Redhumped caterpillars (Schizura concinna) are defoliating pests that feed on a variety of hardwood trees including apple, walnut, and oak.
The Gregarious Orchard Defoliator: Redhumped Caterpillar
The Redhumped Caterpillar (Schizura concinna) is a “noxious” and highly visible pest that targets a wide variety of fruit and ornamental trees across the United States. While it is common in the Pacific Northwest and California, it is a frequent summer visitor to the deciduous trees of Tucson and the Southwest. These “O” pests are famous for their “gregarious” feeding habits; they hatch in large groups and stay together, stripping every leaf from a branch before moving as a “unit” to the next, often leaving young trees completely skeletonized in a matter of days.
Identification: The “Red-Hump” Warning
Identifying Redhumped Caterpillars is straightforward due to their unique anatomical features and defensive posture. For Pestipedia.com users, the bright red coloring is the primary “danger” signal:
- The Head: A distinct, bright cherry-red head capsule that stands out against its striped body.
- The “Hump”: The fourth segment of the body (just behind the legs) features a prominent, bright red hump. This hump is often armed with black, spine-like tubercles.
- The Body: The rest of the caterpillar is marked with longitudinal base lines of yellow, black, and white. They can grow up to 1.5 inches (38mm) long.
- The “Rear-Up” Posture: When disturbed, the entire colony will simultaneously lift their heads and tails into the air in a “U” shape to intimidate birds and predatory wasps.
- The Adult: A nondescript, grayish-brown moth with a wingspan of about an inch. They are nocturnal and rarely seen by homeowners.
The “Skeletonizing” and “Branch-Stripping” Damage
The “noxious” impact of the Redhumped Caterpillar begins subtly but ends with dramatic defoliation:
- Skeletonization: Young larvae lack the jaw strength to eat the entire leaf. They eat only the soft green tissue, leaving behind a “lace-like” network of veins.
- Total Defoliation: As they mature, their appetite explodes. Older larvae consume the entire leaf, including the smaller veins, leaving only the thick midrib.
- Preferred Hosts: In the U.S., they are most commonly found on Apple, Pear, Plum, Walnut, and Liquidambar (Sweetgum) trees. In Tucson, they are frequent pests of ornamental plum and cottonwood.
- Acidic Defense: If handled or attacked, these caterpillars can spray a fine mist of formic acid from a gland on their thorax, which can irritate human skin and eyes.
U.S. Orchard and Backyard Management
In the United States, managing Redhumped Caterpillars is centered on Manual Removal and “Soft” Biologicals. Because they stay in tight groups, they are one of the easiest pests to control without heavy chemicals.
- The “Hand-Pruning” Strike: For Pestipedia.com users, the most effective control is simply finding the infested branch and snapping it off or shaking the caterpillars into a bucket of soapy water. One clip of the pruners can remove 50+ pests at once.
- Bacillus thuringiensis (Bt): Since they are true caterpillars, they are highly susceptible to Bt. This is the preferred U.S. organic treatment because it only kills leaf-eating larvae and will not harm the Lacewings and Damsel Bugs that naturally prey on them.
- Natural Parasitoids: In the U.S., a small wasp called Hyposoter fugitivus is their primary enemy. If you see a caterpillar that looks “mummified” or has a small silk cocoon underneath it, leave it alone—it is a nursery for beneficial wasps.
- Late-Summer Monitoring: In the Southwest, these pests typically appear in July and August. Checking the undersides of leaves for “scale-like” white egg masses during mid-summer can prevent an outbreak before it starts.
They are easily identified by their reddish hump and striped body. These caterpillars feed in groups and can strip leaves rapidly.
Damage includes defoliation, reduced tree vigor, and decreased fruit production.
Management includes pruning infested branches, encouraging natural predators, and applying biological controls such as Bt.