Article written by Madeline Wimmer, UMN Extension Fruit Production Educator.
Images: Managing European red mite (ERM) overwintering eggs is a preventative approach to keeping populations under control, in which ERM eggs are most effectively managed at the ½ inch green growth stage. Left photo credited to University of Georgia Plant Pathology.
Of the three different pest mites affecting apple orchards, European red mite (ERM; Panonychus ulmi) can be one of the most problematic for apple orchards. Technically a spider mite, ERM are about .42 mm and are a reddish-brown color. Their host range primarily includes plants in the Rosaceae, which includes apples.
ERM overwinter as eggs near tree buds and fruiting spurs and hatch in the spring. The first adult generation of the growing season begins to lay eggs around petal fall. ERM populations go through multiple generations throughout the growing season. Their pest habits are unique: their populations and activity often decrease in the middle of the summer due to natural enemy activity. ERM feed on apple leaves, leading to a stippling injury that reduces photosynthetic activity and leaf bronzing, which can result in smaller fruit sizes.
Before discussing solutions for managing ERM, it is important to understand that many mite populations, even when present, can be less damaging when populations are balanced in the presence of predatory mites. There are several predatory mites commonly found in orchards (ex. Neoseiulus fallacis, Amblyseius andersoni, and Typhlodromus pyri).
Many predatory mites are about the same size as a spider mite, with a more pear or oblong body shape and lighter color. A “balanced” predator-to-pest mite ratio in an orchard is an estimated ratio of 1 predator per 10 pest mites to keep pest mite populations at low, non-damaging levels. To protect and cultivate predatory mite populations, avoid applying pesticide products like pyrethroids, organophosphates, and carbamates that can be toxic to predatory mites, especially early in the season.
Growers can scout for ERM overwintering eggs from late fall until early spring, where their small red eggs can be found near buds and fruiting spurs. One effective way to manage ERM at this stage is to use a dormant oil that smothers overwintering eggs, in which case it is recommended to delay the dormant oil spray until the ½ inch green growth stage to be most effective. The Midwest Fruit Pest Management Guide also recommends increasing the oil concentration (1-2%) at this stage.
If ERM is not managed at the egg phase, populations can be scouted for and managed after petal fall or later. Scouting instructions during this time of the year are based on collecting leaf samples and checking leaf undersides for the presence or absence of mites.
The number of leaves with mites is divided by the total number of leaves, and that percentage is used to make a management decision (to be covered in more detail in a future Fruit Update), where the threshold is lower earlier in the season. Because ERM can produce up to eight generations per growing season, early-season ERM management can prevent serious outbreaks. Refer to the Midwest Fruit Pest Management Guide on pages 17-19 for chemical options for post-petal fall ERM management.
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