Zetzellia mali Ewing
Taxonomic placing: Acari, Prostigmata, Raphignathoidea, Stigmaeidae.
Geographic distribution: North America, Europe, Middle East and, Iran.
Host plants: Many fruit trees, especially apples.
Morphology: Body yellow to orange red, about 0.4 mm in length. Prodorsal sclerite reticulated, with 3 pairs of setae, Opisthosomal sclerite also reticulated and bears 5 pairs of setae. Postocular bodies only slightly larger than eyes.
Life history: Zetzellia mali is mainly a predator of eggs and immature stages of various eriophyids but also of spider mite eggs and even of its own eggs, locating these prey only through random encounters. At 21ºC a generation requires about 3 weeks. Females overwinter, sometimes in large groups, within cracks or crevices on tree twigs or bark, or under the empty scales of Diaspididae, often near concentrations of their prey. They emerge in spring and raise 3-4 generations during summer, at times dispersed by winds. During fall in cooler regions they move to overwintering sites, some remaining active on leaves in autumn. The distribution of Z. mali on apple trees depends on that of their prey, especially the apple rust mite. Its ability to feed on various eggs (including its own), and to withstand periods of starvation (about 3 weeks), enables it to survive in orchards longer then other co-occurring predators, such as Phytoseiidae. It is resistant to organophosphates and some other insecticides and tolerant to many fungicides.
Economic importance: Zetzellia mali is an effective predator of Bryobia rubrioculus in Turkey. In North American apple orchards it preys on eggs and immature stages of Panonychus ulmi and on active stages of A. schlechtendali. Predation on the latter is enhanced by high nitrogen levels and is influenced by the specific cultivar on which the interaction takes place. Zetzellia mali may also prey on the eggs of co-occurring phytoseiids. In Italy the predator is considered to be important in controlling A. schlechtendali. Zetzellia mali is usually unable to reduce large populations of pest mites. However, early in the season, or in orchards where pest numbers had been reduced by other means, this predator can keep pest populations for long periods at low levels. It may also hinder or displace the phytoseiid Typhlodromus pyri Scheuten when co-occurring in apple orchards.
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