Leaf blister bites, 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.
These mites are extremely small and cannot be seen without a hand lens capable of 10x-20x or greater magnification.
Quick Facts
• There are two blister mites of concern in Colorado orchards. They are the pearleaf blister mite (Phytoptus pyri) and another blister mite commonly referred to as the “appleleaf” blister mite (Phytoptus mali). Both species are eriophyid mites, which are extremely small and cannot be seen without a hand lens capable of 10x-20x or greater magnification.
• Blister mites cause discoloration and deformities on leaves and fruit.
• Both mite species can attack other trees as well as small shrubs such as mountain ash, snowberry, serviceberry, hawthorn, cotoneaster, and quince.
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
Pear leaf injury caused by feeding of pearleaf blister mites. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Life history and habits
Leaf blister mites typically overwinter under bud scales or near the base of buds as mature females. In the spring, females begin egg laying in buds when the buds start to swell. 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.
Injury
Feeding by leaf blister mites causes several symptoms on apple and pear trees including blistering on leaves, gall formation, 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, as well as protection from natural enemies and pesticides.
Leaf blisters begin to form in the spring and can reach sizes up to about 3 mm (1/8 inch) in diameter. They 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.
Biological control
Leaf blister mites are not usually a problem in orchards with good integrated pest management practices where natural enemies 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.
Chemical control
Leaf blister mites more commonly attack trees in neglected or abandoned orchards. Fortunately, 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.
References
Amrine, J. 2013. Eriophyid mites. Washington State University. Available https://s3.wp.wsu.edu/uploads/sites/408/2015/02/PLS-89-Eriophyid-Mites.pdf
Colorado State University. 2022. Eriophyid Mites. Colorado State University: Extension – Plant Talk Colorado. Available https://planttalk.colostate.edu/topics/insects-diseases/1400-8-eriophyid-mites/
Davis, R. 2011. Eriophyid Mites: bud, bluster, gall and rust mites. Utah State University – Cooperative Extension. Available https://extension.usu.edu/pests/uppdl/files/factsheet/eriophyid-mites2010.pdf
Utah State University. (n.d.). Blister Mites (Appleleaf & Pearleaf Blister Mites). Utah State University Extension. Available https://extension.usu.edu/pests/ipm/notes_ag/fruit-blister-mites#:~:text=GENERAL%20MANAGEMENT,in%20spring%20are%20also%20effective.
Washington State University. (n.d.). Pearleaf and Appleleaf Blister Mites. Washington State University. Available http://treefruit.wsu.edu/crop-protection/opm/blister-mites/