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How to spot the orange-striped oakworm in your yard

The orange-striped oakworm is a caterpillar that feeds on the leaves of oak trees in late summer and early autumn.
The orange-striped oakworm is a caterpillar that feeds on the leaves of oak trees in late summer and early autumn. Photo provided

I was having lunch at the Waffle Shop on North Atherton Street and Candi, one of the hostesses, asked me about a worm that she noticed eating the leaves on her oak tree.

After hearing what the worm looked like, here is what I suspect is Candi’s unwanted visitor: the orange-striped oakworm.

The orange-striped oakworm is a caterpillar that feeds on the leaves of oak trees in late summer and early autumn. This insect is native to North America and occurs from Wisconsin and lower southern Michigan east to New England and southeastern Canada. It is also found in Georgia, Virginia and Kansas, but is more abundant in northern regions. When outbreaks occur, orange-striped oakworms can severely defoliate forest and shade trees. Older caterpillars produce large pellets of frass (fecal pellets) in great abundance, which can be messy when they fall on a sidewalk or on your car.

Caterpillars often wander about as they search for more leaves, and may congregate on sidewalks or the sides of houses. High populations of orange-striped oakworm can be distasteful and annoying to residents in suburban, rural and recreational areas. Accurate identification of orange-striped oakworm and understanding of its biology will help you learn to recognize and manage this insect in your yard.

Young caterpillars are greenish yellow with eight orange stripes that run the length of their bodies. As the caterpillars grow, they become black and the orange stripes become more evident. The caterpillars have two characteristic black hornlike projections just behind the head. Caterpillars rapidly increase in size and may grow to 3 inches in length by the time they finish feeding. Adult moths have bright orange-yellow bodies. The forewing has many dark spots and a single distinct white spot near the long edge.

The orange-striped oakworm has one generation per year in northern North America and two generations per year in southern states. Adult moths may be present from late June to early August. Individual moths can live for about a month. During that time, a female moth deposits up to 500 eggs, usually in a cluster on the underside of an oak leaf.

The eggs hatch in 7 to 10 days and the small caterpillars begin feeding in groups. The young caterpillars skeletonize the leaves, consuming the soft tissue and leaving the veins intact. As the caterpillars mature, they feed in small groups or alone and consume more of the leaf tissue. Large caterpillars leave only the main midrib of the leaf untouched. When they are fully-grown and complete their feeding, the caterpillars crawl to the ground and tunnel a few inches into the soil. Each caterpillar forms an earthen cell and pupates. Pupae remain in the soil until the following summer, when the adults emerge from the earthen cells and burrow up to the soil surface.

They are one the most common “fall defoliators,” feeding on the oak leaves in late summer and early autumn. Though oaks are the preferred hosts, they will occasionally feed on birch, hazelnut, hickory and maple trees. The good news about the orange-striped oakworm and other fall defoliators is that their feeding rarely affects tree health. By late summer when the orange-striped oakworm caterpillars begin feeding, the trees have completed most of their photosynthesis and are preparing for winter dormancy.

Generally, orange-striped oakworm defoliation poses a problem when trees were previously stressed by other environmental factors such as defoliation by gypsy moths or other insects, severe drought, disease or poor site conditions such as compacted soil along a street or sidewalk. Repeated stress can reduce the energy reserves in a tree and lower its ability to tolerate even late summer defoliation.

The good news on the management front is that the orange-striped oakworm has a well-established complex of natural enemies. These natural enemies include diseases and other insects that are predators or parasitoids of the orange-striped oakworm. You can check your leaves on young trees for eggs on the underside of the leaves and clipped them off and destroy them. Another option is to wait until the eggs hatch, then spray the foliage of the oak trees with B.t (Bacillus thuringiensis var. kurstaki). B.t affects only foliage-feeding caterpillars and will not harm beneficial insects such as predators and parasitoids.

I used the old mechanical method of crushing the small caterpillars and also large caterpillars between my fingers but I would certainly use B.t if I found a larger infestation of the orange-striped oakworm on my trees. It is important to maintain the health of your trees by avoiding injuring the trees with your lawn equipment, compacting the soil around the base and under the tree canopy. In dry years watering would also be important to maintaining the health of your tree. Maintaining the overall health of your trees will certainly help your trees tolerate and recover from even severe orange-striped oakworm defoliation.

Bill Lamont is a professor emeritus in the department of plant science at Penn State and can be reached by email at wlamont@psu.edu.