Mites beware

As spring swells into summer, calls to the Entomology Diagnostic Lab bloom right along with the plants. The phone has a noticeably more urgent tone this time of year — mite problems, in particular, surge with the warmer conditions and longer days.

Once established, mites are one of the more difficult pests to manage. The many control options that are now available only add to the complexity. So to help in choosing among the controls and developing your own strategy, it’s a good time to review mites, miticides and mite management.

Of the three groups of mites we commonly see on ornamental plants, two are most often seen in production of greenhouse ornamentals. Spider mites are familiar to most growers, particularly for those who raise foliage plants or other long-term crops or who manage a conservatory or maintain an arboretum.

Two-spotted spider mite (TSSM) is the most common one in this group; it has a very wide host range that includes many common greenhouse and outdoor crops. Adult females are usually pale with a dark green spot on either side of the body.

The Lewis mite is another spider mite sometimes seen in greenhouses. It is usually associated with poinsettia. Slightly smaller and narrower than TSSM, the adult female has multiple small green spots.

Tarsonemid mites are less familiar, but growers will recognize their common names: broad mite and cyclamen mite. Also with wide host ranges, these nearly microscopic creatures are distinguished from one another by some body features, overall color and appearance of eggs.

The third group, eriophyid mites (including rust, bud and gall mites), includes species that are also nearly microscopic but relatively uncommon on greenhouse plants. They tend to be very host-specific. Examples include tomato russet mite on tomato, purple tea mite on spathiphyllum, and fuchsia gall mite on fuchsia.

The deal with detection
Detecting mites while numbers are still low improves chances for control while limiting damage. Accurate identification is important for selecting a strategy or control product, as some miticides are effective against (or are labeled for) only certain kinds of mites.

Mites are rather difficult to see, so we usually rely upon early symptoms of infestation. For spider mites, some sharp-eyed growers turn over older leaves on mite-prone plants to look for the mites themselves. TSSM damage usually appears first as a pale flecking or yellow areas on the oldest leaves, while Lewis mite damage on poinsettia is especially fine and from a distance may easily be mistaken for a nitrogen deficiency. Leaves may eventually turn entirely bronzed and drop off.

As spider mite populations increase, some webbing often appears on or between leaves, particularly when numbers are high. On zonal geranium, TSSM injury may be mistaken for a foliar disease, causing brown spotting, leaf distortion, edema and yellowing.

Tarsonemid mites, which are only really visible with magnification, usually cause terminal growth to become stunted or distorted; buds may even die as symptoms progress. Leaves sometimes take on a darker green color, and the underside may develop a brownish discoloration. Flowers are usually stunted and may develop a pale flecking.

Naturally, expect spider and tarsonemid mites to be more common on older plants and on plants grown from cuttings, compared with seed-grown material. They all appreciate warmer conditions to a point; spider mites thrive in low humidity, but tarsonemids prefer high humidity.

Making management work
When planning a management strategy, give biological control first consideration, particularly for a long-term crop. It may be necessary where risk of miticide resistance is high. Several predatory mites are available; note the favorable environmental conditions (humidity, temperature, daylength) for each and which are used for which mite pest.

Prepare a release plan (say, every 14 days) and budget — be sure to include shipping. Verify the quality of the biocontrols on arrival (they should be alive and in the number ordered), start releases early before a problem is apparent, and check regularly to be sure they are working. Have a game plan should things not work as expected or if other pests or diseases appear.

Koppert and Biobest are two suppliers with side effects lists on their websites to help guide choices should miticide intervention be needed.

Compatibility with biocontrols is only one of several factors to consider when selecting mite-control products. Nearly all miticides can be used for spider mites with limited exceptions (Pylon has been less effective against Lewis mite in our trials but fine for other mites). For tarsonemids, Avid (or generic), Akari, Kontos, Pylon, Sanmite (broad mite), Judo, Magus, Triact 70, and horticultural oil (e.g. SuffOil-X) are labeled.

Miticide resistance is a particular problem with TSSM, so rotate among miticides with different modes of action (MOA) when needed. For rotational purposes Sanmite equals Akari and Ovation equals Hexygon. Product labels now include a Group code for this reason.

If resistant populations do develop, the problem may get more complicated by cross resistance to other miticides, even ones not used yet, with a different MOA. Seek help from a specialist if needed.

Some products (TetraSan, Ovation and Hexygon) are not highly effective against adult mites, so use early or in combination with other products. Most products benefit from good coverage, but TetraSan, Judo, Avid (or generic) and Pylon all have translaminar activity and may be good choices if leaf canopies are dense or where coverage on the undersides of leaves will be difficult. Kontos is the only systemic miticide for use as a drench; it also acts as a systemic when applied to foliage.

Water for mixing should have a pH below 7, especially important for Floramite, and water hardness should be adjusted if cation levels are high, particularly when using M-Pede. Water-conditioning agents such as Choice and ReQuest can be added to correct either of these issues.

Some product labels (Avid, Kontos, Triact 70, Pylon, Judo, Magus, horticultural oil, M-Pede, Shuttle-O; see labels for specific plants) include cautions about application to sensitive plants, and it’s always a good idea to test any new product or tank mix on a small scale on your plants with your own equipment. Maintain good agitation in the tank, particularly with any oil product, and use oils under good drying conditions (sunny, low humidity).

In situations where two-spotted mites have reached high levels, I often suggest first knocking down with M-Pede or an oil-based material, using possibly two applications, though there may be increased risk of injury. Tough plants such as palms can even be lightly power-washed to knock off many of the mites where practical.

Some miticides have labeling for other pests (Pylon for thrips and fungus gnats, Avid for aphids and leafminers, Akari or Sanmite for mealybugs, Magus or Judo for whiteflies, M-Pede or oils for aphids, scales and others). Oil-type products and M-Pede kill only mites present at application, but most other miticides provide some residual control. Compare re-entry intervals and PPE (personal protective equipment) requirements on the labels, and note that some products or uses may be unavailable, limited or restricted in some states or regions.

Editor’s note: Greenhouse Management is always reporting on the latest research on the products that growers use every day. If you have an idea for future articles focused on experts’ field trials and related research, contact astankiewicz@gie.net.

Daniel Gilrein is an extension entomologist at the Cornell Cooperative Extension of Suffolk County.