Urtica ferox

Urtica ferox, commonly known as tree nettle and ongaonga, is a species of nettle endemic to New Zealand. A member of the Urticaceae, U. ferox is covered in spines or trichomes, which contain numerous toxins that are poisonous to humans. U. ferox is a large woody shrub that can grow to a height of 3 metres (10 feet), with the base of the stem reaching 12 cm (5 in) in thickness. Its large spines can result in a painful sting that can last several days or weeks, and has even resulted in one recorded human death. U. ferox is found throughout New Zealand.

The plant was first described by German naturalist Georg Forster in 1786. U. ferox is pollinated by the wind, and its seeds are later dispersed by gravity or the wind. Several butterfly and moth species can be hosted on the plant, and it is the favoured source of food for the larvae of the New Zealand red admiral (Vanessa gonerilla) butterfly. U. ferox had some traditional and medicinal uses for the indigenous Māori people, despite it being a plant usually avoided by people. U. ferox's 2023 assessment in the New Zealand Threat Classification System was "Not Threatened".

Urtica ferox (ongaonga) is a dioecious species of shrub or small tree in the family Urticaceae.^[1] U. ferox can grow to a height of 3 metres (10 feet) with the base of the stem or trunk reaching 12 centimetres (5 in) in thickness.^[1][2] The pale green leaves are very thin like a membrane and the surface of the leaf, stems and stalks are covered in stiff stinging hairs that can grow up to 6 mm (0.24 in) long. These spines are prominent along the salient mid-vein and leaf margin. The leaves range from 3–5 cm (1.2–2.0 in) in width and 8–12 cm (3.1–4.7 in) in length; these are oppositely arranged and there are two stipules per node. The leaf shape is triangular to ovate-triangular with serrated leaf margins. The spines covering the leaf surface, stem and stalk are generally no larger than 6 mm (0.24 in) in length.^[1][3][4]

Flowering occurs from November to March. Pollen grains are collected by the densely packed stigmas on the flowers. The fruit, which are achenes, mature in January, each containing a single seed, 1.5 mm long, which takes one month to germinate.^[5][1][6] Burrows (1996) found that U. ferox is capable of building up large seed banks in the soil which may survive for several years, noting that European species in the genus Urtica produce seed banks.^[7] U. ferox has a diploid chromosome count of 48.^[1]

Urtica ferox was first described by German naturalist Georg Forster in 1786.^[8][9] There are 69 species of the Urtica genus currently accepted by the Plants of the World Online taxonomic database. These species are found throughout the entire world.^[10] There are six species native to New Zealand.^[11] U. ferox is unique among the New Zealand Urtica members due to it being tall, woody, and not closely related to other New Zealand members of the genus Urtica.^[12] Grosse‐Veldmann et al. (2016) constructed a phylogenetic tree of the genus Urtica based on gender characteristics and genetic sequencing.^[13] They found that the South African species, U. lobulata, is the sister species to U. ferox, meaning U. lobulata is the most closely related species to it.^[14][12] It has been presumed that the distinctive morphological evolution and toxicity of U. ferox is a result of the ecological influence of browsing by the now-extinct moa.^[12][15]

The etymology (word origin) of U. ferox's genus name, Urtica, is derived from the Latin word for stinging nettles, which comes from urere, meaning to burn. The specific epithet (second part of the scientific name), ferox, means 'ferocious', 'savage', or 'wild'. In Māori, the word ongaonga is used to describe anything 'annoying' or 'irritating'. Cognates found across different Polynesian languages, including 'hongohongo' and 'okaoka', are words typically used to describe stinging insects.^[16] U. ferox has several recorded Māori names, including: houhi, ongaonga, okaoka, puruhi, taraonga, and taraongaonga.^[17]

Urtica ferox's seeds are dispersed by gravity and the wind.^[18] Like all Urtica species, U. ferox is pollinated by the wind,^[19][20] although bees occasionally visit the flowers to collect nectar.^[21][22] Urtica ferox can be semideciduous or entirely deciduous in some southern localities in winter, an uncommon characteristic in New Zealand's native plants, which only affects about five percent of the country's native woody species.^[18][23]

The plant is a favoured host to the larvae of New Zealand red admiral (Vanessa gonerilla).^[24] The female butterflies lay their eggs on the new growth of U. ferox leaves, which they are essentially immune to. The caterpillar larvae hatch and then spend up to six weeks feeding on those leaves. During this vulnerable stage, the caterpillars use the spiny leaves for protection, wrapping the leaves around themselves similar to a blanket or building a small tent to hide from potential predators such as birds and insects. Populations of V. gonerilla have decreased, possibly in part because of people treating U. ferox as a weed and removing it.^[25] The yellow admiral (V. itea) butterfly can also be present on the plant.^[26] Several other moth caterpillar species feed on the plant, including the Mnesictena flavidalis, Diarsia intermixta, Meterana inchoata, Pasiphila urticae, Epichorista crypsidora and two species from the genus Pseudocoremia and also Udea.^[21][26] Rust fungi from the genus Puccinia can be present on the plant.^[27]

Deer, sheep, and goats eat the leaves of U. ferox despite their stinging spines, and fruits are sometimes eaten by common brushtail possums.^[19][21] U. ferox's stinging spines have killed cattle, dogs, and horses in the past.^[21][16]

The toxins present in the spines contain the neurotransmitters histamine, serotonin and acetylcholine.^[28][29] A 2022 study in the Journal of Biological Chemistry found that two peptides: Δ-Uf1a and β/δ-Uf2a, are likely responsible for the painful sensations. Δ-Uf1a is suggested to create pain by disturbing cell membranes and β/δ-Uf2a the same by modulating voltage-gated sodium channels. Stings have a painful reaction which causes hives, numbness, and itchiness. In severe cases, it can also cause: ataxia, blurred vision, confusion, polyneuropathy, respiratory distress, and hypersalivation. These sting reactions can last for days to weeks.^[28][30] The reaction can potentially be fatal.^[31]

There has been one recorded human death from contact in 1961, a young hunter who died five hours after walking through a dense patch in the Ruahine Ranges.^[32][1] A second death, that of a young man in 2002, was retrospectively attributed to U. ferox by a pathologist some fifteen years later.^[33]

Urtica ferox is endemic to New Zealand. The plant is found throughout the North and South Islands.^[1] The botanist Thomas Cheeseman believed that Otago was the species' southern limit.^[34] However, a more recent revision of New Zealand flora suggests that the species occurs south to Stewart Island.^[35] U. ferox is especially common near Wellington and Christchurch.^[21] The botanist Lucy Moore noted U. ferox is uncommon or not found north of Auckland.^[36] The 2023 assessment of U. ferox in the New Zealand Threat Classification System was "Not Threatened".^[1]

Urtica ferox is typically found in coastal to lowland environments.^[1] It is common in forest margins and shrublands. It occurs from sea level to 800–1,000 m (2,600–3,300 ft) above sea level at maximum elevation.^[21][32] U. ferox is found on various soil types, but it prefers fertile soils, particularly with basaltic rock as a substrate. It can be found on such basaltic formations well inland, for example, in the Canterbury Region on the Harper Hills near Glentunnel, where it is uncommon. U. ferox also occurs on alluvial soils along the valley floor of Taramakau Valley of Westland, within Arthurs Pass National Park.^[37] U. ferox is occasionally found growing on the bases of fallen trees and stream-beds.^[37][38]

Urtica ferox is a plant usually avoided by people, but it did have some traditional and medicinal uses for the indigenous Māori people.^[37][39] In Māori mythology, the plant was placed in forests to prevent people moving freely and to irritate people. The Polynesian explorer, Kupe, was said to have placed several obstacles, including this plant, to hinder pursuers whose wives he had stolen.^[29] In traditional medicinal practices, the bark of U. ferox and the leaves of kawakawa (Piper excelsum), boiled together, were used to treat skin conditions.^[16] The plant was also a traditional Māori food source. The inner stems were sometimes consumed after the leaves and outer bark had been removed. The thin film that makes up the inner bark was also eaten raw and is said to have a sweet taste.^[25][17]

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Websites

• Media related to Urtica ferox at Wikimedia Commons
• Urtica ferox discussed on Critter of the Week, 28 April 2023