Predator mite attacking European red mite. Photo courtesy of Dr. David Biddinger
In the 1980-2000's growers had to spend $50 to $100 an acre to control European red mites. With 10 to 12 generations in a year and the ability to go from eggs to adults in 18 days, red mites required frequent sprays. But, Dr. Biddinger explains, "The best way to take out a mite is to use another mite." The first bio-control used for red mites was Stethorus punctum, a small black lady beetle. It was voracious and tolerant to many pesticides. But it would not even fly into the orchard until red mite levels were at least 8 per leaf and at that level, some injury had to be tolerated. Later they found T. pyri and Amblyseius fallacis predatory mites. A. fallacis is more efficient than T. pyri. However, because it is a pure predator, if it runs out of food it will leave the orchard canopy and go to the ground cover. T. pyri on the other hand, is an omnivore. It eats red mites but it can also live on other things such as fungi, pollen and other insects. It does not retreat to the orchard floor, but always stays on the apple tree where it can immediately respond to pest mite buildup, but this also makes them more prone to eradication if toxic sprays are used. Fortunately, T. pyri appears to be able to develop resistance to many pesticides almost as quickly as the pest, so it is constantly changing and improving.
Be careful what sprays you use and think about timing in order to avoid injury to beneficial mite populations.
You can inoculate your orchard with beneficial mites by bringing in spur leaves from an orchard with well-established populations such as the Penn State Fruit Research & Extension Center. May to June is the best time to do this. Place 2 sets of spur leaves on every 6th tree in high density plantings and every 3rd tree in normal plantings.