data provides an important baseline by which to compare future conservation assessments and hopefully to guide actions. It is not yet clear how many additional “presumed extinct” species would likely warrant removal from a “data deficient” category into the “extinct” category, as additional surveys are still badly needed on all of the Hawaiian Islands. Hawaiian insect conservation lags behind all other groups in terms of support and knowledge, implying that management agencies have determined that they are under less threat, or are less important. Here we clearly show that a lack of funding to collect data, rather than any resilience is to blame for pervasive inaction. Conserving insects for their inherent value has apparently not been a compelling reason to allocate resources or protect habitat; but if investment is not made soon, critical numbers of insect species may go extinct, leaving irreplaceable gaps in ecosystems.
Hawaiʻi as a case study is by no means unusual. The Hawaiian Lepidoptera fauna exemplify unfortunate global trends, but on a much more acute and easily measurable scale. The most pressing issue for most threatened insects worldwide is the lack of basic taxonomic and biological information. Insect species are going extinct faster than they can be described (Medeiros et al. 2013, Rubinoff 2017) and only a small percentage of taxa have received any taxonomic attention since their original descriptions decades or centuries ago (Leandro et al. 2017, Wheeler and Miller 2017, Löbl et al. 2023). Host plant records and other biological information are among the most important data for conserving insects, but are unfortunately lacking for the vast majority of species (Bossart and Carlton 2002, Cardoso et al. 2011). Having a baseline estimate for the total numbers of extinct species is a crucial first step in establishing urgency and protecting what species remain. Similar published “extinction databases” would be greatly beneficial for the conservation of insect taxa elsewhere in the world, especially on islands, where insects are particularly vulnerable to extinction (Howarth and Ramsay 1991, New 2008).