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Species of legume

Wiliwili (Erythrina sandwicensis) is a species of tree in the pea family, Fabaceae, that is endemic to the Hawaiian Islands. It is the only species of Erythrina that naturally occurs there. It is typically found in Hawaiian tropical dry forests on leeward island slopes up to an elevation of
Wiliwili means "repeatedly twisted" in the Hawaiian language and refers to the seedpods, which dehisce, or twist open, to reveal the seeds.

Wiliwili trees grow to a height of with a gnarled and stout trunk that reaches in diameter. The bark is smooth, slightly fissured, and covered in gray or black spines up to in length. The bark on the main trunk of mature trees has a distinct orange cast, which is caused by a terrestrial alga. The wiliwili is summer (dry season) drought deciduous. The dry season usually begins in late April or in May, and trees in the wild typically lose all of their leaves before they bloom. The flowers appear in the first half of the dry season, from April through July. The horizontal raceme and the erect standard are adaptations to pollination by passerine birds. Many other erythrinas are pollinated by hummingbirds, which do not occur in Hawaii. Pods develop and persist on the tree, with the seeds remaining attached long after the pods have opened. The seeds are dislodged by heavy downpours that generally start around November in the islands. Many seeds germinate quickly, and a well established seedling can grow to 4 feet (1 m) in height before the start of the next dry season. That the wiliwili bears spines is unusual for a species that has evolved in the isolated Hawaiian Islands, without the presence of ungulates or other large herbivores. Many plants in the islands have consequently evolved away the protection of spines. The wiliwili is thought to be closely related to E. tahitensis, a tree endemic to the Tahitian Archipelago, and E. velutina, a widespread species found in tropical South America and the Caribbean. Seven other species of Erythrina were under general cultivation in the Hawaiian Islands, but have been mostly extirpated by the alien gall wasp (see Conservation, below). No non native species of Erythrina is known to be naturalized in Hawaii. The wiliwili is distinguished from the other seven cultivated species by a pod with only one to three red or yellow orange seeds, which sink in water; non native Erythrina have pods with larger numbers of brown seeds, which float in water.

Native Hawaiians made a number of items from wiliwili wood because of its low density, such as mouo (fishing net floats), ama (outrigger canoe floats, and extremely long papa hee nalu (surfboards) called olo. Olo, which averaged 18 feet (5 m), were exclusively ridden by alii (royalty). The wood was sometimes used for the waa (hull) of outrigger canoes intended to be used near shore, for recreation, or for training. The shiny orange red seeds were strung into lei. that the Hawaiian wiliwili population was under immediate threat due to an infestation by a gall wasp, Quadrastichus erythrinae, which had been first reported in Hawaii in April of that year. This invasive species appears to have arrived in Hawaii via southern Taiwan, Singapore and southern China within only two years. This species was not previously known to science and was formally named and described in 2004. It is thought to have originated in Africa, and the means of its rapid dispersal across South Asia, the Pacific, and southern North America is not understood. The majority of trees of introduced Erythrina species have died as a result of gall wasp infestation. Native wiliwili forests have also been hard hit, particularly Puu o Kali on the island of Maui prior to 2001, the best remaining example of a Hawai'i low elevation dryland forest ecosystem. The USGS Pacific Island Ecosystems Research Center study site now documents the destructive impact of two invasive insect species (African bruchid beetle Specularius impressithorax and erythrina gall wasp) on Erythrina sandwicensis. A parasitoid wasp, Eurytoma erythrinae, was released by the Hawaii Department of Agriculture in December 2008 as a biocontrol to minimize the damaging effects of Quadrastichus. Eurytoma wasps lay their eggs in the galls created by Quadrastichus wasps. Eurytoma larvae hatch faster than Quadrastichus larvae, on which they exclusively feed. The introduction of Eurytoma has reduced the population of Quadrastichus to a point where the wiliwili trees no longer die, but still have a much reduced production of viable seeds. That's largely because Eurytoma larvae need to feed on multiple Quadrastichus to complete their development, and egg laying female Eurytoma therefore ignore small isolated galls containing a single larva. The Hawaii Departments of Agriculture and of Land & Natural Resources therefore plan to introduce a second parasitoid wasp from Africa, Aprostocetus nitens. The smaller A. nityens completes its development feeding from a single Quadrastichus larva, and it is therefore expected to complement Eurytoma by targeting isolated larvae.

The genus Erythrina was established by Linnaeus in 1753 in his book, Species Plantarum. The name means "red", a reference to the flower color of some of the well known species. In 1786, in his book Encyclopédie Méthodique: Botanique, Jean Baptiste Lamarck originated the name Erythrina monosperma for a Leguminous tree from India and Southeast Asia. That tree has been known as Butea monosperma ever since William Roxburgh created the genus Butea in 1795. Unaware of Lamarck's name, Charles Gaudichaud Beaupré created the name Erythrina monosperma for the Hawaiian Erythrina in 1830 in his book Voyage of the Uranus. In 1841, William Jackson Hooker and George Arnott used Gaudichaud's name in their book, The Botany of Captain Beechey's Voyage. Gaudichaud's name, Erythrina monosperma, was used by most of those who described the Hawaiian species of Erythrina throughout the nineteenth century, including William Hillebrand. In 1932, Otto Degener created the name Erythrina sandwicensis to replace Erythrina monosperma. He stated no reason for the name change. He presented a more complete synonymy than the one given here. Erythrina sandwicensis is the name that has been used since 1932.