Mites in Michigan strawberry fields and their management
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Mites can be significant pests of strawberry, feeding on leaves and reducing the ability of plants to grow, ripen fruit and store energy for the following season. In most Michigan strawberry plantings, mites do not cause economic injury and their abundance is kept in check by predatory mites. However, pest mites can reach levels requiring the application of a miticide when these predators are not present, or conditions are appropriate for the rapid mite development. It is important for strawberry growers to know how to scout for pest and predator mites in their fields, so that damaging populations can be avoided or controlled. Two pest mite species infest strawberry in Michigan: twospotted spider mite and cyclamen mite.
Twospotted spider mite
Twospotted spider mites are the most common mite pest of strawberry and brambles in Michigan. This mite overwinters in and around fields on strawberry and on other broad-leaved plants. The mites start feeding and laying eggs in the spring when temperatures rise, and there are several generations each season. These mites can disperse by "ballooning" in the wind on small silken threads that they secrete while feeding.
The adults are about 1/60 inch in length and yellowish-white with two dark patches. Their eggs are about the same size and are shiny clear spheres, found on the undersides of leaves. They feed by rasping on the underside of strawberry leaves, and this causes strawberry leaves to turn yellow or brown on upper surfaces as feeding continues.
As strawberry rows become green and leaf tissues expand, weekly monitoring for pest and beneficial mites is the best way to ensure that pest mite populations don’t reach damaging levels. Mites grow more quickly during warm and dry weather conditions, but there have been years where abundance of these mites was high in Michigan strawberry fields even after cool conditions.
To monitor for twospotted spider mites in strawberry fields, examine 60 trifoliates per field, taken on a zig-zag pattern across the field, and count the number of leaves with mites. A hand lens is a helpful tool for this job. This sampling method saves you having to count the number of mites per leaf; just the presence or absence of mites is determined. If predators are not present, the threshold is 25 percent of leaves infested (15 of 60 leaves), but predatory mites can efficiently reduce pest mite growth at this density. Even one predatory mite per 10 twospotted spider mites is sufficient to control populations without the need for miticides. If predator mites are not detected at this level, a miticide should be applied to protect the plant with application targeting the underside of leaves.
The cyclamen mite is tiny and not visible to the naked eye, so a hand lens or microscope is required to see the mites. Mature mites of this species measure only about 0.001 inch long. They are pinkish-orange and shiny and the eggs are translucent. Adult females lay about 90 eggs, 80 percent of which develop into females. Cyclamen mites overwinter as adult females in the strawberry crown and can often be found in this part of the plant before they spread to new tissues. During summer, newly hatched mites develop into mature adults within 2 weeks and populations build rapidly soon after they begin to infest a field. At low population densities, cyclamen mites are usually found along the midvein of young unfolded leaves and under the calyx of newly emerged flower buds. At high population densities, these mites can be found anywhere on non-expanded tissue.
Because cyclamen mites require high temperature and humidity to thrive, they are most common in greenhouses and can be brought to a planting on new plants from greenhouses. Leaves heavily infested with cyclamen mites become severely stunted and crinkled, resulting in a compact leaf mass in the center of the plant. Feeding may also result in flower withering and poor fruit production. Identification of infested transplants before planting is the most effective way of preventing their establishment, and these plants should not be used.
There are no well-developed thresholds for cyclamen mite management in eastern North America, but the University of California recommends monitoring newly unfolding leaves and treating with an appropriate miticide if 1 cyclamen mite is found in 10 leaves. This low threshold is an indication of the potential for this insect to reproduce rapidly and to cause significant injury to plants. If cyclamen mites are found at levels above threshold early in the season, application of a miticide immediately before bloom and 10-14 days later is recommended. Some insecticides registered for control of tarnished plant bug and clipper are also active on cyclamen mites, so the spectrum of pest activity should be considered when selecting a miticide.
Predatory mites can be seen on the underside of leaves, where they actively search the leaf surface for pest mites to eat. In one study in southern Michigan, the predator Neoseiulus (=Amblyseius) fallacis was the only species of predator mite present. This mite with a big appetite is a light color and slightly smaller than twospotted spider mite adults. A hand lens is usually required to see them, and their population increase is typically delayed behind that of the pest mites that they feed on.
Preservation of these natural enemies is the best (and cheapest!) way to prevent mite problems, and so growers should consider using selective insecticides and miticides that do not kill the predators. For example, the commonly-used insecticide Sevin and many pyrethroids have been shown to reduce survival of predator mites. Use of selective products is becoming increasingly possible as more are developed and made available to strawberry growers.
Miticide options for use in strawberries
There have been many new registrations of miticides for strawberries in recent years. These miticides vary in the life stage they target, speed and duration of activity, and their pre-harvest interval. The following table is designed to summarize several key variables that can help you determine which miticides registered for use in strawberry are optimal for your Integrated Pest Management program. Selecting a miticide with long residual activity and low toxicity to predators will help provide the best long-term effective control of mites, and there are now some options that can provide this long-term mite control. More information on mite control in strawberry is available in the 2006 edition of MSU’s Fruit Management Guide, publication MSUE-154. This is available online at www.bookstore.msue.msu.edu.
Table: Miticides for control of mite pests in strawberry
|Compound||Life stage target*||Resid. Activity (weeks)||PHI(days)||Toxicity to predators***|
|Acramite, bifenazate (pH 7)||motiles||6-8||1||S|
|Oberon, spiromesifen||egg, motiles||4-6||3||S|
|Stylet oil**||egg, motiles||4-6||0||S|
* Motile forms include mite larvae, nymph and adult stages.
** Organic formulation available.
*** S = safe, M = moderately toxic, T = toxic to mite predators.
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