In Good Thyme - by Mariana Graham

Appeared in WNT, April 25, 2001

Don't worry; those aren't tent caterpillars in your evergreen trees; they're tigers. The fuzzy, tawny-colored caterpillars are larvae of the Silver Spotted Tiger Moth, which feed on Douglas fir and other conifers. They're often confused with tent caterpillars because both create webs or tents. There's a big difference between the two, however.

Tent caterpillars feed on deciduous trees and can devastate entire forests. Tiger Moth caterpillars dine only on conifers, and usually don't do a great deal of damage. Their life cycle begins in July or August, when the golden-brown moth with rows of white spots on its wings deposits its eggs in the needles and twigs of a host tree. It prefers Douglas fir, but may also be found on grand fir, hemlock, cedar, pine and spruce.
In less than a month, the eggs hatch into tiny caterpillars that congregate under loose webs. Often hidden near the trunk and camouflaged by dead needles, the webs or tents are difficult to spot during the fall and winter. The larvae are inactive during cold weather, but as temperatures rise, they leave the tent to feed on needles. Come March or April, the little critters have grown to about an inch long and are really hungry. We get spring fever about the same time they do. We go out in the back yard, look at our trees and are shocked to see that an entire branch has been defoliated. That's when the calls start coming in to the Master Gardeners.

Except in the unlikely event of a major infestation, the advice you'll get from us is "do nothing." Fortunately, the larvae feed on older needles, not buds, so new growth will resume later in the spring. Damage is usually confined to one or two branches. If a branch looks really ragged and ugly and you can reach it, prune it out. The caterpillar will soon pupate and damage will cease.
You may opt to spray with BT (Bacillus thuringiensis), a bacteria that's safe for humans, animals, humans, the environment, but not caterpillars. It's available at all garden centers. Be judicious when using it, however, since BT will also kill larvae of other moths and butterflies.
WEED OF THE WEEK

Our state bird is the American Goldfinch. I love those colorful, delicate creatures and look forward to their arrival each spring. I fill a special feeder with thistle seed to encourage them to stay around my garden. Thistle seed? Yes, goldfinches love it. Fortunately, the seed of the plant sold as bird food, Guizotia abyssinica/niger, is not a true thistle and has been sterilized to prevent germination. Unfortunately, there's enough of the real, wild seed around to create a problem.

Designated as a noxious weed in Washington State, the purple-flowered Canada thistle, Circium arvense, is native to neither Canada nor the U.S. It's thought to have been introduced to North America from Eurasia in the 17th Century. Colonists passed laws in a vain attempt to eradicate the spiny perennial as far back as 1795. Washington Territory settlers were still trying to eliminate it 120 years ago. Thistle obviously won those battles.

Canada thistle is an aggressive weed that spreads via its deep and extensive horizontal root system, as well as by seeds that burst from flower heads in late summer. It colonizes in fields, lawns, forest margins, roadsides, and is a major problem in crop and grazing land. It clogs stream banks, inhibiting water access by livestock. Any hiker who has accidentally walked into a stand of Canada thistle knows just how inhibiting and unpleasant an experience it is. Plowing or rototilling fragments the roots, increasing the number of plants. Repeated mowing is effective if you do so before the buds show color. If you can see purple, that flower can produce seeds, even if you chop off its head!

Although goldfinches feed on thistle seed, there aren't enough birds in the entire state to affect this tenacious weed. The thistle gallfly, however, is a promising biological control agent. It's currently being released experimentally in Island County. Chemical controls are another option. For information on recommended herbicides and their use, contact Susan Horton, program coordinator of the Island County Noxious Weed Control Board, at 679-7327.

Now, when the spiny-tipped rosettes of thistle are coming up Island wide, is the time to act. Perhaps we can do what the colonists and settlers couldn't and at least get Canada thistle under control in Island County.

Mariana Graham is an Island County Master Gardener and member of Garden Writers Association of America. Contact her via Whidbey News Times, 675-6611; fax 675-2732, wnt@whidbey.net.