Predator mite attacking European red mite. Photo courtesy of Dr. David Biddinger
A little history...
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.
Scouting for red mites in the early spring
- Look for presence or absence of red mites.
- Look at the underside of leaves on 10 random trees of a susceptible variety in a block.
- Generally you can tolerate 2.5 red mites/leaf until the middle of June.
- Generally you can tolerate up to 5 mites per leaf after that until mid-July and up to 7.5 mites per leaf in August through harvest.
Scouting for beneficial mites
- Sample several trees in each block by examining the underside, mid-veins of 25 leaves/tree for fast-moving, tear-drop shaped mites with a hand lens (10 to 15X lens).
- They will appear to be clear or slightly reddish and tear drop in shape, but not red or bright yellow in color or have spots.
- If you have a ratio of predator to prey of at least 1:10, biocontrol will do the job. At 1:20 it will still most likely do the job.
Make sure not to wipe out beneficial mites
Be careful what sprays you use and think about timing in order to avoid injury to beneficial mite populations.
- A single application of a pyrethroid can kill beneficial mite populations. Pyrethroids can also stimulate red mites to reproduce more rapidly and increase the number of generations they have in a season. "You can end up with more than 80x more over-wintering red mite eggs after a pyrethroid spray in the late summer or fall," Dr. Biddinger explains.
- Several other insecticides including Rimon, Lannate, and multiple late season applications of neonicotinoids such as Beleaf or Provado can also result in mite flare-ups.
- Delegate can be a little rough on benefical mites. It is best to use in the spring when beneficial mites are reproductive and populations can bounce back. For example, some growers would use 2 applications of Delegate in the spring and 2 of Altacor in the fall.
- Lannate is very toxic to beneficial mites, but has a short residual time which helps reduce the effect on beneficial populations.
- Indigo, which is part pyrethroid, kills all the beneficials and resulted in more than 25 red mites per leaf in one study.
- Agrimec plus oil is effective on red mites when applied during the short 7-10 day interval after petal fall. T. pyri has developed a tolerance to it.
How to get beneficial mites in your orchard
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.
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