Garden Detective

Garden Detective: Red-humped caterpillar can be hungry pest

What’s attacking this blueberry plant? Those are red-humped caterpillars, a voracious pest that feasts on young leaves.
What’s attacking this blueberry plant? Those are red-humped caterpillars, a voracious pest that feasts on young leaves.

Q: What are these little creepy things? They were found on our blueberry plant.

Carol Smith, Sacramento

A: According to UC master gardener Annie Kempees, the photo you sent appears to show red-humped caterpillars. The red-humped caterpillar (Schizura concinna) is found throughout much of California. It can be a serious problem in the warm Central Valley. This pest most commonly attacks landscape and fruit trees, especially where insecticides applied to control other pests have killed their natural enemies.

Red-humped caterpillars seem to be more abundant after a warm winter. Upon hatching, caterpillars feed in groups on lower leaf surfaces and skeletonize the leaves. As the larvae become larger, they tend to disperse and consume the entire leaf, leaving only the tough, woody veins.

When infestation is light, larvae eat leaves on only a few branches, but occasionally a heavy infestation develops that defoliates entire trees or shrubs. Most severe defoliation occurs during the summer. Even if completely defoliated, trees that are otherwise healthy usually recover without the need of chemical applications.

The red-humped caterpillar has four stages of development: egg, larva (caterpillar), pupa and adult (moth). Adult moths lay eggs, which are nearly spherical and pearly white to cream colored, in groups of 25 to 100 on the underside of younger leaves.

Caterpillars are 1 to 1  1/2 inches long when fully grown and have a base color of yellow. Longitudinal white, reddish brown, or sometimes black stripes mark the body. The head is usually orange or brick red, as is the fourth body segment, which is distinctly humped and has two prominent, black tubercles or spines. Each body segment also has less distinctive black tubercles. Caterpillars rest with their hind end elevated.

The pupa is reddish brown, a little more than a  1/2 -inch long, and enclosed in a silken cocoon in the soil or in the layer of organic debris or mulch covering the soil.

Adult moths have a wingspan of 1 to 1 3/8 inches. The forewings are reddish to grayish brown and often are darkest along the hind margin. The hind wings are off white to light gray or brown. In autumn, caterpillars drop to the ground and spin silken cocoons. They remain inside the cocoons during winter and transform into pupae in spring.

Moths begin emerging from pupae in April and May. They mate, and each female can lay more than 200 eggs. The eggs hatch into tiny caterpillars that feed, grow, and then drop to the ground to pupate. There are often as many as four or five generations of this insect per year.

A number of parasitic wasps, including Cotesia (Apanteles) species and Hyposoter fugitivus, use the red-humped caterpillar as a host and often provide effective natural control. General predators including spiders, lacewings, bigeyed bugs, and damsel bugs also feed on eggs and caterpillars. However, in some instances additional control measures are necessary.

The simplest of these is to cut off the foliage that contains caterpillars while the insects are still young and active. At this stage, you’ll need to prune off only small branches in order to destroy a large group of caterpillars.

Crush the caterpillars, or drop infested leaves into a can of soapy water.

If you choose to use insecticides, choose ones that are least toxic to the caterpillar’s natural enemies. Bacillus thuringiensis – which is sold as a variety of products including Safer Brand Caterpillar Killer or Greenlight BT Worm Killer – is an effective spray for red-humped caterpillar control. Spray with Bt after the first skeletonized leaves appear. Bt is most effective on the smallest caterpillars. Be sure the caterpillars are present before spraying.

Spot treatments of affected trees are all that are generally necessary, even on young trees. Use with caution. Bt will kill all caterpillars including butterfly producing ones.

Experience has shown that sprays for red-humped caterpillars are often applied too late to have any effect. Unless many non-parasitized caterpillars are present on trees, it’s best to delay spraying until the next generation appears and is feeding on the leaves. If you find significant numbers of parasitized red-humped caterpillar pupae, biological control alone will likely control the pest and sprays should not be needed.

Additional information on the red-humped caterpillar is available online at www.ipm.ucanr.edu.

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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