Eriophyid mites are now active on conifers

Large numbers of eriophyid mites have been found on spruce, hemlock and Fraser fir in 2016.

Eriophyid mite feeding can result in a variety of symptoms on deciduous plants including galls, swollen or thickened growth and leaf blistering. In conifers, we usually find yellowing or bronzing of the needles and they are often referred to as rust mites. Eriophyid mites are different than other mites we deal with in conifers in that they are very, very tiny and carrot- or hotdog-shaped with only four legs on one end. They can be clear, tan, cream or orange.

Eriophyid mite Eriophyid mites

Left: An eriophyid mite on a spruce needle. Notice this one is carrot-shaped. Photo credit: Howard Russell, MSU Diagnostic Services
Right: Eriophyid mites on white pine. These are hotdog-shaped. Photo credit: Howard Russell, MSU Diagnostic Services

Eriophyid mites

Eriophyid mite eggs on spruce. Photo credit: Howard Russell, MSU Diagnostic Services

Immature and adult eriophyid mites suck on the sap in the needle. This feeding can discolor and distort foliage of many conifer species. We find these mites on a number of conifers including spruce, white pine, Scotch pine, Fraser fir, concolor fir and hemlock.

Mite eggs on spruce close up

Hemlock rust mite.

When heavy mite populations are present, their damage gives the needle a dusty, bronze- to rust-colored appearance. Similar damage can occur from winter injury, nutrient deficiency, drought or herbicide damage. To tell if the damage is caused by these mites, you will need to look at the needles with a hand lens and even then you may overlook them if you don‘t know what you’re looking for. To scout for these insects, zig-zag through your fields randomly checking trees for mites and looking for trees with bronzing or distorted foliage. Look at both the new and previous years’ growth.

Eriophyid mite damage

Eriophyid mite bronzing on Black Hill spruce. Photo credit: Jill O’Donnell, MSU Extension

Often, growers ask Michigan State University Extension educators at what levels they should consider spraying. To determine if you need to treat your field, North Carolina State University recommends for rust mites to keep track of the percentage of shoots that have mites, as well as the greatest number of mites on an individual needle, adding the number on both the upper and lower surface of the needle.

Treatment threshold

To determine if a pesticide is necessary, both of the following criteria must be met:

At least 80 percent of the shoots have mites on them. In most cases, it is not necessary to treat until the majority of the trees have at least a few mites on them. This percent incidence is determined by dividing the number of shoots with at least a single mite somewhere on the shoot by the total number of shoots examined.

There are at least eight mites on a single needle on one shoot. Only one needle on one shoot has to meet this criterion to reach treatment threshold if 80 percent incidence has been reached. Count mites both on the front side and back side of the needles to reach this sum.

This mite likes cool, dry weather and numbers can increase quickly if the conditions are right. Keep scouting weekly until you see mite numbers begin to decrease.

The use of broad-spectrum insecticides, particularly synthetic pyrethroids, may trigger flare-ups of eriophyid mites. Products that are used to control spider mites may not control eriophyid mites that are biologically different. Successfully treating for eriophyid mites requires using a miticide or insecticide effective against eriophyid mites, such as Avid (abamectin), Sevin (carbaryl) or Envidor (spirodiclofen). Remember to continue to scout trees even after treating trees to determine if mites were killed.

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