Leaf blister mites, Phytoptus spp.
Order: Acari
Family: Eriophyidae
Description
Like all eriophyid mites, adult leaf blister mites have cylindrical bodies that taper at both the head and rear. They have two pairs of legs near the head and are extremely small, measuring roughly 0.2 mm in length as adults. These mites cannot be seen without a hand lens or microscope capable of 10x-20x or greater magnification. Eggs of the pearleaf blister mite are about ¼ the size of adults, oval shaped, and translucent to white. The first stage nymphs are about half the size of adults, while the second stage nymphs are about the same size as adults. Given their extremely small size, the presence of feeding injuries is often used to indicate that an infestation is present.
Pear leaf injury caused by feeding of pearleaf blister mites. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Eriophyid mite adult. All leaf blister mites are eriophyid mites. Leaf blister mites cause blistering on leaves and fruit. In Colorado, leaf blister mites can attack apple and pear trees in orchards or home gardens. These mites are not usually considered a major pest in orchards and can be managed using integrated pest management practices. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Close up of leaf blisters caused by feeding of P. mali, commonly referred to as the appleleaf blister mite. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Quick Facts
- Leaf blister mites cause blistering on leaves and fruit. In Colorado, leaf blister mites can attack apple and pear trees in orchards or home gardens. These mites are usually not considered a major pest in orchards and can be managed using integrated pest management practices.
- There are two blister mites of concern in pear (Pyrus spp.) and apple (Malus spp.) orchards in Colorado. They are the pearleaf blister mite (Eriophyes pyri) and another blister mite commonly referred to as the “appleleaf” blister mite (Eriophyes mali). Both species are eriophyid mites, which are extremely small and cannot be seen without magnification.
- Blister mites cause discoloration and deformities on leaves and fruit.
- Management involves conserving resident populations of natural enemies and applying pesticides when infestations are persistent and intolerable.
Life history and habits
Adult leafblister mites overwinter in colonies under the fruit scales and leaf buds host trees. They tend to be most abundant in terminal buds. In the spring, females begin feeding and laying eggs in buds when the buds start to swell. A single female can lay as many as two dozen eggs throughout her life. The nymphs develop within blisters and exit the natal blister through a small hole in the center after reaching adulthood. The adults migrate to form new blisters on the host plant, and mites only survive for several days outside of leaf blisters. Both mite species can attack other trees as well as small shrubs such as mountain ash (Sorbus americana), snowberry (Symphoricarpos albus), serviceberry (Amelanchier arborea), hawthorn (Crataegus monogyna), Cotoneaster spp., and quince (Cydonia oblonga).
The average development time from egg to adult takes 20-30 days in the spring, but only 10 to 12 days in the summer when temperatures are more favorable for mite development. Leaf blister mites can migrate to growing terminals of the plant in response to overcrowding or when leaves sustain heavy damage and migrate to overwintering sites before leaves drop in the fall. At least two generations are produced each year in Colorado.
Injury
Feeding by leaf blister mites causes several symptoms on apple and pear trees including blistering on leaves, leaf discoloration, deformation and scarring of fruit which makes them unmarketable. Blistering occurs when plant cells die and separate in the center of the forming blister. At the same time surrounding cells enlarge and create a hole, which provides mites with access to plant tissue deeper in the leaf for feeding. Mites that reside within blisters are protected from natural enemies and pesticides. Therefore, the timing of applications is important.
Beginning in early spring, blisters are formed on leaves along the midvein and can reach sizes up to 3 mm (1/8 inch) in diameter. Initially the blisters are pale, green, pink, or red, but graduate darken to black or brown. The blisters may be less visible earlier in the season, but young leaves infested with leaf blister mites will have noticeable rough areas that are light green to light red in color. Leaf drop and lower shoot growth may occur when damage to foliage is severe. Unlike leaf blisters, mites do not live in blisters on fruit. For examples of blistering damage on fruit, see the Washington State University webpage. These mites can also feed on fruit blossoms, which kills them are reduces fruit yield. Fruits that do develop are often abnormally shaped with brown scabs and scarring on the skin. However, certain pear varieties naturally produce fruit with russeted surfaces, such as Bosc, Hardy, Nelis, and Winter, and do not show scabbing.
Biological control
Natural enemies of leaf blister mites include predatory mites (Acari: Phytoseiidae), predatory thrips (Thysanoptera: Aelothripidae), and certain lady beetles (Coleoptera: Coccinellidae) such as the spider mite destroyer, Stethorus punctum picipes. Leaf blister mites are not usually a problem in orchards with good integrated pest management practices where natural enemies are conserved and provide protection against mites in residential trees. To protect natural enemies, broad spectrum insecticide applications should be avoided, ant populations should be controlled, and young trees should be hosed off periodically to remove dust.
Chemical control
Leaf blister mites more commonly attack trees in neglected or abandoned orchards. When an infestation is present, there are many options for chemical management since blister mites are not resistant to pesticides. Leaf blister mites are most susceptible to chemical treatments when on fruit and terminal buds prior to bud swell in the spring. Therefore, pesticides should be applied before bloom or after harvest to prevent fruit damage. Summer applications of fuming or systemic pesticides can provide some control, however they will not prevent fruit damage.
Post-harvest applications of horticultural oils or neem oils can be effective when mite damage is persistent and intolerable. It is recommended that plants be irrigated before oil applications during periods of low rainfall. In addition, spray applications of micronized or wettable sulfur can be effective when plants are treated after buds swell but before most buds have opened. However, to prevent inadvertent toxicity toward plants, sulfur should not be applied to plants within one month before or after applications of oil, or vice versa. It is worth noting that feeding injuries cannot be reversed after fruits have set, although treating infested trees can help reduce feeding injuries the following season.
References
Amrine, J. 2013. Eriophyid mites. Washington State University. Available here.
Claudia, D., C. Linder, and E. Wyss. 2007. Autumn acaricide applications as a new strategy to control the pear leaf blister mite Eriophyespyri. Crop Protection. 26(10): 1532–1537.
Colorado State University. 2022. Eriophyid Mites. Colorado State University: Extension – Plant Talk Colorado. Available here.
Davis, R. 2011. Eriophyid Mites: bud, bluster, gall and rust mites. Utah State University – Cooperative Extension. Available here.
UCIPM. 2021. Pearleaf blister mite – Eriophyes (= Phytoptus) pyri. University of California – Agriculture and Natural Resources. Available here.
Utah State University. (n.d.). Blister Mites (Appleleaf & Pearleaf Blister Mites). Utah State University Extension. Available here.
Washington State University. (n.d.). Pearleaf and Appleleaf Blister Mites. Washington State University. Available here.