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Spruce spider mites on arbor vitae

Spruce spider mite damage on thirty foot tall white cedars was observed during the week of July 13th in Bemidji. Spider mites are most active during warm, dry weather on a variety of conifers including spruces, pines and cedars. The spruce spider mite, Oligonychus ununguis, are so named because of the silk they produce from silk glands near their mouths. Initial symptoms of mite damage to foliage often appear as bronzed and/or sticky foliage. From a distance the leaves appear grayish-bronze due to mottled chlorosis of the leaves that is a result of mites sucking plant fluids from the leaf tissue. Heavily defoliated trees are weakened by the loss of plant fluids being rerouted (to producing high populations of mites instead of being stored in the roots of the host trees). This insidious damage goes unnoticed for a long time before the problem is recognized.

This premature loss of foliage may kill twigs, especially in shaded areas, and can lead to the death of entire branches or trees if infestations persist. Mites are usually controlled by their predators, but mite populations can build up so fast that miticide application may be required on ornamental trees. Cygon (dimethoate), Orthene, among others, will control mites. Miticides should be applied at eight to ten day intervals because mites can generate a new population in ten days during July and August. Please read and follow label directions.

White pine bark aphids

White pines infested with pine bark aphids (actually adelgids) were observed near Bemidji on July 24th. These insects usually feed on the undersides of limbs and on the trunk by inserting their stylets (mosquito-like mouthparts) through the bark into the phloem tissues. They may be recognized by the white cottony material that collects in patches where they are feeding. Depending on the summer, several generations can be produced during the growing season. White pine bark aphids can be serious pest, causing twig and branch dieback and even tree death. To control the aphids, apply a "superior" type dormant oil spray in early spring and spray with Cygon or Diazinon in late April, mid- May and again in early July.