Poisonous plants native to New Zealand are not numerous but, with the many toxic species now naturalised here, plants poisonous to man and animals must be seriously considered.
The Maoris recognised that the kernels of karaka (Corynocarpus laevigatus) caused paralysis and took steps to prevent these poisonings; they realised, too, that tutu (Coriaria arborea) was toxic. Severe, if not fatal, stinging must have been experienced from ongaonga, the tree nettle (Urtica ferox), while the poisonous nature of green poroporo berries (Solanum aviculare and S. laciniatum) would have been recognised.
The introduction of livestock with European colonisation soon revealed the toxicity to animals of tutu, ngaio (Myoporum laetum), Strathmore weed (Pimelea prostrata), waoriki (Ranunculus rivularis), and bracken (Pteridium aquilinum var. esculentum). With the expansion of agriculture and the introduction of many species that soon became widely naturalised, hemlock (Conium maculatum), St. John's wort (Hypericum perforatum), and ragwort (Senecio jacobaea), the cause of Winton disease, were additional important poisonous plants. Increasing horticultural importations yielded such poisonous plants as yew (Taxus baccata), laburnum (Laburnum vulgare), and Jerusalem cherry (Solanum pseudocapsicum). Toxic species which have escaped from horticulture and are now naturalised include Cape tulip (Homeria collina), oleander (Nerium oleander), and species of cestrum. The most recent addition to the list of poisonous plants is the fungus Pithomyces chartarum, the spores of which are responsible for facial eczema of sheep, a photosensitivity disease of quite long standing in New Zealand.
Connor (1951) lists almost 200 species of known and suspected poisonous plants. Of 118 species definitely known to be poisonous only 12 are native, the remainder being naturalised species. Some 72 species have been suspected of causing plant poisoning, and these include 22 indigenous species. Plant families with large numbers of toxic species include Ranunculaceae (14), Papaveraceae (8), Euphorbiaceae (8), Papilionaceae (15), Umbelliferae (8), Compositae (14), Solanaceae (8), Gramineae (8); the remaining 43 families are represented by a few species each. Most species are toxic only when eaten, but nettles are contact poisons, as also are the cultivated poison ivy (Rhus toxicodendron) and primula (Primula malacoides).
A wide range of poisonous principles is present – alkaloids in hemlock, laburnum, ragwort, poroporo, yew, and Jerusalem cherry; glycosides in karaka, buttercups (Ranunculus), and foxglove (Digitalis purpurea); cyanogenetic glycosides (those which yield prussic acid) in cherry laurel (Prunus laurocerasus) and titoki (Alectryon excelsus). The poisonous principle in tutu is tutin, a picrotoxin-like substance which is found in all parts of the plant except the black, swollen petals. Tutin, on passing through the passion vine hopper (Scolypopa australis), is converted to mellitoxin and other honey toxins, which remain in honey dew secreted by the hopper on tutu plants. Honey dew, when gathered from tutu by bees and incorporated into honey, is poisonous to man. St. John's wort contains hypericin, a photo-sensitising pigment. Buttercup glycosides yield protoanemonin, a volatile substance which causes acute irritation and inflammation of the stomach; this toxin is destroyed when the plants dry. Nitrates accumulated by such plants as oats (Avena sativa), mangels (Beta vulgaris), and variegated thistle (Silybum marianum) may be broken down in the stomach by bacterial or enzyme activity into toxic nitrites. Ngaio contains a toxic essential oil.
Toxic properties may be restricted to specific parts of plants – in some members of the Cruciferae only the seeds, which contain mustard-oil glycosides, are poisonous. The kernels of karaka are toxic, but only when uncooked. Ripe seeds of laburnum are the most toxic part of this plant, while in many species all parts of the plant are poisonous.
by Henry Eamonn Connor, M.SC., Botany Division, Department of Scientific and Industrial Research, Lincoln.
- New Zealand D.S.I.R. Bulletin, No. 99 (1951), “The Poisonous Plants in New Zealand”, Connor, H. E.