Insects of Hawaii


by
Francis G. Howarth
Gordon Nishida
Bishop Museum, Hawaii
Adam Asquith
U.S. Fish and Wildlife Service
Insects are the dominant animals in most terrestrial ecosystems, especially on isolated oceanic islands where many larger animals are absent. In Hawaii, many of the original colonizing species evolved into perhaps 10,000 or more new species and adapted to live in the diverse island habitats. In addition to their importance as pollinators of native plants, recyclers of nutrients in ecosystems, and food for native birds and other animals, insects are also excellent subjects for evolutionary research. The isolation and habitat diversity of the Hawaiian Islands make them wonderful natural laboratories for studying ecology and evolution. Many important research projects have featured Hawaiian insects, such as the native Drosophila (see Foote and Carson, this section) and crickets (Otte 1989).
Because insects are important components of ecosystems, insect surveys can be used to assess the health of native ecosystems, and reserve managers often need to be able to determine the status of insects to properly manage other natural resources. Such assessments, however, are daunting tasks: although about 5,100 native insect species have been described in Hawaii, probably at least as many more remain undescribed or unknown. In addition, about 2,600 insect species have been established through human activities. Many native species are declining from the combined effects of invasive non-native organisms and human alteration of habitats.
Information on the status of Hawaiian insects came from a data base compiled at the Bishop Museum of all published records on the taxonomy, biology, and distribution of Hawaiian arthropods (Nishida 1992). Further information on the status and trends of selected rare species was obtained from label data of preserved specimens, especially those in the research collections at Bishop Museum and University of Hawaii, Honolulu, as well as from personal communications and observations of researchers in the field. Population surveys are in progress to determine the status and trends of a few insect groups such as the damselflies (Megalagrion; Polhemus 1993) and cave species.

Insects of Hawaii
Fig. 1. Comparison of native and non-native insects in the larger orders (i.e., represented by more than 75 species) in Hawaii. Source: Hawaiian Terrestrial Arthropod Database, February 1994.
Only 16 out of 30 insect orders recognized worldwide are represented in the native fauna. Another 11 orders have become established through human activities (Figs. 1 and 2). The beetles (Coleoptera), flies (Diptera), bees and wasps (Hymenoptera), and moths (Lepidoptera) are the largest groups in the Hawaiian Islands. Most native species are found on the high, main islands, but each of the northwest Hawaiian Islands harbors a few interesting species (Fig. 3). Oahu currently has the most known species, but this stems from collecting bias because most entomologists have lived and worked on Oahu. Maui and Kauai, in particular, should have comparable numbers. Western Maui, for example, was missed in the early insect surveys, and its insect fauna remains poorly known. About 63% of the species occur on only one island, and many have extremely restricted ranges within their island. This limited distribution and lack of information on how many species there are and where they survive have important consequences in planning for their conservation.

Trends
Fig. 2. Comparison of native and non-native insects in the smaller orders (i.e., represented by fewer than 75 species) in Hawaii. Source: Hawaiian Terrestrial Arthropod Database, February 1994.
Profound changes are occurring in the Hawaiian insect fauna. Increasing contact with the outside world has broken the isolation that allowed the evolution of native species. The changing composition of the Hawaiian insect fauna is readily apparent from the contrast between historical collections and reports (e.g., Perkins 1913; Zimmerman 1948) and more recent records and surveys. This change is particularly obvious in lowland areas where land conversion (Cuddihy and Stone 1990) and the establishment of alien species have eliminated or drastically reduced the abundance and diversity of native arthropods. For example, Asquith and Messing (1993) found that less than 10% of the insect fauna of a lowland agricultural area on Kauai is composed of native species, and, at a low-elevation site on the island of Hawaii, even the arthropod community on the native tree 'Ohi 'a lehua (Metrosideros polymorpha) is composed primarily of alien species (Gagné 1979).
Fig. 3. The island distribution of native insect species.
At higher elevations in more intact vegetative communities, invasive alien arthropods have become dominant in some guilds, such as honey bees as pollinators and millipedes and isopods as detritivores. The effects of predatory species, such as the Argentine ant (Linepithema humilis; Cole et al. 1992) and the western yellowjacket (Vespula pensylvanica; Gambino et al. 1990), in the decline of some native groups are well documented.

Introduced Parasites
Although the native Hawaiian faunas naturally bear some pressure from parasitoids, with endemic taxa of wasps in the families Ichneumonidae, Bethylidae, Diapriidae, Eucoiliidae, and Eulophidae, and the fly family Pipunculidae, the taxonomic composition and therefore the ecology of parasitism itself have been altered by the addition of alien species. For example, the Hawaiian Islands originally had no native species of braconid wasps, but now harbor 76 species in 42 genera (Nishida 1992). Many of these parasitoids are not confined to disturbed habitats or alien hosts. By using a Malaise trap (a tentlike net left in place to capture flying insects), A. Asquith and M. Kido (USFWS and University of Hawaii, Kauai, unpublished data) recently sampled the parasitic wasp community in a high-elevation native mesic forest on the island of Kauai for a full year. Of the 17 species of Braconidae and Ichneumonidae captured, all but one are parasitoids of moth larvae and pupae, and all but two are known to attack native Hawaiian moths. No known species of native ichneumonid wasp in this forest is extinct, and the endemic taxa still contribute the most to species diversity (Fig. 4). Human activities, however, have essentially doubled the number of species parasitizing native Lepidoptera. Furthermore, parasitoid abundance in this community is dominated by non-native species (Fig. 4), with less than 1 in 10 parasitoids being native to Hawaii. On a numbers of individuals per species basis, the two species introduced for biological control (Eriborus sinicus and Meteorus laphygmae) are more invasive in this forest than the supposed inadvertently introduced species of parasitoids. Not only have these introductions increased the number of species and individuals parasitizing Hawaii's native Lepidoptera, but also the new species have searching and immobilizing behaviors to which the native fauna is unaccustomed.

Fig. 4. Species and total catch composition of wasps in the families Braconidae and Ichneumonidae in a high-elevation mesic forest on the Hawaiian island of Kauai. Wasps were sampled from January 1992 to January 1993 by using a Malaise trap.
Populations of the native stink bug (Pentatomidae) genera Coleotichus and Oechalia dramatically decreased after 1962 following the purposeful introductions of a tachinid fly and several scelionid wasps for biological control of the non-native pest, the southern green stink bug (Nezara viridula). The koa bug (Coleotichus blackburniae) is the largest and most spectacular native Hawaiian true bug and was, until the early 1970's, common on all the major Hawaiian islands, including within the city of Honolulu, where it could frequently be found on introduced acacia trees. Numerous specimens were deposited in the insect collections of the Bishop Museum and University of Hawaii every decade from 1890 to 1970, but very few specimens have been seen since 1978. Because the koa bug is conspicuous, and its rarity has been publicized (Howarth 1991), its population decline seems real and not an artifact of survey effort.
Since the koa bug is gregarious and hundreds of individuals could be collected from a single tree, it was used as an alternate host for rearing introduced parasites before their release. Thus, circumstantial evidence implicates these biological-control introductions in the demise of these native bugs. Recent observations suggest that small populations of the koa bug still survive on most of the major islands, but quantitative status surveys and protection for this insect may need to be initiated to ensure its continued existence.
These examples support the arguments of Gagné and Howarth (1985) and Howarth (1991) that alien parasitoids are the major factor contributing to the decline and extinction of many native insect species. Lepidopteran caterpillars were an important food source for native forest birds and other native organisms; thus, their decline may affect other parts of the forest community. The ability of non-native arthropods to invade intact native communities demonstrates that conservation efforts aimed at habitat preservation, or the selection and management of nature reserves based on plant diversity or endemism, may not provide sufficient protection for some insects and their associated biota because of the continued emphasis on biological control and insufficient quarantine control in Hawaii. The effect of invasive alien arthropods means that we could save the forest and still lose the bugs, but we would eventually lose the forest as well because of the loss of pollinators and other functional groups of insects.

Extinctions
With at least 50% of Hawaii's native birds (Stone 1989) and mollusks (Solem 1990) extinct, it is likely that Hawaii has also lost a significant proportion of its terrestrial arthropod fauna. While 36 arthropod species are recognized as extinct by the U.S. Fish and Wildlife Service, populations of two species, a damselfly (Megalagrion nesiotes) and a sphinx moth (Manduca blackburni), have recently been rediscovered. The lack of intensive surveys for most of Hawaii's rare arthropods makes their status equivocal and weakens arguments for the allocation of conservation resources for these animals.
One of the few areas in Hawaii where arthropod extinctions are reasonably well documented is on Laysan Island in the northwestern part of the chain. While only 3.8 km2 (1.5 mi2) in size, it harbored at one time a native arthropod fauna of at least 77 taxa with at least 14 endemic species (Conant et al. 1984). With intensive surveys during the 1960's and 1980's, we now know that 35% of Laysan's endemic species are extinct. Other evidence of arthropod extinctions comes from those species associated with endangered or extinct plants. In 1917 a new species of Proterhinus weevil was collected from the last remaining tree of Hibiscadelphus giffardianus on the island of Hawaii. While the tree has been given a reprieve from extinction by propagation of individuals from seed, the weevil, which breeds in dying branches, was doomed with the death of the last wild tree. Many Hawaiian insect groups are similarly extremely host-specific; for example, some species of long-horned beetles (Plagithmysus), with 139 known species, and leaf bugs (Nesiomiris), with 50 species, occur on rare hosts and face a similar fate.

Survey Needs
Waiting for confirmations of extinctions or the discoveries of relict populations is ineffective, reactive conservation and will not preserve Hawaii's remaining arthropods. We need to identify species early in their decline or at least before they slide beyond recovery (Howarth and Ramsay 1991). This report is limited to the insects, but other native invertebrates deserve mention, including the spiders and relatives (arachnids), sandhoppers and relatives (crustaceans) (Howarth and Mull 1992; Nishida 1992), and mollusks (Solem 1990; Cowie et al., in press). The worms and smaller invertebrate groups are even less well-known than the arthropods.
The koa bug (Coleotichus blackburniae), the largest Hawaiian true bug (three-fourths inch long), was common, but few have been seen since 1978. Courtesy W.P. Mull
The urgency and effectiveness of status surveys are exemplified by one being conducted for Hawaii's damselflies. On the island of Oahu alone, two damselfly species are believed extinct, and three additional taxa are severely reduced from their historical ranges and in danger of extinction. For example, sometime between 1983 and 1985, Megalagrion nigrohamatum nigrolineatum disappeared from its usual haunts along streams near Honolulu. Surveys begun in 1990 have found it in only three isolated localities near the headwaters of Oahu streams. This represents a greater than 99% reduction in range in a decade. Most of its former habitat still appears suitable and the reasons for its decline are uncertain, but researchers suspect the decline results from the effects of non-native species, as well as habitat destruction (Polhemus 1993).
Status surveys of additional selected groups of arthropods should be a top priority so that appropriate conservation measures can be planned. Studies on the systematics of Hawaiian biota, including descriptions of new species, are also urgently needed. Whether a population represents a native or non-native species or 1, 10, or 20 closely related species has bearing on effective conservation strategies in reserves (Howarth and Ramsay 1991).
For further information:
Francis G. Howarth
Bishop Museum
Department of Natural Sciences
PO Box 19000-A
Honolulu, HI 96817
References
Asquith, A., and R.H. Messing. 1993. Contemporary Hawaiian insect fauna of a lowland agricultural area on Kaua'i: implications for local and island-wide fruit fly eradication programs. Pacific Science 47:1-16.

Cole, F.R., A.C. Medeiros, L.L. Loope, and W.W. Zuehlke. 1992. Effects of the Argentine ant on arthropod fauna of Hawaiian high-elevation shrubland. Ecology 73:1313-1322.

Conant, S., C.C. Christensen, P. Conant, W.C. Gagné, and M. Lee Goff. 1984. The unique terrestrial biota of the northwestern Hawaiian Islands. Pages 77-94 in Proceedings of a Symposium on Resource Investigations in the Northwestern Hawaiian Islands. University of Hawaii, Sea Grant College Program, Honolulu.

Cowie, R.H., N.L. Evenhuis, and C.C. Christensen. Catalog of the native land and freshwater molluscs of the Hawaiian Islands. Bishop Museum Bull. in Zoology. In press.

Cuddihy, L.W., and C.P. Stone. 1990. Alteration of the native Hawaiian vegetation: effects of humans, their activities and introductions. University of Hawaii Press, Honolulu. 138 pp.

Gagné, W.C. 1979. Canopy-associated arthropods in Acacia koa and Metrosideros tree communities along an altitudinal transect on Hawaii Island. Pacific Insects 21:56-82.

Gagné, W.C., and F.G. Howarth. 1985. Conservation status of endemic Hawaiian Lepidoptera. Pages 74-84 in Proceedings of the 3rd Congress of European Lepidopterology Cambridge, 1982. Karlsruhe: Societas Europaea Lepidopterologica.

Gambino, P., A.C. Medeiros, and L.L. Loope. 1990. Invasion and colonization of upper elevations on East Maui (Hawaii) by Paravespula pensylvanica (Hymenoptera: Vespidae). Annals of the Entomological Society of America 83:1088-1095.

Howarth, F.G. 1991. The environmental impacts of classical biological control. Annual Review of Entomology 36:485-509.

 Howarth, F.G., and W.P. Mull. 1992. Hawaiian insects and their kin. University of Hawaii Press, Honolulu. 160 pp.

Howarth, F.G., and G.W. Ramsay. 1991. The conservation of island insects and their habitats. Pages 71-107 in N.M. Collins and J.A. Thomas, eds. The conservation of insects and their habitats. Academic Press, London.

Nishida, G.M., ed. 1992. Hawaiian terrestrial arthropod checklist. Bishop Museum Tech. Rep. 1. 262 pp.

Otte, D. 1989. Speciation in Hawaiian crickets. Pages 482- 526 in D. Otte and J.A. Endler, eds. Speciation and its consequences. Sinauer Associates, Sunderland, MA.

Perkins, R.C.L. 1913. Introduction to fauna Hawaiiensis. Vol. l. D. Sharp, ed. Cambridge University Press, Cambridge. xv-ccxxvii.

Polhemus, D.A. 1993. Damsels in distress: a review of the conservation status of Hawaiian Megalagrion damselflies (Odonata: Coenagrionidae). Aquatic Conservation: Marine and Freshwater Ecosystems 3:343-349.

Solem, A. 1990. How many Hawaiian land snail species are left? and what we can do for them. Bishop Museum Occasional Papers 30:27-40.

Stone, C.P 1989. Native birds. Pages 96-102 in C.P. Stone and D.B. Stone, eds. Conservation biology in Hawaii. University of Hawaii Press, Honolulu.

Zimmerman, E.C. 1948. Insects of Hawaii. Vol. 1. University of Hawaii Press, Honolulu. 206 pp.