INTERESTING INSECTS

By N A Martin (2017)

Flax Spidermite, New Zealand flax spidermite

This endemic spidermite was first found on leaves New Zealand flax, Phormium tenax (Hemerocallidaceae), in 1949 at Paiaka by Dr FJ Newhook. It was later rediscovered in 1969 in a flax plantation on the Moutoa Estate, near Shannon in Southern North Island. This led to its formal description and naming in 1970. It has since been found in Auckland and probably occurs throughout New Zealand. It occurs on both species of flax plants in city gardens and parks as well as native ecosystems.

Conservation status: Probably widespread, not threatened.

These mites are very small. Adult female mites are 0.6-0.8 mm long and are longer than wide. Adult males are slightly smaller and have a pointed end to their abdomen. Adult mite bodies have large lateral dark areas and a brownish green background colour. The four pairs of legs are whitish or tinted orange. Adult female mites lay pale orange spherical eggs (0.143-0.169 mm diameter) on the underside of flax leaves. The larva that hatches from an egg looks like a tiny adult, but only has three pairs of legs and lacks the reddish colour and black spots. The mite larva moults (changes skin) into a nymph which has four pairs of legs like the adult. The first nymphal stage moults into a second nymph. Both nymphal stages have lateral black spots. The last juvenile stage moults into an adult mite. Adult males mate with newly emerged adult females.

Feeding

The mites have pointed mouth parts that puncture the surface cells of flax leaves. They suck the contents of the cells. The black areas in the abdomen are part of their gut where the plant cell contents are digested.

Walking and web spinning

The mite uses its legs for walking. As they walk they produce a web from their abdomen which is attached to the leaf surface. When a dense colony forms the surface of the leaf becomes covered by webbing.

Dispersal to new leaves and new plants

When a dense colony develops or a plant leaf is no longer suitable, mites may walk to other parts of a leaf or to new leaves of the plant or adjacent plants. Spidermites are also dispersed by wind. Mated female mites will climb to the top of the plant produce a strand of silk and stand waiting for a gust of wind to take them away. The mites form new colonies on the underside of flax leaves. New colonies are often formed where there is some shelter, such as a moulted insect skin.

This mite requires special procedures and taxonomic knowledge to identify specimens. However, it is the only species of spidermite known to live on flax, Phormium species, so it can be recognised by its presence in colonies of the mites on its host plants. Mite feeding creates pale areas on the underside of leaves, while dense colonies appear red due to high numbers of eggs.

Predators

At least two kinds of predators have been found feeding on flax spidermites, ladybirds and predatory mites. It is likely that they are also fed on by predatory fly larvae and adults and nymphs of predatory sucking bugs (Hemiptera).

The name of only one of the predatory mites is known. Phytoseiulus persimilis (Acari: Mesostigmata: Phytoseiidae) is the commonest predator of flax spidermites around Auckland. This predatory mite comes from Southern Europe and was imported into New Zealand to assist control of two-spotted mite, Tetranychus urticae (Acari: Tetranychidae), a pest of greenhouse and outdoor crops. Adult female Phytoseiulus persimilis are bright red and larger than flax spidermites. They lay larger oval red eggs. It's larvae, nymphs and adult males are also red.

Several species of spidermite feeding ladybirds, Stethorus (Coleoptera: Coccinellidae), are present in New Zealand. The name of those that eat flax spidermites are not yet known. The adults are tiny and black. Oval eggs, white, tan or reddish are laid in mite colonies. The larvae may be white, white with dark spots or dark grey. The pupae are black.

Table: Predators of Flax spidermite, Tetranychus moutensis (Acari: Prostigmata: Tetranychidae), from Plant-SyNZ database (20 April 2017). The reliability index shows the quality of evidence for the host association (0-10, 10=high quality).

Scientific Name	Common Name	Classification	Enemy Type	Reliability Index	Biostatus
Phytoseiulus persimilis Athias-Henriot, 1957	(Mite)	Acari: Mesostigmata: Phytoseiidae	predator	10	adventive
Stethorus sp.	Spidermite ladybird (Beetle)	Coleoptera: Coccinellidae	predator	7	unknown

Flax spidermites are found on both species of native flax, Phormium. Colonies of mites form where there is shelter on the underside of leaves. The mites have pointed mouth parts that puncture the surface cells of flax leaves. They suck the contents of the cells. Feeding removes the green cell contents and old and active mite colonies can be recognised by the pale colour of the leaf in and around mite colonies. The mites also cover the surface of the leaf with silk webbing.

Table: Host plants of the Flax spidermite, Tetranychus moutensis (Acari: Prostigmata: Tetranychidae) from Plant-SyNZ database (20 April 2017). The reliability score shows the quality of evidence for the host association (1-10, 10=high).

Common Name(s)	Scientific Name	Family	Reliability Index	Biostatus
Coastal flax, Mountain flax, Kōrari-tuauru, Wharariki	Phormium cookianum Le Jolis	Hemerocallidaceae	9	endemic
Flax, Lowland flax, New Zealand flax, Swamp flax, Harakeke, Harareke, Kōrari	Phormium tenax J.R.Forst. & G.Forst.	Hemerocallidaceae	10	endemic

Occasionally large colonies of flax spidermite develop on the underside of flax leaves, especially on plants in modified environments such as gardens and parks. These colonies do not appear to weaken plants. Predatory mites and tiny ladybirds will control populations.

Flax spidermite irritation to flax workers

This species of mite was rediscovered in 1969 after workers cutting New Zealand flax for processing for fibre complained of 'itching' on the face, neck, and hands. They said it was due to the mites that occurred in large numbers on the underside of leaves. Skin scrapings taken from the affected areas showed that mites were present. During cutting and handling of mite-infested flax, the mites had become squashed and smeared on the exposed skin causing considerable 'itching' to susceptible people. In some instances, medical treatment was necessary.

Biological control

Predators from other countries have been imported and released into New Zealand to control pests of crops and other valued plants. Up to the 1980s, the primary concern when selecting a predator to import was its likely effectiveness within New Zealand. Since the mid-1970s there has been increasing concern about the potential impact of any imported organism on New Zealand native organisms and valued flora and fauna. A predator can impact not only directly on native herbivores, but also on native predators by competing with them for food. Today legislation requires that the benefits of any imported organism, including predators, outweigh any potential harm to native or valued organisms.

The predatory mite, Phytoseiulus persimilis (Acari: Mesostigmata: Phytoseiidae), is from southern Europe. It was originally discovered in in the late 1950s in Chile as a very effective predator of two-spotted mite, Tetranychus urticae (Acari: Tetranychidae), that damages greenhouse and outdoor crops. Since the 1960s it has been used for control of two-spotted mite in greenhouses as part of Integrated Pest Management (IPM) programmes for those crops. It was imported into New Zealand in 1967 for that purpose. After it was discovered that it existed outdoors in Southern Europe, an outdoor strain of the predator was imported into New Zealand in 1977. This predator is now known to feed on native spidermites, such as the flax spidermite, and on gorse mite, Tetranychus lintearius Dufour, 1832, which was imported for biological control of gorse. It would be difficult to import a predator such as Phytoseiulus persimilis under today's laws where the benefits have to clearly outweigh risks to native and valued fauna.

In addition to the deliberate importation and release of predators and parasites, they can accidentally arrive and establish in New Zealand. Two of the named species of spidermite ladybirds, Stethorus species (Coleoptera: Coccinellidae), are endemic, while one is an Australian species.

Cumber RA. 1954. Injury to Phormium caused by insects, mites, and molluscs. New Zealand Journal of Science and Technology. 36 A ( 1): 60-74.

Manson DCM. 1970. The spider mite family Tetranychidae in New Zealand. V. Tetranychus (Tetranychus) moutensis a new species of spider mite from flax (Phorium tenax Forst.). New Zealand Journal of Science. 13: 323-327.

Plant-SyNZ: Invertebrate herbivore-host plant association database. plant-synz.landcareresearch.co.nz/

Thomas WP, Walker JTS. 1989. Tetranychus urticae Koch, two-spotted mite (Acari: Tetranychidae). Pp. 245-250. In: Cameron PJ, Hill RL, Bain J, Thomas WP (eds). A review of biological control of invertebrate pests and weeds in New Zealand 1874-1987. CAB International, Wallingford, Oxfordshire, UK.

The New Zealand Institute for Plant & Food Research Limited (Plant & Food Research) for permission to use photographs.

1 December 2018. NA Martin. Changed symbol used for apostrophes.