Garden Q&A

Posted: June 11, 2010

Question: Last summer, the leaves of my red-twig dogwood were almost entirely eaten by masses of little caterpillars. Is there anything I can do if it happens again?

- P.J.

Answer: The evil little eaters are the larvae of the dogwood sawfly, a non-stinging cousin of wasps. For the last two years they have attacked a small pagoda dogwood I have, skeletonizing most of the foliage so that only the rib veins remain.

Here's the bad news: They aren't caterpillars, which are the larvae of butterflies and moths. It's bad because Bt (Bacillus thuringiensis), the natural control that is so safe and useful for cabbage loopers and tomato hornworms and their kin, will do nothing to stop sawfly larvae.

Knowing the life cycle is helpful in attempting control. Adult sawflies emerge from decaying wood in May or June. Using a piercing organ called an ovipositor, the females lay eggs inside the leaf tissue of certain dogwoods. After hatching, the larvae go through three phases, all of which are spent living on and dining on the dogwood leaves. In succession, the larvae appear translucent yellow, then covered with chalky white material, and finally yellow with a black head and spots, about an inch long.

On my pagoda dogwood, I noticed the rapidly disappearing leaves in July and found scads of the larvae on remaining leaves, all coiled in a bunch. This is your assignment: Check your red-twig dogwood daily for the next several weeks. As soon as you see leaves being eaten, kill the larvae by squeezing them with your fingers or spray with horticultural oil or insecticidal soap, both of which smother the larvae and are far more benign than insecticides. Given the tenacity of the sawfly larvae, I'd opt for horticultural oil.

If you don't kill them, they will mature, stop feeding (hey, the leaves are gone anyway!), drop to the ground, and find decaying wood to burrow into for the next nine months or so. Therein lies your other assignment: Remove decaying wood, such as logs, near your plant; replace any rotting wood that's part of a shed or other nearby structure.

There is slightly good news: Since the foliar feeding takes place well into the growing season, the plant typically survives with minimal ill effects. Yes, it looks all eaten up, but it will fully leaf out the next spring.

Gray dogwood (Cornus racemosa), C. sericea (one of several red-twig dogwoods), and pagoda dogwood (C. alternifolia) are the most susceptible plants. Flowering dogwood (C. florida) appears unaffected.

Q: To what extent is it possible or practical to amend soil in a garden to accommodate certain plants (that would really prefer another habitat)? In my case, I keep falling for things that seem to like alkaline, well-drained soils, not like what we have around here. - Vince Harris

A: Kathryn Belville of the Bartlett tree company once gave a presentation at the annual Woody Plant Conference at Swarthmore College that included a most demonstrative, even passionate exhortation: "It is very, very, very difficult to change soil pH!" (Except I think there may have been more uses of very.)

Soil pH is the measure of alkalinity/acidity. Neutral is 7.0, with acid soils less than 7, alkaline soils higher.

Belville was talking from a tree (and shrub) point of view, which means far more soil than an alkaline-loving perennial would involve. But her point is quite valid, for the overall pH in an area is the norm to which even amended soil is bound to return, due to the inherent composition (such as the presence of limestone) and rainfall. Abundant rain leaches salts from the soil, making it more acid, and our area gets much more rain than most of the Western and interior states. Thus, in my native central Texas, acid-favoring dogwoods are puny and sparse at best and fresh blueberries were unheard of in the days before mass refrigerated shipment of food.

Another bit of evidence: Lawns do better in less-acid soil, so in these parts lime is often applied. Indeed, it is applied year after year, because the soil quickly returns to its normal pH.

To grow alkaline-inclined perennials, a relatively small raised bed or a rock garden (use chunks of limestone!) is your best bet. Have the soil tested, and add horticultural lime based on the test results. Gravel, in the form of crushed marble, will increase drainage and provide a long-term boost in pH. In fact, I'll bet that improved drainage - especially winter drainage - would be as much a boon for your preferred plants as pH adjustment.

- Michael Martin Mills


Send questions to Michael Martin Mills, The Inquirer, Box 41705, Philadelphia 19101 or gardenqanda@earthlink.net. Please include locale. Read his recent work at http://go.philly.com/michaelmartinmills

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