Should you spray for eriophyid mites on spruce?

Eriophyid mites are being found in Colorado Blue and Black Hills spruce. Here’s advice for scouting and deciding whether to spray.

Immature and adult eriophyid mites suck on the sap in the needle. This feeding can discolor and distort foliage of many conifer species. Currently, we are finding these mites on Colorado Blue and Black Hills spruce, but they can cause damage in other conifers.

Eriophyid mites are also known as rust mites. This group of mites is very, very small and are often difficult to see with a hand lens. Eriophyid mites are very tiny, and have a carrot-shaped body with only four legs on one end. They can be clear, tan, cream or orange. In order to see them, you need at least a 15x hand lens. Even then, the insects are barely noticeable. When many mites are present, their damage gives the needle a dusty, bronze to rust-colored appearance.

Eriophyid mite
Orange, wedge-shaped eriophyid mite on spruce needle.

Eriophyid mite eggs
Close up of eriophyid mite eggs on a spruce needle.

Treatment threshold

Growers often ask at what levels they should consider spraying. One threshold developed by North Carolina State University recommends for hemlock rust mite on Fraser fir, white pine and hemlock. To scout for these insects, zigzag through your fields and randomly pick a tree every 10 to 15 trees. Typically, we find these mites more on the current year’s growth. 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.

Two criteria to determine if a pesticide is necessary

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.

Keep scouting weekly until you see mite numbers begin to decrease. Products that are used to control spider mites may not control eriophyid mites that are biologically different. Successfully treating for eriophyid mites requires using an insecticide or miticide 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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