Eriophyoid mites are commonly known as ticks, rust mites or gall mites. In scientific classification, eriophyoid mites belong to the superfamily Eriophyoidea, the subclass Acari, the class Aracnida and then the phylum Arthropoda. Arthropods are animals that have a segmented body and an exoskeleton. Members of the class Arachnida, such as spiders, have a body divided into two main parts (propodosoma and opisthosoma) and have four pairs of legs. Acari have an even simpler body structure. Their propodosoma and opisthosoma are only divided vaguely by a transverse recess. There are around 3,600 species of eriophyoid mites worldwide (Amrine and Stasny, 1994; Kuang, 1995;Amrine, 1996), but it is estimated that the total number of species may amount to more than 50,000 (Amrine, 1996). Except polar areas, eriophyoid mites can be found anywhere that has plants. There are not many scholars studying them (Huang et al., 1996), so their species are little understood and there are still many questions associated with their biology.
Eriophyoid mites are extremely small, ranging in length from 0.1-0.3mm, so they are often invisible to the naked eyes. Their appearance is rather worm-like. Unlike most mites that have four pairs of legs, eriophyoid mites have only two pairs of legs, located at the front end of the body. This is also the most evident difference between eriophyoid mites and other mites. The second difference is the location of genitalia. The genitalia of most mites are located at the end of the abdomen while that of eriophyoid mites are behind the coxa at the anterior end of the body, a location equivalent to man¡¦s chest. Eriophyoid mites have some superficial rings on their body with only few setae. They vary greatly in body design and appearance. Some are worm-like. Some have very long setae and a big wide shield, like wearing armor. Some secrete wax to enclose themselves. Some have reticulate design on their shield.
Eriophyoid mites are plant feeding mites with their piercing-sucking type of mouthpart, causing malformation of or obvious damage to host plants. Researchers in early days named eriophyoid mites as gall mites, bud mites, blister mites, erineum mites and rust mites based on the damage they caused to host plants. In fact, most eriophyoid mites do not induce apparent damage to host plants. They are like vagrants, wandering around on leaves and feeding on them. They are often too small to be noticed by humans.
Among different types of damage caused by eriophyoid mites, galls often attract the most attention. That is because their odd shapes often arouse researchers¡¦ curiosity. Galls are abnormal growths of plants induced by other organisms, including enlargement and multiplication of plant cells. Galls are used by their producers as havens or food sources. Most mite galls occur on leaves, sometimes on branches, buds or flowers, but so far no galls on roots have ever been found. Galls may occur on different plant species and come in a variety of forms, from open patches of hairs to semi-open or closed, rectangular, round or irregular swellings. Galls are ¡§closed¡¨ environment which is defined in terms of biology instead of physics because they are used, for example, as food sources, havens to hide from enemies and places to find mates (or spermatophores). Galls are like isolated islands. They provide eriophyoid mites with security and do not limit their movement like a jail. Even a closed-type gall has an opening on the other side of the swelling to allow eriophyoid mites go in and out freely. Around 1,000 out of more than 3,000 species of eriophyoid mites cause galls (or hair-like patches) to their host plants. In other words, gall-producing eriophyoid mites account for one-third of the total species. However, based on preliminary researches in Taiwan, only less than 1/20 species are gall-making species. Such a great disparity in ratio does not come from Taiwan's unique environment. Instead, it is caused by the sole emphasis on eriophyoid mites-causing damage by early scholars and contemporary western researchers, who have inclined to ignore vast majority of free-living eriophyoid mites.
In the 18th century, galls caused by eriophyoid mites were once considered as fungi and were named accordingly. People in the 19th century believed galls induced by eriophyoid mites were formed by insect larva, so they considered them as insect-forming galls. It was not until the early 20th century that researchers, with assistance of advanced microscopes, started to understand some galls were induced by eriophyoid mites.
Two main types of life cycle occur in eriophyoid mites. The first is called a simple life cycle: eggs, first nymph, hibernation, second nymph, and then adult. The second is called a complex life cycle, which involves two types of females, the protogyne or primary form, and the deutogyne or wintering (summering or migrating) form. Eriophyoid mites practice direct reproduction. That is, the male produces spermatophores and then the female finds and deposits them in its spermatotheca. Before eggs lay, the female squeezes sperms out of the spermatotheca to fertilize the eggs. The fertilized eggs then develop into female mites. When the sperms in the spermatophores are used up or when the female does not find any spermatophore, the unfertilized eggs it lays develop into male mites. This type of reproduction is called arrhenotokous parthenogenesis. Therefore, all male eriophyoid mites are haploid.
The more common and important eriophyoid mites in Taiwan can be identified through the structurally different galls they cause:
- air-like galls: Galls on litchis are induced by Aceria litchii (Keifer, 1943) and Acaspina litchii (Haung et al., 1990). In early days, these galls were mistakenly identified as caused by fungi and named accordingly as erineum . Galls on longan trees are induced by Aceria litchii (Keifer, 1943) and Neoepitrimerus longanus (Huang, 1996). Hair-like galls on Litsea hypophaea Hayata are induced by Aculodes kostermansii . Presently Aceria litchii are seldom found on lichiis and longan trees. The reason probably lies in the use of pesticide which eradicates the dominant gall mites, the Aceria litchii , that in turn are replaced by less dominant gall mite species. Such scenarios are commonly seen among e riophyoid mites.
- Blister-like galls: Galls on Schefflera actophylla are induced by Notalox sp., which can be found all over the island. E riophyoid mites wander around on swelling blisters and feed on them. When the blisters become flat, which probably signifies their final stage, no more e riophyoid mites will be found.
- Open galls: Galls on Cryptocarya chinensis are induced by Aculodes sp. They become the most abundant in around early March in spring. The hair inside the gall is transparent and becomes brown after spring. E riophyoid mites also disappear gradually after spring.
- Closed galls: Galls on Photinia niitakayamensis Hayata are caused by Stenacis stranvesis (Huang, 1996). The galls are green with their top turning to red first. At last, the whole gall will turn brown. Galls on Cudrania cochinchinense are induced by Aculodes heterophylles (Haung, 1996). They are round and flat and look withered and yellow. At the peak period, there are often 80 eriophyoid mites inside every gall. These mites are fat and maggot-like. When the gall is damaged and opened, the eriophyoid mites are creamy in color but will turn reddish brown when in contact with sunlight for a while. Galls on Cordia dichotoma are induced by species of Aculodes . They look like mosquito bites and are yellowish green in color. Galls on Lycium chinese are induced by Aceria tjyingi (Manson, 1972). There are only some juices instead of hair inside the galls. Galls on Hibiscus tiliaceus L. are induced by Aculodes hibiscus (Huang, 1992). When the trees are damaged most severely, galls, gray in color, can be found on both dorsal and ventral sides of the leaves. Galls on willows are induced by Aculops taiwanensis . The galls are green in early spring and turn pink later when eriophyoid mites peak in number. These galls gradually turn brown and scab-like in autumn.
- Bud galls or flower galls: Galls on the flower of Elaeagnus thunbergii Serv. are induced by Aculodes sp. The flower can not blossom and hence wither.