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Garden Q&A: That’s no cocoon, it’s an egg sac, and what to do about sour mulch

This is the egg sac of Basilica spiders, an beneficial garden spider that eats many pests. - Original Credit: For The Baltimore Sun
This is the egg sac of Basilica spiders, an beneficial garden spider that eats many pests. - Original Credit: For The Baltimore Sun (Ellen Nibali / HANDOUT)

What kind of caterpillar will come out of this cocoon? I see several hanging on my azaleas, from thread tough as fishing line.

Though shaped like the fanciful caterpillar in picture books, these are actually the egg sacs of a tiny but impressive spider known as the Basilica spider.

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Differing sources claim it is named for its many bright colors — red, black yellow, green, white and orange — reminiscent of stained glass windows or for its unusual web. As a member of the orbweaver family, the young basilica spider weaves the iconic web of graduated circles within circles. However, once it matures, the basilica spider adds a dome shaped web below, like a basilica dome (or the U.S. Capital.)

Inside the dome, sticky threads dangle to catch its prey, including pest insects. So basilica spiders are a good sighting. Basilica spiders are most active in late spring-early summer.

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Black blotches appeared on my Japanese anemones two days after a mulch application. Mulch was put right up to the stems, over two inches deep, and now my coleus, azaleas and hydrangea are looking bad, too. What caused this? What is the treatment?

This damage may be due to wood alcohol in the mulch, known as sour mulch. Water thoroughly to leach any compounds that have not yet evaporated. Cut back damaged plant parts and remove any dead plants (most apt to be annuals or perennials).

Search ‘toxic or sour mulch’ on the Home and Garden Information Center website for more information.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Maryland’s Gardening Experts” to send questions and photos.

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