Mite infestation leaves its mark on bamboo
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Q: My bamboo has odd streaks on the leaves - any ideas?
A: Many bamboo stands in the maritime Northwest show signs of infestation by a mite, the bamboo spider mite. These very small members of the arachnid class (meaning they have eight legs like spiders) damage bamboo leaves severely. The mites are difficult to see, but the damage is prominent.
Mite infestations often cause leaves to turn yellow, an effect that results when the mites suck plant juices. The leaves appear stippled, with tiny dots of yellow or brown that merge to cover the entire leaf.
Look for fine webbing on the underside of the leaf. New leaves will come in green but show damage as the mites affect the unfolding growth. (Keep in mind some color variations on bamboo are natural, not insect-created. A common example is Sasa veitchii, which has green leaves edged in white.)
The problem is almost ubiquitous on bamboo. It particularly affects the showiness of the larger-leaved variety. When bamboo is grown as a hedge or screen, it's possible to ignore the mites and just accept the fact that the bamboo isn't a uniform green.
In my garden, a large clumping bamboo (Bambusa multiplex "Golden Goddess") has gradually become more and more infested and looked worse over the past few years. I've chosen not to treat it because the plant is too big and because the infestation doesn't affect its screening effect.
However, if the bamboo is in a prominent place in the garden, or is a container or specimen plant, you may wish to try controlling the mite infestation.
First, avoid using a broad-spectrum insecticide if you wish to encourage beneficial predatory mites, which attack the problem. (The beneficials cannot, however, stay ahead of a bad infestation.)
Insecticidal soap is registered as a least-toxic chemical control for the spider mite. It's necessary to add a "spreader-sticker" to the soap when mixing it, because the chemical doesn't adhere well to the leaves. Check with your nursery or garden center for this product, which is often sold under various brand names. These are not generally harmful to beneficial insects. Be sure to coat the under surface of the leaf thoroughly.
The nursery specialists at Bamboo Gardens of Washington in Redmond (425-868-5166) suggested two cultivars that demonstrate some resistance to bamboo mite: a tall, running bamboo named "Okuboi" - good for screening as it can get up to 12 to 15 feet tall - and a small, dwarf bamboo: Pleioblastus viridistriatus.
Q: Are home garden pesticides a threat to bee populations?
A: Yes. As we head into summer, gardeners will experience various insect infestations and may choose insecticides that can kill bees. (Don't forget that your vegetables require pollination just as fruits do. All squash, such as succulent summer or pattypan types, depend on pollinators to ripen.) Here are a number of ways to avoid affecting these valuable garden pollinators.
• Make sure bugs are really your problem. Many plant problems occur because plants don't have their basic needs met. Be sure the plant is first receiving its necessary cultural care: soil, water and sunlight.
• Identify the insect before reaching for a toxic control. Many insects aren't particularly harmful - you may see one that's just passing through and isn't harming a plant. If you see holes or other leaf problems, be sure that the infestation is serious enough to treat. Many ornamental plants, if otherwise healthy, can lose up to 25 percent of their leaf surface without suffering harm.
If you need help getting the insect identified, take it to a nursery or to a Washington State University Master Gardener clinic. (Call your local WSU Cooperative Extension office to find locations of summer clinics. In King County, call 206-205-3100. The clinic at the Center for Urban Horticulture is closed indefinitely because of a fire that destroyed all materials there on May 23. Other summer clinics are on regular schedules. In Snohomish County, call 425-357-6010; in Pierce County, 253-798-7170.)
• Direct your insecticides appropriately. Many common garden chemicals will carry warnings on the label stating that they harm bees. Check your rose insecticides, in particular. Labels often suggest spraying late in the evening when bees aren't working and keeping sprays off plants in flower. Some insecticides should never be used where bees work. One of the worst is the granular form of an insecticide with the common name carbaryl (sold as Sevin). Bees can carry the granules back and annihilate the entire hive.
• Try other methods. Now is the time hatched aphids gather on rose buds. The most bee-friendly way to deal with them is to wash the aphids off with a gentle spray of water. Be careful, because the new buds are brittle and break easily. This won't get rid of all aphids, but it does protect both the bees and the beneficial insects such as ladybugs that dine on aphids.
If you choose a chemical to deal with aphids, try an insecticidal soap. Read the label carefully before applying any chemical.
For more information, call 206-205-3100 and request the fact sheet "Before you Spray, Read This."
Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension.