Dan GilreinHundreds of samples come into the Cornell University diagnostic laboratory each year. Of the greenhouse inquiries, most begin and are resolved at the pathology lab.
Many of the insect and mite infestations are relatively easy to identify and don’t usually require the services of a diagnostician. The few exceptions include a large number of broad mite problems associated with spring annuals and cut flowers. Broad mite infested plants are usually submitted as a suspect disease problem. The broad mite symptoms are easily mistaken for those caused by pathogens, herbicides or environmental factors. There are some clues that can help in early recognition before broad mite infestations have spread and damage is severe. Controls need to be carefully selected because not all that work against spider mites are effective on broad mites.
Identifying broad mites
Unlike the more familiar two-spotted spider mite, broad mite (Polyphagotarsonemus latus) belongs to a separate family of “thread-footed” (tarsonemid) mites, which also includes cyclamen mite. Full-grown broad mites are much smaller than two-spotted spider mites and almost invisible to the naked eye (a 10-20X magnifier is helpful).
Adult mites are pale white to almost translucent in color and they have hind legs that are somewhat threadlike in appearance. The females are a bit “broader” in the middle compared with the cyclamen mite.
The “polyphago” part of the Latin name is a clue to the mite’s wide host range including many common greenhouse and outdoor plants. Plants on which broad mites are commonly seen include gerbera, begonia, New Guinea and standard impatiens, English and Algerian ivies, salvia and zonal geranium. Less common hosts include chenille plant, plectranthus, marigold, celosia, hydrangea, fuchsia and jasmine.
There have been complaints of broad mite on peppers (both greenhouse transplants and field production), greenhouse cucumbers and even on potatoes. The host range extends well beyond the crops mentioned. There is no evidence so far that this subtropical mite overwinters outdoors in colder areas like New York.
Signs of broad mites
Broad mites produce translucent or pale white oval eggs, which are uniquely ornamented with small, raised dots that distinguish them from cyclamen mite. Two-spotted spider mite eggs are much larger and spherical. Broad mite eggs are surprisingly large, nearly the size of the females. They are one of the key pieces of evidence when confirming a broad mite infestation.
After the eggs hatch, the flimsy “eggshell” left behind collapses onto the leaf surface. These remnants can confirm the broad mites presence even after an infestation has been controlled.
Usually the mites and eggs can be found on the undersides of leaves, but they are sometimes found on the upper surfaces as well. During warm, dry conditions the mites seem to be isolated to the terminal growth or leaf undersides, or associated with areas where the dense plant canopy results in higher humidity. In some cases layers of terminal leaves can be peeled away to locate mites or eggs or numerous leaves can be scanned to confirm the presence of the pest. It is helpful to submit several symptomatic plants when requesting a diagnosis.
Infestations are usually seen when growers notice distortion and stunting of new growth. Growers frequently mention that the symptoms appeared suddenly. Leaves may be strap-like or cup downward and sometimes flowers are also affected with petals stunted and discolored or with streaks and fine flecks similar to thrips damage.
On standard impatiens, the leaf cupping is also a symptom of downy mildew. Leaf distortion on New Guinea impatiens also looks very much like cultural or environmental (cold temperature) issues, or sometimes resembles thrips damage. Occasionally growers have submitted samples suspecting ethylene or 2,4-D (herbicide) injury.
Two other fairly distinctive symptoms include the fine bronzing under leaves (e.g. gerbera, begonia, celosia and others), and a russeting (brown, corky bark-like texture) on stems of many plants. Both of these symptoms also resemble injury from insecticidal soap sprays.
Broad mite infestations have been most common on vegetatively-grown crops and rarely observed on plugs and seed-grown plants. I suspect this points to the origin of many infestations and has much to do with the increased numbers of complaints since the late 1980s, corresponding to changes in production methods. Broad mite is nearly impossible to detect early and sometimes the source is within a grower’s own range. Growers should work cooperatively with propagators to address the situation when incoming material is the suspected source of infestation.
Also, be aware of the potential of broad mites to hitchhike on the legs of whiteflies. This raises the need to monitor whitefly infestations where broad mites are also found.
Easy to control
The good news is that broad mite is relatively easy to control. In greenhouse vegetable production, biological control with preventive releases of Neoseiulus fallacies, a predatory mite, is often used. This beneficial mite prefers a greenhouse environment that is warm and not too dry, with temperatures in the upper 60°Fs to 70°Fs and humidity ranging from 60-70 percent.
Miticides work well and are more commonly employed for curative use to “clean up” a problem. Not all miticides control broad mites.
Currently labeled products for ornamental greenhouse plants include Avid (and generic versions), Sanmite, Akari, Judo and Pylon. In New York, Akari is not yet labeled for use on broad and cyclamen mites. Note label warnings about plant sensitivity and product information bulletins. Unless labels indicate otherwise, start with two applications around a week to 10 days apart. Make sure to obtain good spray coverage with contact miticides.
Examine new plant growth carefully a week after the last application. Plants should start to appear normal if the treatments are successful. If repeat applications are needed, avoid rotating Sanmite with Akari, as they have similar modes of action. Fortunately, miticide resistance has not yet been a problem with broad mite.
Dan Gilrein is extension entomologist, Cornell University Cooperative Extension, Long Island Horticultural Research and Extension Center, (631) 727-3595; firstname.lastname@example.org.