If the new growth on trees appears to be chewed off, the culprit is likely the brown escargot snail. Yes, this common pest is exactly the same species served up in garlic butter in French restaurants. It was imported by a Frenchman, M. A. Dumas, into California in the 1880s. It is now found up the snow line, competing rather nicely with our native, shell-less banana slugs for fresh greenery. Susceptible tree species include citrus and wisteria, not to mention nearly all our favorite little accent plants.

Both snails and slugs become active in moist, warm spring weather, harboring under planks and leaves and continuing to roam in hot summers where irrigation keeps them happy enough to bear young six or seven times a year. A favorite place to hide is on the bottom of bonsai pots.

Fortunately, bonsai are portable, and moving your susceptible plants to higher ground usually solves the problem. If your pots are already elevated, as on an outdoor shelf, you should check underneath planks for the critters and scrape them off. Wrapping copper stripping (Snail-Barr) around the base of raised benches does a good job of keeping snails away, because something in this metal reacts to the gastropod’s slime and gives the creatures a tiny electric shock.

No, I am not making this up! University of California entomologists have done the research. If you visit any commercial orange grove (where snails are a huge problem) you will see copper collars around the trunks of all citrus trees to keep the snails from climbing up and spoiling the fruit. You, too, can make a little copper collar for the trunk of your bonsai tree. You can also wrap the very bendable copper strips around pots, removing them at show time. A simple twist of copper bonsai wire seems to be too narrow to be a deterrent. I’ve tried it. In any case, Snail-Barr is not expensive.

Other creepy-crawlies: Spring leaves can also be chewed off broadleaf tree species by caterpillars, large and small. If you see chewed-looking leaves, investigate the trunk and branches and any “folded” leaves. Common pests in San Francisco are inch-long caterpillars and the smaller Oak Moth larvae. Both are brown and hard to see. Run your finger on a branch and you can find them by touch.

Hand-picking is an easy way to control caterpillars, since the majority are attracted only to specific host plants. They do not eat unrelated species as they have no appetite for them. If you have a large number of oaks and maples apparently infested, you can dust with Bt (Bacillus Thuringiensis). Bt is a microbial parasite that kills all types of caterpillars. It should not used indiscriminately if you enjoy seeing butterflies in the garden as you work on your trees.

(April 1994)

Tag: pests