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When Mite Makes Right
Biological Control of Two-spotted Mite (downloadable PDF)
Want a reliable worker, willing to work long hours, maintain plant quality, reduce chemical use, with many years experience working in Oregon nurseries? One of the best candidates for the position is versatile, native to the northwest, and pear-shaped. It also has eight legs and stands about a quarter micron tall. I’m talking about predatory mites, tiny little animals whose appetite for spider mites makes them a great tool for suppressing and managing mite outbreaks. Growers in Oregon have been using predatory mites for control of spider mites in field and container nurseries since the mid-1990’s and have found them both affordable and efficacious.
Spider mites are an ideal pest to manage with biological control since they are commonly occurring animals with a short life cycle and many generations which allow damaging populations to build quickly. They often require multiple applications of miticides to control them, a costly situation which may also lead to pesticide resistance. Additionally they reside in difficult to spray locations, usually on the leaf underside or deep within the plant canopy.
The benefits of using predatory mites are many, including reducing or eliminating the need for miticides, no re-entry interval s for workers, nor chemical exposure. They don’t cause phytoxicity and can eliminate spider mites before any plant damage occurs. They can disperse on their own, including under the leaves and into tight spaces. They can increase their populations over time and overwinter in nurseries, giving control not just for the season but possibly seasons to come.
The types of predatory mites most commonly used in biological control of spider mites are in the family Phytoseiidae. There are many different species commercially available. Some Phytoseiid mites are specialists like Phytoseiulus persimilis, which feeds exclusively on spider mites. Others, such as Amblyseius (Neoseiulus) fallacis, are adaptable feeders, utilizing alternative prey, even pollen. Both types have their benefits. P. persimilis, can increase its population rapidly in response to surging spider mite populations, but is susceptible to population crashes as its food source becomes scarce. A. fallacis does not control high spider mite populations but does well at low spider mite levels, and survives on alternate food, allowing growers to use it early in a crop cycle to suppress mite outbreaks and avoid damage.
Given these and other attributes, research here in Oregon has shown A. fallacis to be one of the best species for managing mites in our nurseries. It is native to our region and capable of overwintering. It is commercially available by several insectaries. A. fallacis has controlled a range of spider mites in nurseries here including bamboo spider mite, citrus red mite, southern red mite, and two-spotted spider mite. Although we had success establishing A. fallacis on conifer crops targeting spruce spider mite, consistent, acceptable suppression of this pest has been elusive. A. fallacis will eat cyclamen mites based on research on strawberry plants and will also feed on eriophyid mites such as the tomato russet mite.
Growers in other regions or in specific sites might benefit from other Phytoseiid species. Adapted to hotter, drier conditions are Galendromus occidentalis, the western predatory mite, and N. californicus. One greenhouse cut flower rose grower in our area uses both A. californicus and P. persimilis on the same crop, using the former to suppress outbreaks and the latter, to work on hot spots. Amblyseius andersoni is popular in Europe where it is available in slow release sachets embedded in long ribbons easily applied to crops.
A. fallacis naturally occurs in many agricultural fields. They overwinter in dead leaves, on stems, and on green foliage. Research on overwintering by Dr. Paul Pratt and Brian Croft at OSU found that A. fallacis was most abundant on conifers; intermediate on evergreen shrubs; and minimal on herbaceous perennials, deciduous shrubs, and shade trees. Covering plants with protective plastic negatively affected overwintering survival.
The lower temperature threshold of A. fallacis is near 50°F (10°C). At 70°F (21°C), A. fallacis eggs hatch in 2 days, and adults develop in 7 days. At cooler temperatures (55°F) adults develop more slowly, in 16 days. The female predatory mite produces about 30-60 mites, about the same as spider mites. A. fallacis also prefers humid environments as found when the plant canopy is dense.
Around October in our area, females enter diapause in response to short days (daylength at 14 hours or shorter). A time of hibernation, they suspend activity and move into overwintering sites. Indoors diapause may not occur if temperatures remain 18°C (64°F) or above.
Monitoring for spider mites in the early season is critical. Those using predatory mites want to catch activity of spider mites early in the season. Early spider mite populations are more scattered but this extra vigilance allows releases prior to an increase of spider mites and damage. Use a 10X or stronger hand lens to look at the leaf underside, particularly on older leaves or on plants with prior infestations. It is useful to note the presence of the round spider mite eggs (seeds of the future) and also to distinguish the oval eggs of the predatory mites (a sign they are reproducing). Sampling should occur more frequently with warmer temperatures. Bi-weekly or weekly sampling is recommended once mite activity resumes in the spring shifting to weekly once temperatures are warm.
Many growers skip actual counting of spider mites. Instead they monitor regularly and act promptly at first sight (presence/absence sampling) of spider mite activity. They order their mites and make releases immediately upon arrival of the predators. This reduces some of the labor required for monitoring but also ensures availability of prey for the newly released predators.
Several factors should be considered when determining the number to release. More rapid control can be expected with relatively high (innundative) release rates (7000-10,000 predators per acre) but another strategy is low (inoculative) release rates from 1500-4000 predators per acre. Release rates as low as 1400 predators per acre have been used in apple tree nursery stoolbeds. This allows the predators to build up and disperse over time. This approach is more affordable but also requires releasing the predators at lower spider mite populations. The actual release rate depends on the density of two-spotted mites, the density of foliage in the block, the time of year and temperatures, the period of time allowable before control is desired, and prior use of the mites (slow build up of resident predators over time). A mite release calculator developed for strawberries allows growers to explore different release rates, dates, and other factors that affect control (see Resources).
Based on OSU research trials in a variety of ornamental production systems beginning in 1995, the best cropping situations for successful releases were deciduous and evergreen shrubs and herbaceous plants. In particular, a continuous canopy seemed to aid dispersal. This is because one of the main ways mites disperse is by wind. Should predators blow onto a nearby plant, the landing spots provided by the leaf canopy aids establishment. When mites land on soil or gravel, they usually will not survive. Less successful were trial releases with tall vertical growing plants with little foliar canopy such as shade trees. In that situation, limited humidity may be a factor and as well as reduced dispersal success.
If beginning spider mite populations are high, a knock-down spray with a compatible miticide prior to predator releases may be necessary to reduce spider mites to a manageable level for the predatory mites. Information about websites that list pesticide compatibility for predators can be found under Resources.
Obtaining and handling predators
There are several insectaries that raise A. fallacis. The predators are commonly shipped in small containers filled with predators, a carrier and food source which resembles corn grits. More information on suppliers can be found in the Resource section.
Quality control of predatory mites has improved over time but it is still important to inspect the product received with a hand lens. Look for live vs. dead mites, and size of the predator sent (Adult mites and larger females). Most insectaries will stand by their product and work with growers if a shipment is not satisfactory. Predator mites will not survive storage well and should be released as soon as possible (refrigerate only for a short time if immediate releases are not possible - temperatures around 50° F). It is best to avoid high temperatures when making releases (try early morning or in the evening).
In general, space release sites evenly throughout the field, especially towards the upwind side and wherever spider mite densities are highest. If possible, scout the field and flag spider mite hotspots just before or during predator releases. Apply predators more heavily wherever spider mite densities are high and fewer predators where spider mites are less dense. Predators will build up slightly faster wherever spider mite eggs are abundant. Predators will move more quickly wherever spider mites are sparse. Release sites with more restricted air flow benefit from more release points as dispersal of the mites is slower without wind movement.
Follow up releases with regular monitoring. Be patient, it may take several weeks to find released predators after inoculative releases. Flagging release sites can help staff track the predators’ dispersal.
Pre-planning and communication at the nursery site is essential for success of the program. Decide where you will order predators prior to needing them and check on availability. Nurseries may want to post signs to highlight release areas so employees know not to spray incompatible pesticides. Have contingency plans for compatible pesticides should another pest show up. Start small with a new program. Expand with success.
Many of our original cooperators in the predatory mite research trials have continued using the predators to this day. They have found the program affordable and effective, often seeing increases in plant quality while reducing or eliminating use of pesticides and re-entry restrictions.
Sometimes a little mite, makes right.
Applied Bionomics Technical Manual: http://www.appliedbio-nomics.com/technical-manual.html
L.S. Osborne, L. E. Ehler, and J. R. Nechols . 1999. Biological Control of the Twospotted Spider Mite in Greenhouses: http://www.mrec.ifas.ufl.edu/lso/SpMite/b853a1.htm
Pratt, P. Croft, B. and J. DeAngelis. 1999. Biological Control of Spider Mites in Ornamental Nurseries: http://www.ent.orst.edu/prattp/introduction.html
Pratt, P. and Croft, B. 1999. Biocontrol of spider mites by Neoseiulus fallacis in ornamental plants that represent a range of morphological types: http://www.ent.orst.edu/prattp/plant.html
Pratt, P.D., R. L. Rosetta, and B.A. Croft. 2002. Plant-Related Factors Influence the Effectiveness of Neoseiulus fallacies (Acari: Phytoseiidae), a Biological Control Agent of Spider Mites onLandscape Ornamental Plants: http://www.ars.usda.gov/SP2UserFiles/person/11988/Prattdocs/Pratt%20et%20al%202002%20plant%20factors.pdf
Coop, L., R. Rosetta, and Brian Croft. 1997. Release Calculator and Guidelines for using Neoseiulus fallacis to Control Two-Spotted Spider Mites in Strawberry: http://uspest.org/ipm/mcalc.html
-Mite Calculator: http://uspest.org/ipm/mitecalc.html
Control Methods for Pests in Commercial Greenhouses
of Minnesota Extension: Understanding Biological Control in Greenhouses
Pratt, P. and B. Croft. 1999. Compatibility of the Predatory Mite Neoseiulus fallacis with Pesticides Registered for Use in Ornamental Nurseries: http://www.ent.orst.edu/prattp/pesticides.html
Koppert Side Effects Webpage: http://side-effects.koppert.nl/
The Association of Natural Biocontrol Producers: http://www.anbp.org/
of Beneficial Organisms in North America
Two-spotted spider mite adult, larva, and eggs
on Ajuga reptans
Two spotted mite damage on Ajuga