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Two-Spotted Spider Mites Flourishing

07 August 2012

It’s smaller than a pinhead. Even so, the voracious two-spotted spider mite could be the world’s poster pest for dry, hot weather.

“They’re doing real damage in Kansas - even worse than we saw in summer 2011. Two-spotted spider mites can attack an amazing number of plants. As often as not, today’s scorched-looking arborvitae foliage and yellowing rose leaves are reflecting both heat-wave stress and spider mite damage,” said Raymond Cloyd, K-State Research and Extension entomologist.

Two-spotted spider mites produce multiple generations per growing season, Cloyd added. But, when the weather’s dry and the temperature’s high, they can multiply really fast. Between 70 F and 85 degrees, for example, the time they need to progress from egg to adult shortens by half - from 14 to seven days.

Each adult female in every generation lays an average dozen eggs per day (100 to 300 total). She doesn’t even have to mate first, the entomologist said.

Spider mite on a pecan plant. Louis Tedders, USDA Agricultural Research Service, Bugwood.

As a result, infestations easily expand, yet include spider mites in all stages of development. The mix of stages alone makes controlling the pest difficult.

Adding to the problem, the mites have repeatedly proved they are able to develop pesticide (miticide) resistance. That has eliminated multiple tools for gardeners, farmers and pest control pros, Cloyd said.

“Mites aren’t an insect. They’re in a whole different family, along with spiders, ticks and scorpions,” he said. “Spraying an infested plant with an insecticide simply eliminates the mites’ natural enemies. Applications of carbaryl (Sevin, Adios, and Slam) have paved the way for some notably severe spider mite outbreaks.”

Some products labeled for spider mite control also can cause unwanted results. Such organophosphates as Malathion, Orthene and Dursban may actually stimulate female spider mite reproduction.

Fortunately, with enough rain, a naturally occurring fungus helps keep two-spotted spider mites in check, Cloyd said. When temperatures are moderate, beneficial insects and predatory mites also serve as a natural control. Their job outstrips their abilities, however, when the weather heats up.

“The classic test for spider mites is to shake a branch over a sheet of white paper and see what falls onto it. Mites look sort of like a typed period – a half-millimeter dot -- that moves,” he said. “If they move fairly slowly and leave a green stain when crushed, they’re the two-spotted kind. If they’re fairly quick and leave a red stain, they’re the beneficial mites that eat other mites.”

Human control efforts are more effective if started when an infestation is small, he said.

Unfortunately, two-spotted spider mites can go unnoticed as the growing season warms up and they become increasingly active – and numerous. The pests not only are small but also tend to congregate on the undersides of leaves.

Besides, they seem to prefer older leaves, Cloyd said. For instance, they can cause extensive damage in a dwarf Alberta spruce before they get around to attacking the external, more visible needles. (Spruce spider mites also attack these popular evergreens during spring and fall.)

On deciduous plants – pear trees, tomatoes, geraniums and the like – two-spotted spider mites’ damage shows up as small silver-gray to yellow leaf speckles. The mites’ mouthparts are like a blood-drawing needle. They puncture plant cells and suck out the green chlorophyll.

With enough of that damage, infested leaves yellow, turn bronze and drop. Plants can get weaker, lose large sections, become deformed or even die.

What’s a Plant Owner to Do?

Curbing two-spotted spider mites in the landscape takes patience and multiple approaches.

“You have to remember this is about management. Total control is impossible. If nothing else, new spider mites can blow in from a neighbor’s yard, ballooning on the webbing they spin,” the entomologist said.

Cloyd listed the following as helpful tools in limiting attacks:

  • With a magnifying glass or sheet of white paper, start scouting for spider mites in mid to late spring.
  • Fertilize enough to keep plants strong and healthy. But, don’t overdo it, especially with soluble forms of nitrogen. Over-fertilizing promotes soft, juicy tissues that are easier for mites to drain.
  • Remove weeds and old garden debris. Either can serve as a breeding ground or winter haven.
  • Don’t let plants go without needed water. (Irrigate in the morning to avoid disease problems.)
  • A forceful water spray can dislodge spider mites in all life stages – eggs included. So, hose down infested plants two or more times a week, targeting the underside of leaves. That can be enough to keep a light infestation in check.
  • Both organic and inorganic sprays can help control heavy infestations. Read labels to see if products target a certain life stage. Repeat spray applications, as your chosen label suggests. Be sure to cover the underside of leaves thoroughly. Don’t use the same product season-long or year after year.

“All of that is important. If you carefully read the label on insecticidal soaps or horticultural oils, for example, you may find that frequent applications could harm roses or other plants,” Cloyd said.

  • Remember that plant injury can result if you apply any pest control material – miticide, insecticide or fungicide – when the air temperature is above 85 degrees.

Among the contact sprays that still work on two-spotted spider mites are: bifenazate (Floramite), hexythiazox (Hexygon), fenpyroximate (Akari), acequinocyl (Shuttle), fenbutatin-oxide (Hexakis), insecticidal soap and horticultural oil. The products that penetrate plant tissues, so work past their application time include: abamectin (Avid), spiromesifen (Forbid) and etoxazole (TetraSan).

August 2012

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