Joan Swenson: Voracious caterpillar has appetite for destruction
| Monday, Aug 25 2008 06:22 PM
Last Updated Friday, Mar 27 2009 06:05 PM
Images
Dead redhumped caterpillars in Gregg Knowles' hand. Knowles used a pesticide on the worms. (Photo courtesy of Joan Swenson)
Partially eaten redbud leaves. (Photo courtesy of Joan Swenson)
Gregg Knowles with worms that attacked his redbud tree. (Photo courtesy of Joan Swenson)
Gregg Knowles’ redbud tree has been attacked twice this summer by redhumped caterpillars. One or two more attacks may be coming, too, before the hot weather is over.
Knowles planted his redbud last year and sometime later discovered that the tree was being denuded of its leaves by small yellow striped caterpillars with a big red head and a red hump with two protruding spines on its fourth body segment.
The culprit is the redhumped caterpillar, which is known to have three or more generations of worms per summer. Trees preferred by the caterpillar are liquidambar walnut and plum, but it will also feed on other fruit trees as well as cottonwood, birch, willow and redbud leaves.
I haven’t seen a redhumped caterpillar in many years, not since friends who lived on the then-edge of town off White Lane watched their liquidambar trees get stripped by this voracious eater that can feed shoulder-to-shoulder and skeletonize tree leaves.
I suspect that the redhumped caterpillars were imported into Knowles’ yard as eggs on the redbud leaves when he brought the tree home and planted it last year. There are a number of liquidambar trees and redbuds in the neighborhood that don’t seem to be affected by the redhumped caterpillars.
These caterpillars hatch from groups of eggs laid on the bottom of leaves. When they start feeding, they do so in groups. As the caterpillars grow larger, they move as individuals throughout the tree, sometimes defoliating it completely, as was the case with the last year’s infestation on Knowles’ tree. Most trees recover from the attacks, depending on the severity and health of the tree.
After the caterpillars finish feeding, they drop to the soil and, according to the University of California Integrated Pest Management Program, spin cocoons, where they repeat the process until their final generation in the fall. Moths emerge from the cocoons in the spring and mate, laying eggs on favorite trees and the cycle starts again. Females can lay 200 eggs apiece. The redhumped caterpillar can have up to five generations a year, as they drop, cocoon and emerge repeatedly.
The redhumped caterpillar is parasitized by several types of wasps, but they may not control the pest completely. The UC recommends keeping an eye on trees that have been infested and cut off foliage that has young caterpillars feeding closely together. Crush the caterpillars and you may be able to stop another generation from hatching.
Bacillus thuringiensis can be effective, too, if applied thoroughly when only a few leaves have been skeletonized. Bt, as the bacterial concoction is often known, will not damage beneficial insects as knock-down pesticides will. Bt will make the worms sick and they’ll stop eating after feeding upon it. Redbud trees are generally small enough that treatment with Bt is possible — bigger trees make application difficult. When using Bt, treat as early as possible. You may have to treat a second generation.
Some caterpillars that feed heavily on desirable plants turn into pretty butterflies, but that’s not the case with the redhumped caterpillar. Spray with Bt and squash caterpillars with abandon — the moths are small red-and-brown creatures that don’t add beauty to the garden.
PALM MEETING
The University of California Cooperative Extension is sponsoring a one-day meeting on palm selection and care Sept. 9 in the UC Extension office auditorium. Topics will include UC research on palm tie-up and transportation, nutrition, species selection, irrigation, palm pruning and other topics. Speakers are experts on palms, Dennis Pittenger from UC Riverside, palm writer Don Hodel, and plant pathologist Jim Downer, who specializes in palms.
The program begins at 8 a.m. It is being held in conjunction with Abate-A-Weed, which is handling registration. For more information, contact John Karlik at jfkarlik@ucdavis.eduor 868-6220 or Abate-A-Weed, 589-0615.
