Mistletoe research group
Background on Mistletoes
The name “mistletoe” is given to plants that are stem hemi-parasites. Hemi-parasites can produce their own food through photosynthesis, but they also use specially adapted roots called haustoria to extract water and nutrients from the xylem tissues of their host plant.
There are about 1400 mistletoe species around the world. Mistletoes belong to four plant families in the order Santales: Eremolepidaceae, Misodendraceae, Loranthaceae and Viscaceae. The first two of these families are small and confined to the Americas, while the latter two are larger and have a world-wide distribution. All New Zealand mistletoes belong to Loranthaceae or Viscaceae.
Viscaceae contains 400 mistletoe species with small, insect-pollinated flowers, including Viscum album, the European mistletoe favoured by druids. Loranthaceae contains 950 species in 50-80 genera. Most of these species have large, bird-pollinated flowers and are primarily distributed in the Southern Hemisphere.
New Zealand is home to eight native mistletoe species. All but one of these species are endemic (found nowhere else in the world). Ileostylus micranthus is also found on Norfolk Island. A ninth species, Trilepidea adamsii, was last seen in 1954 and is now presumed extinct. A tenth species, Muellerina celastroides, may have been found in the 1800s in the Bay of Islands, but there have been no confirmed records of this common Australian species in New Zealand since then, so it is uncertain whether this early record was an error.
Three New Zealand mistletoe species (Peraxilla tetrapetala, Peraxilla colensoi and Alepis flavida) are known as “beech mistletoes” because their primary host trees are southern beech (Nothofagus spp). The beech mistletoes belong to the family Loranthaceae and they have hermaphroditic, bird-pollinated flowers (see Pollination Research). All of the beech mistletoes flower in December or January (New Zealand summer).
Peraxilla tetrapetala (also called ‘red mistletoe’) has bright red flowers and insect galls on its leaves. Its most common hosts are mountain beech (Nothofagus solandri var. cliffortioides) and tawheowheo (Quintinia serrata) in the North Island, mountain beech (Nothofagus solandri var. cliffortioides) in the central and southern Alps and silver beech (N. menziesii) in South Westland, Fiordland and the Catlins.
Peraxilla colensoi (or ‘scarlet mistletoe’) is similar to red mistletoe but has larger leaves and flowers. It is the largest mistletoe species in New Zealand, sometimes reaching up to 3 m in diameter. Scarlet mistletoe has been recorded growing on 16 different host species, but its most common host is silver beech.
Alepis flavida (or ‘yellow mistletoe’) is easily differentiated by its smaller, yellow-orange flowers. It also tends to grow on branches further out from the host trunk, and its leaves have a faint red margin. Yellow mistletoe is the most host-specific mistletoe species in New Zealand, nearly always growing on mountain beech.
A fourth beech mistletoe species, Trilepidia adamsii, was last seen in 1954 and is presumed extinct. It probably had a very limited natural distribution even at the time of European settlement. This limited distribution probably made T. adamsii vulnerable to habitat loss, which would have resulted in small, fragmented populations that were susceptible to factors such as damage from over-collecting and herbivory. Habitat loss might also have led to simultaneous reductions in bird populations, resulting in reduced pollination and dispersal.
Analysis of flower structure (from preserved material and historical drawings) shows that Trilepidea had a more complex two-stage explosive flower mechanism as seen in a number of African mistletoes, though previously unknown in Australasia. This may have made it particularly vulnerable to pollination failure (see Ladley and Kelly 1996).
Beech mistletoes are now uncommon in many parts of New Zealand. However, they are still locally abundant in some parts of the Southern Alps such as Craigieburn, Lake Ohau, the Lewis Pass, Mavora, Haast and the Eglinton Valley.
Two New Zealand mistletoe species are called “green mistletoes” or “pirita”: Ileostylus micranthus and Tupeia antarctica. The green mistletoes also belong to the family Loranthaceae, but unlike their beech mistletoe cousins, they have small (2-5 mm), greenish-yellow flowers and are pollinated by insects like bees and flies.
I. micranthus usually has slightly larger leaves (20-80 mm long) than T. antarctica and its young branches are flattened sideways and green. Tupeia is distinguished by its round young branches, opposite leaves 20-30 mm long and its pale, whitish bark. It rarely grows larger than 1 meter in diameter, compared to I. micranthus which can reach 2 m across.
Both green mistletoe species have small, greenish-yellow flowers that appear in spring. The flowers are pollinated by a range of small insects such as flies and moths. These flowers open by themselves when they are mature, unlike the beech mistletoe flowers, which must be opened by birds or bees (see Pollination Research). I. micranthus fruits are bright yellow, while T. antarctica fruits are white with purple markings. Birds must remove the fruit flesh and disperse the seed onto a host branch for the seed to germinate.
Unlike the beech mistletoes which grow almost exclusively on southern beech tree hosts, green mistletoes grow on a wide range of host plants. I. micranthus has been found on more than 206 different native and exotic host species. It mostly grows in low altitude shrubland, but totara (Podocarpus totara) is a common host in Northland, Nelson and Southland.
T. antarctica mostly grows in eastern shrublands. It can grow at higher and colder sites than I. micranthus, and it grows on over 37 different host species. Five finger (Pseudopanax arboreus), Pittosporum species and putaputaweta (Carpodetus serratus) are its most common native hosts, while in urban and agricultural areas it often grows on introduced plants such as tree lucerne (Chamaecytisus palmensis).
Both green mistletoe species grow throughout the North and South Islands. I. micranthus is also found on Stewart Island and Norfolk Island. Green mistletoes are the most common mistletoes in New Zealand, but they are currently declining in many areas, especially in the North Island.
The “dwarf mistletoes” are three endemic New Zealand mistletoe species belonging to the genus Korthasella. Unlike the rest of New Zealand’s mistletoes, they belong to the family Viscaceae. K. salicornioides grows mainly on manuka and kanuka, while K. lindsayi and K. clavata grow on a wide range of shrubs and small trees.
Dwarf mistletoes mimic the shoots of their host, and at less than 10 cm long, they can be difficult to locate. Their flowers are tiny and inconspicuous, and their fruits are small, single-seeded, fleshy berries. There is some evidence that their fruits may be explosive, shooting their seeds for several metres. However, it has been suggested that the seeds may also be dispersed over longer distances by birds.