Small Fruit Mite - Broad Mites on Blackberries

Broad mites were first found on blackberries in PA in 2013 and at that time were thought to be an isolated occurrence. Since then we've found that they can contribute to a nearly total crop loss on primocane-fruiting blackberries. Broad mites have been found in plantings in other states across the eastern U.S.  Floricane-fruiting varieties have been less affected, perhaps because of differences in how they are managed.

Typical damage from broad mites is tissue distortion, reduced terminal leaf growth, either downward or upward curling or cupping of leaves (Photo 1) and flower clusters that appear compressed (Photo 2) or blossoms that dry up. Symptoms on flower clusters may not show up until the second year of infestation. Broad mites build up to hundreds per leaflet on younger terminal leaves.

These mites are very tiny - less than 0.2 mm (about 1/100th of an inch) as are their distinctive eggs dotted with white spots (Photo 3). Magnification of 30 to 60x is needed, so it will be necessary to enlist from your local extension office or someone with a microscope.  

Because of the small sizes of broad mites and the eggs, symptoms of leaf curling and dying terminal foliage (Photo 4) and flower clusters that occur after populations are very high may be what is noticed first.  These symptoms could be mistaken for fire blight, a bacterial disease.

With citrus, the mites are found in depressions on the fruit where the females lay their eggs, and as is evidenced by the number of mites and eggs on a young blackberry fruit (Photo 5), it appears that blackberries provide a similarly desirable fruit surface.

At first, we didn't know why we were seeing this pest on blackberries now, or where it came from.  However, this mite has a very wide host range, and is commonly a problem in greenhouse vegetable or ornamental crops.  It is thought that initial infestations may have occurred in primocane-fruiting blackberry plants propagated in a greenhouse.  However, broad mites can also be moved around in other plant material, or on the tools workers use as they move from patch to patch. Interestingly, broad mites can hitch rides from place to place by attaching themselves to other insects such as whiteflies, aphids, and possibly thrips.

Its ppearance in grower fields also coincided with the adoption of primocane-fruiting blackberry cultivars.  Primocane-fruiting blackberries perform best in areas with warmer temperatures and in protected locations such as high tunnels, which encourage higher mite populations in general.  However, primocane-fruiting cultivars are not tipped in mid-summer like floricane-fruiting cultivars are.  This mite is mainly found in the terminal portions of the plant, and so could multiply on primocane-fruiters unimpeded, but likely gets pruned out of floricane-fruiting cultivars regularly if inadvertently.    

So, what else can one do to control broad mite infestations? First, keep watch for the symptoms, and if you notice just a plant or two exhibiting suspicious symptoms, rogue it out along with a couple of plants to each side of it. Practices similar to those that would assist with controlling two-spotted mites (conserving natural enemies, releasing predatory mites early enough and at timings that would allow them to establish in the planting, and avoiding use of broad-spectrum insecticides) may be beneficial. Unfortunately, this is difficult to do given the frequent insecticide sprays needed to control spotted wing drosophila. 

According to the University of Arkansas, if symptoms are noticed, an effective way of scouting is to pull 10 leaflets from the top portion of the plant where young leaves are just beginning to unfurl and lay flat. If 1 to 5 mites are found per leaflet, it's time for a miticide application. Agri-Mek, Portal and Magister are effective on broad mites.  The PHI for Agri-Mek and Magister is 7 days, so timing for their use may be problematic.  The PHI for Portal is 1 day.  Regardless of which product is used, only one application per year is recommended because of a high likelihood of resistance development.  Then conduct additional scouting to determine if a second application is necessary and spray only if the above-mentioned threshold is reached. 

Acknowledgement

Thanks to Sara May at Penn State's Plant Disease Clinic for assistance in diagnosing this problem in Pennsylvania.

Additional Reading/References

Fasulo, T.R. 2000, revised August 2010. Broad Mite. Featured Creatures, Univ. of Florida.

Johnson, D. 2014. Broad Mites Damaging Blackberries. Arkansas Fruit and Nut News, Vol. 4, Issue 7, Sept. 5, 2014

Johnson, D. 2015. Do You Have Broad Mites on Blackberry? Arkansas Fruit and Nut News, Vol. 5, Issue 5, June 8, 2015.

Myers, M. and Bogash, S. 2015. Broad Mites in Fruiting Vegetables. Penn State Extension, posted June 22, 2015.

University of California IPM Pest Management Guidelines: Citrus, Broad Mite. Updated 2015. UC ANR Publication 3441.