The following is an article on Pear Blister Mite, a pest to be watching for in the landscape
Over the last couple of years we have been seeing more pear blister mite damage on Asian pears, European pears and ornamental pears. These mites are very small and you will need at least a 15 to 20X hand lens to see them. They are white, long, slender, and striated. They also have a few long hairs. Eggs are spherical and pearly white.
Monitoring: Look under bud scales during winter where these mites feed. This feeding can cause buds to dry and fail to develop in spring. In the spring, pear blister mites feed on emerging leaves, flowers and developing fruit. Feeding results in blistered leaf spots that start red and eventually turn black. The spots on the fruit are usually depressed with a halo of clear tissue that tend to run together. Eggs are laid inside the blister which is where young mites start feeding so they are rarely seen.
Control: In late October the females will begin to seek out places on the branches to overwinter. They like to settle down into the bud scales and bark cracks. At this time of year they are moving about on the twigs and are susceptible to pesticide applications. You have a relatively small window in which to treat for this pest. A 2% rate horticultural oil can e applied in late October. The kill rate can be improved with addition of Abamectin (Avid) to the horticultural oil.
Pear blister mite damage.
Article and photo from the October 23, 2009 edition of the TPM/IPM Weekly Report for Arborists, Landscape Managers & Nursery Managers from the University of Maryland Cooperative Extension http://www.ipmnet.umd.edu/09Oct23L.pdf