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Plant Answer Line Question

Keywords: caterpillars, Senecio jacobaea, Biological control, Noxious weeds--Washington, Weed control--Pacific Northwest

I was just walking through a wild area in Seattle with lots of weeds, and came across some strange caterpillars. They are mostly hairless (to the naked eye, anyway), and are striped black over orange with black legs. Can you tell me what they are?


I believe you may have seen the Cinnabar moth caterpillar, Tyria jacobaeae, which was introduced to the United States to control a noxious weed, tansy ragwort (Senecio jacobaea). Here are pictures to compare, and more information:
Flickr image

Both Washington State's noxious weed control board and King County Noxious Weeds have information on weed control with Cinnabar caterpillars. (See page 5 of the document). Tansy ragwort is a Class B noxious weed, and control is required in King County.

Date 2018-06-27
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Garden Tip

Keywords: Biological control, Insect pests--Control, Pesticides and wildlife, Trees--Diseases and pests

Gypsy moth is often in the news and with it comes the promise of aerial spraying of Btk by the department of agriculture. While the idea of the government spraying pesticides over an entire neighborhood may be frightening, a gypsy moth out-break would be devastating to the trees of the Emerald City or any city. Gypsy moths defoliate over 500 species of trees, both deciduous and evergreens.

Bacillus thuringiensis var. kurstakiis a bacterium that affects only caterpillars. It is considered an acceptable pesticide by organic gardeners, provided it used only when really needed. The major caterpillar pests in our area include:

  • the larvae stage of the gypsy moth;
  • cutworms that feed in winter and spring on primroses, chives and other perennials;
  • tent caterpillar often seen later in the spring on apple trees;
  • keep in mind that sawfly larvae which can strip a flowering red currant bare in a few weeks are not caterpillars, and Btk will not control them.

Btk will kill caterpillars of butterflies, which is why it must be used with caution only when pest populations are high or the potential damage is intolerable. Btk is typically sold as "caterpillar killer" where other pesticides are sold.

Date: 2007-04-20
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