Many species of mites and ticks are severe pests of crops and stored products or ectoparasites of man, livestock and pets. Furthermore, many species are potential carriers of diseases. A large number of plant-parasitic mites considerably reduce yield of crops and might eventually kill their host plant. Particularly in agricultural systems, spider mites became severe secondary pests. Due to their stylet-like piercing-sucking mouthparts, the mites are able to pierce and suck juices from individual plant cells. Thus, mites can avoid most of the defence mechanisms of the plant. In addition, short generation times of one to three weeks paired with a high reproductive potential easily allow the animals to over-exploit their hosts. The piercing-sucking action of mites causes typical signs of damage on the affected plant tissues.

Pests of crops are spider mites, red spiders or twospotted mites (Tetranychidae) and gall, rust and blister mites (Eriophyidae). Some species of the latter family cause an abnormal development of the epidermal plant cells forming galls or other deformations of the leaf surface. The latter kind of damage is referred to as erineum and can be confused with fungous growth. The piercing-sucking action of mites can also result in grey or brown mottling of the foliage. In PNG, one Eriophyidae species reportedly damaged Terminalia brassii.

Important agents for biological control belong to the predacious families Phytoseiidae and Stigmaeidae. Mites like Typhlodromus and Phytoseiulus are reared commercially in large quantities. After their release in an infested area, these predators effectively suppress plant-parasitic mites and other small insect pests.