Urtica ferox, commonly known as tree nettle and in Māori: ongaonga, taraonga, taraongaonga, оr okaoka, is a species of nettle endemic to New Zealand. Unlike the other species in the genus Urtica found in New Zealand, all of which are herbaceous, ongaonga is a large woody shrub that can grow to a height of , with the base of the stem reaching in thickness. It has large spines that can result in a painful sting that lasts several days.
Ongaonga is the main source of food for larvae of the New Zealand red admiral butterfly or kahukura, Vanessa gonerilla.
Description
U. ferox can grow to a height of 3 m (9.8 ft) with the base of the stem reaching 12 cm (4.7 in) in thickness. 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 long. These spines are prominent along the salient mid-vein and leaf margin. The leaves range from in width and in length, these are oppositely arranged and there are two stipules per node. The leaf shape is ovulate-triangulate with a serrated leaf margin each bearing a spine of up to . The spines covering the leaf surface, stem and stalk are generally no larger than in length, the spines on the older darker bark are smaller and softer - these don't cause a sting. This nettle is winter deciduous in cold climates, evergreen in mild climates and can lose its leaves in drought conditions if it is growing in shallow soils.
Flowering occurs from November to March and because U. ferox is a dioecious shrub it will cross pollinate, with transferral between the flowers enabled by the wind. Pollen grains are collected by the densely packed stigmas on the flowers and seed dispersal is carried out by rolling, wind, and by birds. The fruit, which are achenes, mature in January, each containing one 1.5 mm long, brown coloured, ovoid-shaped seed which takes one month to germinate. In his study on the germination behaviour of 5 different vascular seeding species, C. J. Burrows found that U. ferox had the lowest germination rate of 59% compared to >85% for the other similar species. He surmised that it is capable of building up a large seed bank in the soil which may survive for several years, citing that European species in the genus Urtica produce seed banks.
Toxin
The toxin present in the spines is triffydin (or tryfydin). This toxin contains histamine, serotonin and acetylcholine, the latter causing powerful stimulation of the parasympathetic nerve system. Multiple stingings can have a very painful reaction which causes inflammation, a rash, and itching. In high concentrations it can also cause: loss of motor movement, paralysis, drop in blood pressure, convulsions, blurred vision, confusion and in extreme cases, death.
Acute polyneuropathy can occur due to U. ferox stings; and there has been one recorded human death from contact—a lightly clad hunter who died five hours after walking through a dense patch. There is also one other likely death, with the mystery of the death only being solved by the pathologist years afterwards.
Distribution and habitat
U. ferox is endemic to New Zealand and inhabits coastal and lowland forests and shrublands in the North, South, and Stewart Islands. It is commonly found in clearings or forest margins, often forming large patches, from 0–600 m above sea level. It has also been recorded on the Hen and Chicken Islands along stream-beds.
Ecology
Although U. ferox is a toxic shrub, it also plays a significant part in the phenology of a native butterfly, namely the red admiral (Vanessa gonerilla). The leaves of the tree nettle is the preferred food and provide protection for the butterfly larvae. When the larvae arrives in the leaves, it will curve the tip of nettle leaf, use the silken threads to make the leaves stick together, and construct a secure place where it can eat the food. Due to the caterpillar's activities, the leaves of shrub can fall off. The eggs of butterflies, also including the yellow admiral (Vanessa itea), are laid on the leaves of nettle during the spring and summer. It takes eight to ten days for the eggs to incubate. Beside these butterflies, some mammalian pests also eat the leaves of tree nettle, like the Common brushtail possum, goats and deer. However, because no non-flying, non-marine mammals existed in New Zealand prior to human arrival in the last one thousand years, it is unadapted for them.