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Also see publication Two-spotted Spider Mite: Management in Greenhouses and Nurseries
MANHATTAN, Kan. – 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.
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).
Brief: If It’s Green, Look for Spider Mites
MANHATTAN, Kan. – Two-spotted spider mites have few prejudices. They’re perfectly willing to feed on an estimated 1,100 different plant species.
“They’re also willing to feed from the time they hatch until they die – through every one of their developmental stages,” said Raymond Cloyd, K-State Research and Extension entomologist.
The two-spotted mites are houseplants’ No. 1 pest, sometimes enclosing whole leaves in their webbing. In fact, Cloyd said, the pests are so tiny that webbing can be the first clue of an indoor infestation. (Wind and rain typically dispose of webbing spun outdoors.)
Two spotted spider mites also are major damage-causers from commercial greenhouse to farm field --where they attack corn, cotton and soybeans They like wild and weedy plants, too, ranging from berry thickets, Jimson weed and violets to henbit, wild mustard and wood sorrel.
The weeds, in turn, can serve as undisturbed breeding grounds that help the mites spread.
Entomologists at central U.S. land-grant universities have come up with the following, however, as some of the landscape hosts that two-spotted spider mites seem to prefer:
Annuals, ornamental - dracaena spike, fuchsia, geranium, hollyhock, impatiens (New Guinea and garden), marigold, pepper (sweet and hot), petunia, snapdragon and viola.
Annuals, vegetable – beans (various), sweet corn, cucumber, eggplant, muskmelon, squash, tomato and watermelon.
Perennials, mixed - lemon balm, chrysanthemum, columbine, coral bells, daylily, Michaelmas daisy, grape vine, English ivy, rose mallow, mint, monarda, oregano, salvia, creeping phlox, primula/primrose, scabiosa, speedwell/veronica, strawberry (ornamental and fruiting) and vinca/periwinkle.
Shrubs - arborvitae, azalea, burning bush, butterfly bush, boxwood, cotoneaster, wall germander, holly, hydrangea, juniper, maple (amur and Japanese), privet, pyracantha, rhododendron, rose and viburnum.
Tree – ash, birch (white/gray/river), cherry, citrus, Kentucky coffeetree, elm, black locust, silver maple, pear, persimmon, plum and poplar.
“The list is handy. But, I’d get a hand lens and take a closer look at any plant that starts losing leaves or looking scorched -- particularly during hot, dry weather,” Cloyd said.
Brief: Spider Mites: World-Class ‘Adapters’
MANHATTAN, Kan. -- An international team of 55 scientists recently found why the two-spotted spider mite is such a survivor. They decoded its genome – its sequence of genes.
The team reported their resulting “gene map” is rather simple for an arthropod (which includes the insects, crustaceans, and families of centipedes and spiders). Yet, the genome includes more “detoxification” genes than ever seen before -- genes the mite can turn on or off, as needed.
“The team chose the two-spotted spider mite because it’s a destructive, global pest. It can fairly easily develop pesticide resistance. Plus, it can feed on an amazing number of host plants – which the team put at more than 1,100 plant species,” said Raymond Cloyd, K-State Research and Extension entomologist.
The mite prefers hot, dry weather, he added. But, it’s a persistent pest for houseplants and commercially grown greenhouse plants, as well as for landscapes, vegetable gardens, market farms, orchards and row-crops. The mite also thrives on and can spread from a number of weed species.
Cloyd said the team’s kind of research is vital to a world of limited resources and growing populations. It allows scientists to target specific gene segments as they seek ways to limit a pest’s damage.
Among this study’s findings: The spider mite has 39 protein-encoding genes (some previously unknown) in one “resistance” family. That’s many times the nine to 14 “detox” genes currently identified in both insects and vertebrate animals.
“As part of their study, the team switched the mites from one host to another to see what happened,” Cloyd said. “They found this mite can activate some of those genes and deactivates others, to adjust to a new situation. Undoubtedly, that’s also a factor in how the mites can actually adapt to toxic plants, as well as develop resistance to pesticides.”
(Note: More about the study is at Big Pest, Small Genome: Two-Spotted Spider Mite Genome Decoded.)
Strange Facts. . .
* An international team of scientists found a mystery when they mapped the DNA of the two-spotted spider mite: Some of the mite’s genes are similar to those found in bacteria and fungi.
* Spider mites got their name because, like spiders, they spin webby strands of silk. While strong, however, the mites’ silk is 185 to 435 times thinner. Spider mites are vegetarians, so they use their silk for protection and travel, not entrapping prey.
* Scientists are predicting two-spotted spider mites will be a growing concern in a warming climate because the mites multiply extremely fast at high temperatures -- 90 degrees F or more.
* The spider mite genome contains about 90 million “base pairs” of DNA. So far, that’s the smallest gene sequence that scientists have “mapped” for any arthropod (i.e., spineless animal with an external skeleton, segmented body and jointed legs). Other arthropods’ gene maps have included up to 7.1 billion base pairs.
* When scientists switched two-spotted spider mites from one host plant to another, they discovered that one-half of the mites’ cytochrome P450 family of genes responded by either turning on or turning off. The mites had adapted with the biggest basic-level change ever seen in an animal.
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