By Aaron Cato, Extension Specialist, Horticulture IPM, University of Arkansas
The buzz about broad mite seems to have picked up across the Southeast after reports of significant infestations in North Carolina during the 2019 and 2020 growing seasons. Broad mite has been a known pest of Southeast blackberry plantings for over a decade. The majority of observed issues were centered around Arkansas, and little occurrence and injury has been reported in many of the states closer to the Atlantic coast.
With the buzz of broad mite reports, growers have understandably been in seek of remedies. Broad mite can be a fickle pest species that may or may not show up, and often environmental conditions can shift the occurrence and seriousness of this pest from year to year. Outlined below is a summary of observations and research regarding broad mite that should lead to successful management of this pest.
What is Broad Mite?
Broad mite, Polyphagotarsonemus latus (Banks), is a tarsonemid mite that feeds on new leaf material, flowers, and fruit. Unlike other mite pest species, such as the two-spotted spider mite (tetranychid mite), broad mite is microscopic (0.1-0.2 mm) (Figure 1) and generally goes unnoticed until injury on new growth and reproductive structures is observed. Broad mite is distributed throughout much of the world and occurs as a pest mainly in tropical or subtropical regions such as the Southeastern United States. Broad mite has a large host-range and is most notably a pest in greenhouse production of food and ornamental crops.
Broad Mite in Blackberry
Broad mite was first reported as a pest of blackberry in the United States in 2007 and was further realized as a serious threat to commercial blackberry production in 2014 (Vincent et al. 2010, Johnson & Garcia 2015). Commercial plantings in Northeast Arkansas exhibited large levels of estimated yield loss ($15,000 to $20,000), and infestations were observed in many states across the Southeastern region. Broad mite was initially found to infest greenhouse propagation of blackberry plants, and later infestations of established plants were observed during the early summer months.
Broad mite is a tropical pest species that does not emerge until summer in climates with cold winters. Broad mite emergence likely is different for each blackberry growing region in the Southeast, and growers should be on the look-out for small pockets of damage and adults present on leaves. In Arkansas we usually begin to see populations increase in late-May and we don’t generally observe injury or significant infestations until late-June. This varies from year-to-year and sometimes we don’t see any injury until August. A good rule of thumb is to begin scouting primocanes when you have green fruit across your plantings, and don’t let up until it starts to cool down.
Broad Mite Injury in Blackberry
Broad mite feeding on blackberry is often reminiscent of injury from auxin herbicides and stunts plants in a similar manner. Malformation of plants is due to the toxic nature of the mite’s saliva. Feeding leads to stiff, curled leaves with cupped margins, a decrease in internode length, and potentially leaf death and tip dieback in serious infestations (Figures 2, 3). Broad mite will also feed on and damage the fruit and flowers of primocane-fruiting cultivars (Figure 4). Significant yield loss has been observed to primocane crops through a direct effect on developing flowers and fruit. Broad mite also effects the growth of primocanes on florican fruiting varieties and likely leads to significant yield loss in the following year (Figure 5).
Broad Mite Management in Blackberry
Scouting is key is to broad mite management. Any miticide that is applied before broad mite is present is likely to have no positive effect and could potentially lead to increased issues in the future. It’s likely that any pyrethroids or other insecticides used could also promote broad mite issues, as they kill predatory mites. Growers should scout for signs of injury in their plantings throughout the year, especially once the green fruit stage is reached. Damage will generally pop-up in a small area before it spreads throughout plantings. Once any suspected broad mite injury is observed, pull around 10 unfurling leaflets (second-node from the top, leaves should be just starting to lay flat) from surrounding primocanes. Ambered-colored adult broad mites can be seen at about 30x-60x magnification, which is usually available at your local extension office. Also be on the lookout for their distinctly polka-dotted eggs, which indicate that it is time to spray.
Broad mite numbers often build very rapidly and work by Dr. Donn Johnson has indicated that reaching an average of 1-5 mites per leaflet is the sweet spot for control (Johnson and Garcia 2015). Once mites exceed an average of 10 per leaflet, damage is usually widespread and populations can be difficult to effectively manage. Finding eggs in samples along with adult mites is also a good indication that it is time to apply a miticide for control. After applying any miticide for broad mite, continue scouting to assure effectiveness and for the potential of new infestations. Farms in Arkansas that have major broad mite issues often necessitate two applications a year, especially in years when the first infestations begin early.
Broad Mite Control Options
Currently there are many options to control broad mite, but only two that can safely be used in the heat of the summer (above 80-90°F). Products such as M-Pede (potassium salts of fatty acids), Microthiol (sulfur), JMS Stylet Oil (paraffinic oil), or Neem Oil all offered sufficient suppression of broad mite (Lefors et al. 2017). These products can be risky to use in the heat of the summer and can damage blackberry plants if applied when it is too hot. It is important to note that these products were not always found to be effective in efficacy trials (Johnson and Garcia 2015).
Effective miticides that are safe to use in the summer include Magister SC and Agri-Mek SC + NIS (Figure 6). With these two products, growers effectively can make 3 effective applications in a single year for broad mite (2 Agri-Mek + NIS and 1 Magister). In most years only 1-2 applications will be necessary, but we have seen instances where infestations were hard to knock back for more than a few weeks at a time. These products both have a 7-day preharvest interval which may complicate their use in primocane fruiting cultivars.
Effective Management Plan for Broad Mite
Broad mite shows up too late in Arkansas to affect the floricane crop, but this may not be the case across the entire Southeast. Control efforts generally need to be focused on limiting damage to this years primocanes, which could translate to yield loss in primocane fruiting varieties and lowered yield potential in next year’s floricane production. Scout for leaf injury and confirm that it is broad mite damage by sending in samples to your local extension service. If you are observing damage and there is more than 1 broad mite per leaflet across a significant portion of a plant, Agri-Mek + NIS is a good first option. Save Magister for a second shot as a rotation tool if necessary. You will need thorough coverage (75-100 GPA is preferable) to get acceptable control as this pest is often feeding deep inside terminal leaf material.
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Johnson & Garcia. (2015). Broad mite biology and management on blackberry. SRSFC Report. https://smallfruits.org/files/2019/07/2015-06.pdf.
LeFors, J. A., D. T. Johnson and T. Woodruff. 2017. Acaricidal Control of Broad Mites in Blackberry, 2016. Arthropod Management Tests, 2017: 1-2.
Vincent, C. I., M. García, D. T. Johnson and C. R. Rom. 2010. Broad Mite on Primocane-fruiting Blackberry in Organic Production in Arkansas. Hort. Tech. 20: 718-723.