European Red Mites

Winter months are an excellent time to prune fruit and nut trees. Naked and dormant, it is easy to see each tree’s structure. This is also a good time to inspect for common pests, such as scale insects and European red mites.

While you will certainly want to get rid of any San Jose scale, walnut scale, Italian pear scale, or frosted scale insects you see, you should leave the European red mites where they are. 

​Why in the world would you want to leave pests on your trees?

European red mite being attacked by a predatory mite (CSIRO) CC BY 3.0

Like many other plant-sucking pests, European red mites overwinter as eggs, tucked tightly against the bark of twigs, stems and leaf and bud scars. When they hatch, they begin feeding. The thing is, these tiny pests have small populations, at first. They don’t cause significant damage until late summer. By that time, they will have attracted an army of hungry, beneficial predators.

​European red mite description and lifecycle

If you look closely, you may be able to see the reddish-orange eggs of European red mites. If you use a hand lens, you might even be able to see that these eggs have a spike. While winter eggs are attached to bark and stems, summer eggs are generally found on leaves, near leaf veins, and on fruit. Around the same time that walnut trees start leafing out, these winter eggs will hatch and bright red, tiny mites will appear. 

European red mite egg clusters can be found in the crooks of tree branches (University of Georgia Plant Pathology Archive, University of Georgia, bugwood.org) CC BY 3.0

These mites will go through 3 moltings, or instars. First instars have 3 pairs of legs, while older instars have 4 pairs of legs. Freshly molted mites may sometimes have a green tint. Adult females are reddish, with 4 rows of curved hairs emerging from white spots. Males are smaller than females and brown in color. Females measure in at 1/72 inch. Males are 1/80 inch, which means you could line up 4 of them on the edge of a dime.

Females can lay eggs without mating, but these offspring will all be male. [This is called arrhenotokous parthenogenesis. Most parthenogenetic offspring are female, as with aphids, so this is different.]

​If populations become especially high, European red mites will disperse themselves in a behavior called ballooning.

Damage caused by European red mites

While it is all well and good to let these pests have their day in early spring, as temperatures rise and populations increase, they can cause serious damage to apple, cherry, pear, plum, prune, and walnut. They may also be found on almond, chestnut, currants, gooseberries, grapes, peach, and raspberries, as well as roses, privet, lilac, elm, alder, and black locust.

Young mites prefer feeding in the relative safety of the undersides of leaves. Older, more mature mites feed on both sides. Light feeding will cause leaf stippling. Heavy feeding can bronze leaves. Bronzing may be fine for baby shoes, but it makes photosynthesis impossible. Whereas other mites produce webbing and cause leaf drop, the European red mite produces little or no webbing and no leaf drop. Persistent, heavy mite feeding can also cause transpiration burn (dead spots), reduced fruit size and quality, shoot growth, trunk and limb growth, and root growth.

If populations of European red mites become significant, you can apply delayed dormant horticultural oil, but that oil may cause sunburn damage. It’s a tough call. Since European red mites have demonstrated resistance to miticides (a type of pesticide geared toward mites), it is better to avoid chemical sprays. Spraying these pests with a hose does nothing.

If your garden or landscape has a lot of biodiversity, odds are pretty good that there will be enough predators to control European red mite populations. Also, keeping plants dust-free makes the environment less hospitable to these pests.