INTERESTING INSECTS

By N A Martin (2018)

Cabbage tree mite

Little is known about this distinctive unnamed endemic spidermite. It has only been found on the common cabbage tree, Cordyline australis (Asparagaceae) and only in two cities in the North Island. I suspect that it occurs in low numbers on wild plants, but that populations are kept low by its natural enemies.

Conservation status: Rarely found, status unknown.

These mites are very small and live in colonies on the underside of leaves. Adult female mites are about 0.5 mm long. Adult males are slightly smaller and have a pointed end to their abdomen. The adult females have a pale head, front of body and legs. Most of the abdomen is a dark red. The male and juvenile mites are a pale orange-red. Adults are typical mites, having four pairs of legs.

Adult female mites lay brown, spherical eggs in dense clusters on the underside of leaves. The larva that hatches from an egg looks like a tiny adult, but only has three pairs of legs and is a pale orange-red. 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 are orange-red. The large nymphs become darker red like the adult female. 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 leaves. They suck out 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 another leaf on the same branch or a nearby branch. 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 leaves.

This mite requires special procedures and taxonomic knowledge to identify specimens. However, it is the only species of spidermite known to live on cabbage trees. It also forms colonies with large groups of brown eggs that leave large groups of white egg shells.

Predators

Several species of spidermite feeding ladybirds, Stethorus species (Coleoptera: Coccinellidae), are present in New Zealand. One of which has been found feeding on a colony of Cabbage tree mite. Both adults and larvae feed on the mites.

The mite may also be eaten by predatory mites and predatory fly larvae.

Cabbage tree mite have only been found on the common cabbage tree, Cordyline australis (G.Forst.) Endl. (Asparagaceae). The mites live on the underside of leaves and may form large dense colonies.

The mites have only been found on young plants in cities. In native habitats they may live on mature plants, but populations are probably kept low by natural enemies, hence the lack of observations. The mites may also live on other species of cabbage tree.

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.

Occasionally large colonies of Cabbage tree mite develop on the underside of leaves of young cabbage trees away from native ecosystems. These colonies do not appear to weaken plants. Predatory mites, tiny ladybirds and larvae of predatory flies will control populations.

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

Zhang Z-Q, Henderson R, Flynn A, Martin NA. 2002. Key to Tetranychidae of New Zealand. Landcare Research Contract Report. LCO102/144: 1-62.

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