There are currently lots of options to control spider mites in orchards; we have an unprecedented number of different compounds and different modes of action. Modes of action is an important issue because we have had a long history of miticide resistance in orchard mite populations. That doesn’t mean that resistance is inevitable in the future. With judicious use of miticides, used only when necessary and rotating modes of action from year to year, we should be able to preserve our current tools.
What’s out there?
European red mites are the most common pest in our apple orchards currently, although twospotted and McDaniel spider mite are also seen occasionally. On pear, twospotted spider mites are the most common species, but European red mites are also seen occasionally. Apples can tolerate considerably more damage than pears (Anjous are quite susceptible), but apples are more likely to show the effects of damage if they are stressed by a heavy crop load, hot weather, and insufficient moisture. The latter has not been a problem so far in 2010!
Preventing outbreaks and miticide options
While miticide options are plentiful, prevention is always the best first choice. The key to preventing mite outbreaks is to avoid sprays that are toxic to both the predators (Typhs, aka Galendromus [=Typhlodromus] occidentalis) or their alternate prey (apple rust mite). Unfortunately, quite a number of products that have multiple uses in our overall pesticide program are toxic to one or the other. DAS provides information on the negative impact of WSU-recommended insecticides on Typhlodromus occidentalis and apple rust mite in the full WSU Spray Guide (click on Category: 3. Natural Enemies to view details). You may not be able to avoid all of toxic pesticides, but try not to pile them on. It’s a robust system, but eventually it will break.
Some of the products to use sparingly include sulfur containing products (lime-sulfur, ATS, dry flowable sulfur); carbaryl (Sevin); the neonicotinoids (Actara, Calypso, Assail, Clutch, Provado); Carzol. Delegate, Ultor and Warrior are also toxic to Typhs, but they are also fairly miticidal, so flare-ups of spider mites are less likely. One of the “gotchas” for some of the new materials is that their harmful effect is expressed through sublethal (rather than lethal) effects. In other words, the Typhs don’t die, but they aren’t eating the pest mites or laying eggs. So, depending on the material, the number of predators you see on the leaves may not be a good indicator of the potential for biological control the way it used to be.
If you do want to control mites, and don’t have confidence that predators will do it in time, there are a number of options. The ovicides (toxic to eggs) Savey and Apollo are most useful when used early in the season or at low thresholds– they won’t kill the adults, so control may take some time. They do tend to last a long time, so mixing with an adulticide (toxic to adults) can improve control. Acramite, Nexter, and FujiMite will be toxic to adults, so you will see the effect faster. Zeal and Envidor are toxic to eggs and larvae, and will be a little slower than the straight adulticides. Wait a week or 10 days before evaluating the level of control. Lastly, on the “soft” side, a summer oil will suppress mites, but you will likely have to make multiple applications (about 2 week intervals), because oil is a contact materials, with little residual effect.
Mites are difficult to detect initially due to their small size, but the effects of substantial feeding will be evident without getting out of the car. Mites are visible on foliage, but most people will need a hand lens to distinguish the species. Reddish stains on the fingers after handling infested leaves are usually diagnostic for European red mite.
There are several approaches to monitoring mite populations. One is an in situ examination. Take a hand lens and examine 10 leaves from each of 10 trees per block (total of 100 leaves), and count the number of mites. This is laborious and subject to counting error if populations are high. However, if other equipment is lacking, it can give a reasonable assessment of mite populations.
A standard approach is to collect leaves in the field and bring them back to where a leaf brushing machine and a microscope can be used. Another technique called binomial sampling has become more popular recently because it simplifies the estimating procedure with the use of statistics. To read more about the various monitoring techniques for the different spider mite species visit the Orchard Pest Management Online.
Because mites are on the tree from the pre-bloom period through fall, it is important to make repeated assessments for mites. You cannot take a single sample at a critical point in its life cycle and be finished for the season. Mites can build up at different times in different blocks or years. Where biological control occurs, many people do not monitor populations. However, it is advisable to sample at least once in mid- to late June to insure that rust mites and predators are present in adequate numbers.
In cases where nonselective pesticide use has destroyed T. occidentalis (Typh) populations, it may be possible to re-establish them more quickly by seeding the orchard to hasten recolonization. Predators may be reintroduced by taking shoots from an orchard with rust mites and Typhs, and placing them in the affected orchard. Because the movement of Typhs between trees is limited, at least one Typh-infested shoot per tree should be used. If a source orchard is not available, T. occidentalis can be purchased from commercial suppliers of beneficial insects. Strains have been selected in the field and laboratory for tolerance to commonly used orchard pesticides, so make sure that the strain is appropriate for the area of introduction.
(Elizabeth Beers, WSU-TFREC)