Garden Detective

Backyard horror story: Attack of the redhumped caterpillars

Garden Detective: What can eat all the leaves off a redbud tree? Redhumped caterpillars have a notorious appetite.
Garden Detective: What can eat all the leaves off a redbud tree? Redhumped caterpillars have a notorious appetite. Bee file photo

Experts tackle readers’ garden questions.

Q: I have a rental home in Sacramento. A tenant called, distressed; a red bud tree had been “stripped” by caterpillars in a very short time period. We are curious about them and wonder if you can identify the type. I did go to Green Acres to purchase a product we sprayed on the tree. So far, it appears there has been no more activity.

Ann McKeel, Roseville

Sacramento County Master Gardener Sherrill Neidich: The culprit causing damage to your redbud tree appears to be a redhumped caterpillar (Schizura concinna). I had a similar experience with this caterpillar on a western redbud tree in Yuba City. I heard a crunching/chewing sound coming from my tree and upon inspection I found the back half of the tree was defoliated and there were redhumped caterpillars everywhere. It looked like a scene from a horror movie!

The redhumped caterpillar is found throughout most of California and can be a serious problem in the warm Central Valley. This pest most commonly attacks liquidambar, walnut and plum trees, but also can be found on a wide range of ornamental and fruit-bearing trees.

Adult caterpillars are easy to identify. They are approximately 1 inch long when fully grown, and yellow with white, reddish brown or black horizontal stripes. What makes this caterpillar distinctive from others is its head, which can be orange or brick red, as well as a fourth body segment that is humped and has two black spines. Each body segment also has smaller black spines.

The redhumped caterpillar has four stages of development: egg, larva (caterpillar), pupa and moth. Moths begin emerging from the pupae in April and May. During this time, they mate and each female can lay more than 200 eggs, which are round and pearly white to cream colored, and can be easily seen on the undersides of younger leaves. The eggs hatch into tiny caterpillars that have a yellow or reddish body with a black head.

In autumn, the caterpillars drop to the ground and spin silk cocoons, in which they remain during winter and then transform into pupae in spring. The cocoons, resembling little reddish-brown footballs, can often be seen on the ground and easily destroyed.

There can be as many as four or five generations of caterpillars per year and the most severe damage to trees usually occurs during the summer months.

The caterpillars are voracious eaters and feed in groups on lower leaf surfaces. As the larvae become larger, they move throughout the tree and consume entire leaves, leaving only the tough woody veins. When infestation is light, larvae will eat leaves on only a few branches; however, a heavy infestation can defoliate an entire tree in a short period of time. If a tree is otherwise healthy, it may look worse for wear but it usually recovers.

Redhumped caterpillars are often controlled by two parasitic wasps, Hyposoter fugitivus and Apanteles schizurae. Female wasps lay their eggs in the caterpillars and the wasp larvae feed inside. Other predators, including spiders, lacewings, bigeyed bugs and damsel bugs, also feed on the eggs and caterpillars. If a lot of redhumped caterpillars have been parasitized, this biological control alone may be all that is needed to rid the tree of these pesky insects.

Additional control measures include cutting off the foliage that contains the caterpillars while the insects are still young and active. Pruning off small branches may be all that is necessary to destroy a large group of caterpillars. The infested foliage can be placed in a bucket of soapy water or the caterpillars can be crushed.

If the entire tree is infested, an insecticide, such as Bacillus thuringienis (commonly called Bt), which is less toxic to the caterpillar’s natural enemies, can be used. After a redhumped caterpillar eats a leaf sprayed with Bt, it will die within a couple of days. Products, such as Safer Brand Caterpillar Killer or Greenlight BT Worm Killer, that contain Bt can be found at local garden stores. Be sure that caterpillars are present before spraying and always read product instructions thoroughly before applying.

Another environmentally friendly insecticide that controls redhumped caterpillars is spinosad (Monterey Garden Insect Spray). Like Bt, the entire tree will need to be sprayed for effective control.

For additional information on the redhumped caterpillar, including photos, check out the University of California Integrated Pest Management (UC IPM) Pest Note 7474 at http://ipm.ucanr.edu/PMG/PESTNOTES/pn7474.html.

For information on natural enemies that kill or reduce the numbers of another organism, visit the UC IPM’s Natural Enemies Gallery at http://ipm.ucanr.edu/PMG/NE/index.html.

Sherrill Neidich is a UC Cooperative Extension Master Gardener for Sacramento County.

Garden questions?

Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to h&g@sacbee.com. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:

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