Monitoring and controlling bud mites in highbush blueberry

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.    

With harvest ending in some west Michigan blueberry fields, and bushes forming flower buds for the 2011 season, it is time to consider bud mite management for fields that are infested with this pest. The first step is to sample fields that had poor bud development this season, to determine whether treatment for bud mites is required. This article describes identification, sampling approaches and the available control options.

Blueberry bud mite (Acalitus vaccinii) has been identified as the cause of some problems with poor growth and low yield in Michigan blueberry fields. Sampling by crop scouts, MSU Extension and the Berry Crops Entomology program has detected this pest across most of the major blueberry production regions in our state. However, only some fields have sufficient populations to cause economic levels of injury, and only some cultivars are susceptible. For example, in Grand Junction, we have seen Rubel bushes with high infestation and damaged growth growing next to Bluecrop plants that showed no visible symptoms. Because of this, bud mite management is warranted only in fields where 1) poor growth/damage have been seen, AND 2) high bud mite populations are verified by magnified analysis of bud samples.

This mite is microscopic (left image), white or clear, and feeds inside buds in the winter (middle image), causing damage to developing tissues and resulting in symptoms that include blistered red bud scales in spring, misshapen flowers, small leaves and fruit, or few berries per cluster (right image). Berries on infected shoots may also appear roughened and malformed.

Bud mite

The wide variability in symptoms among varieties adds to the difficulty in diagnosing this pest injury. It is important to take shoot samples in the late summer and fall as buds are being set or early spring to identify infestations. Bud mites move to fruit buds formed this year to find places to spend the winter, so fields should be sampled by taking 10 randomly-selected shoots and sampling the top five fruiting buds on each shoot for a total of 50 buds per field. These should be examined to verify that bud mites were the problem with the bushes, because some poor fruiting/growth symptoms are quite similar to the catch-all category of “winter damage.” Sampling can be done with a hand lens if you know what to look for, or can be done under a microscope by trained personnel. Send samples to your scout, local extension office, crop consultant, or to the MSU diagnostic lab (www.pestid.msu.edu) for checking. While there has been no research to develop a specific economic threshold, if 10 percent of the sampled buds are infested with bud mite, and the field is a susceptible variety, chemical control should be considered.

This pest can be challenging to control with pesticides because of its small size and the difficulty of getting miticide residues into the tiny cracks and crevices it inhabits. The immediate post-harvest timing is recommended for targeting this pest because the mites are relatively exposed before the buds have formed completely for the winter. Effective control is extremely difficult once the mites are protected under bud scales, and so prompt action is needed if a planting requires control of bud mites.

Chemical control options for bud mite

Registered miticide options for blueberry bud mite are limited, but there are effective registered miticides available (Table 1). Endosulfan-containing products such as Thiodan 3 EC, Thionex etc. are the most effective miticides for this pest, and these should be applied immediately post-harvest, with reapplication two to three weeks later in heavily infested fields. This will control the mite populations and prevent colonies feeding on buds through the winter. Although the label recommends waiting six to eight weeks between the sprays, this was developed for southern US conditions, and in Michigan we often do not have that long between the end of harvest and formation of next year’s buds. That’s why we recommend growers tighten up this period between sprays to get the second Thiodan spray on before complete bud formation. The label recommends that sprays be applied at high pressure (150 to 200 psi) and high gallonage to obtain effective coverage and penetration. Unless the interior spaces of the bud scales are wetted, it is unlikely that good control will be achieved. Use of a surfactant to improve the spreading and penetration of the spray is expected to increase control of bud mites.

Upcoming label changes for Thiodan/Thionex etc.

During 2010, the U.S. Environmental Protection Agency completed a reassessment of the risks associated with all food-related uses of endosulfan (Thiodan/Thionex etc.). In a July 23 announcement, the agency announced remaining time allowed for use on many crops, including highbush blueberry. Due to the importance of bud mite control in blueberries, there is a phase-out period negotiated through July 2015. The new restrictions will not take effect until next year, and there will be a four-year phaseout before complete restriction to allow time for alternatives to be developed and registered.

Other miticides

Trials of new alternatives to Thiodan including Sulforix have been completed at MSU in recent years. We have also tested some highly-effective new miticides that are not yet registered in blueberry. From our recent trials we expect these to provide control equivalent to endosulfan once they are labelled.

We have found that Sulforix provides moderate control of bud mites when applied in the fall. Many growers are using this for a disease control spray at the end of the season and can expect some level of mite suppression if used at this timing. However, applications timed for leaf drop because of the focus on disease control are later than the ideal timing for bud mite control. By this timing most of the mites will be inside the bud scales and much harder for the spray material to reach.

An additional option for population suppression of bud mites is the application in spring of a delayed-dormant application of oil. A high grade, ultrafine oil applied at 0.5-1% by volume can help to reduce populations in the spring.

Our pesticide trials at the Trevor Nichols Research station have compared the various options for bud mite control in recent years. Table 1 shows the average level of control (compared to untreated bushes) found in these trials for the main registered options for bud mite control.

Table 1. Miticide rates, timings, and efficacy for blueberry bud mite

Compound
Rate / acre
Application Timing
Avg. % control
Thiodan 3 EC, Thionex, etc.
2 qt
Post-harvest
93%
Sulforix
1 gal
Pre- or Post-harvest
60%
Summer oil
1% v/v
Delayed-dormant
27%

 

Other management options

Pruning infested shoots from bushes is a cultural control that should be done to reduce infestation. In some southern states, bushes are “topped” to cut off bud-mite infested shoots. Many growers leave prunings in the row middles and chop them in the row, but in fields infested with bud mite, the removed wood should be taken out of the field and burned or buried. Chopping this wood in the row middles may spread the mites back onto the bushes.

Biological control agents have been observed feeding on bud mite colonies. These include predatory mites and predatory thrips. There are also some fungi that specialize in feeding on mites and these have been found inside bud scales sucking the juices out of these tiny mites. While we still know little about the ability of these beneficial insects to control bud mites, it is likely that they are helping to suppress pest mite populations in Michigan blueberry fields.

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