Heidi Lindberg, Michigan State University Extension - July 13, 2017

Scouting for broad mites poses challenges for growers in that the damage is often noticed 20 to 30 days after the initial infestation.

In “Broad mites in ornamental crops – Part 1: Challenges and treatments,” I covered the challenges of controlling broad mites on ornamental plants. First, broad mites cannot be spotted with the naked eye or even with a 5x or 10x hand lens. Significant damage to the plant tissue is often only noticed 20 to 30 days after initial infestation. In addition to these factors, scouts often wonder: Where is the best place to find the broad mites?

A study by Gobin et al. (2017) in Belgium found that the “manifestation of broad mite damage lagged behind the actual infestation numbers, with broad mite populations often higher on plants next to plants showing symptoms.” Therefore, growers scouting for broad mites should examine the plants in the immediate area of the plants with damage. On the plants themselves, 50 to 60 percent of broad mites were found on the top shoots while 40 to 50 percent were found on the lower foliage. The broad mites often hid in the crevices between leaves and the stems of plants near the apical meristem and were also found on flower buds. The broad mites entered the flower buds once the flowers began to open, allowing them to hide between the petals.

Growers who have experienced extremely high broad mite pressure or are growing a significant number of one of their preferred crops might want to consider a more intensive strategy to find broad mites and prevent crop damage. For example, the greenhouse in Belgium (who mainly grow azaleas and rhododendrons) adopted an apical meristem sampling program with assistance from ornamental plant researchers.

In the joint project, growers randomly snipped apical meristems throughout the crop and placed the shoots in a 70 percent ethanol solution. After shaking, they then would remove the plant shoots and use vacuum filtration to isolate any broad mites. While this is a labor-intensive process, it reduced plant losses and proved to be a strategy to monitor broad mite populations and alter their releases of A. swirskii accordingly.

In order to implement this intensive broad mite scouting strategy, Michigan State University Extension recommends dipping shoots into alcohol. Growers can buy a box of scintillation vials (100 vials for $25 at the MSU stores) and then dip 10 shoots into one vial. The vial should be labeled with the location information that indicates to the grower where the samples were taken. Growers will need a Buchner funnel, an aspirator and filter paper. Shake the vial and pour over the center of the filter paper, then examine that area of the filter paper under the microscope. This method also works well for thrips.

For those using chemical controls and do not desire to undertake this extensive scouting strategy should use a recommended miticide when you start to see broad mite damage. Leave a couple of plants untreated and compare the treated and untreated plants. If the untreated plants continue to get worse and the treated plants do not, growers likely have broad mites. The most accurate method, however, would be to send plants into MSU Diagnostic Services for evaluation.

The study referenced in this article is: Gobin, B., E. Pauwels, E. Mechant, and J. Audenaert. 2017. Integrated control of broad mites in ornamental plants under variable greenhouse conditions. IOBC-WPRS Bulletin Vol. 124: 125-130.

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