In the last two weeks or so orangestripped oakworms (Anisota senatoria) hatched and have been feeding on urban oak trees. Last week I found caterpillars that had just hatched and some that were 3 or 4 cm long on trees at the same site. Orange striped oakworms have one generation per year. Adults emerge in mid summer and lay clusters of yellow eggs on the bottom of oak leaves. In Raleigh eggs usually hatch around August first. Young caterpillars are yellow and feed gregariously. They start skeletonizing leaves, leaving most fine veins intact. As they grow, larvae become black with increasingly noticeable orange stripes. Large larvae consume entire leaves, leaving only the midvein.
Orangestripped oakworms can partially or entirely defoliate trees but often only one or two branches are affected. Even though they are actively feeding now orangestripped oakworms become most obvious later in August when they start crawling down trees and along the ground looking for pupation sites in soil and leaf litter. Also, later in the summer damage becomes more noticeable and people begin to notice frass pellets on their decks and walkways. By that point though they are usually about done feeding for the year so treatment is not warranted.
Unlike most pests of ornamental plants, orangestripped oakworms have been fairly well studied. This is thanks to Mark Coffelt and Pete Schultz who studied this critter at Virginia Tech in the 1990’s. They studied the life cycle, damage, parasitoids, and developed a sampling plan, aesthetic injury level, and threshold.
The authors (Coffelt and Schultz 1990) used a survey with photographs of trees with 15, 25, 50, 75, or 100% defoliation. Most respondents (70%) were willing to accept some defoliation and 42% responded that 25% defoliation was aesthetically acceptable. Moreover, they found that 25% defoliation did not reduce root starch reserves which is a measure of tree vigor.
To determine which trees may exceed the aesthetic threshold, Coffelt and Schultz (1993) developed a threshold based on egg masses per tree. They found 5m trees with 12 cm dbh will reach 25% defoliation if with one egg mass and 100% defoliation with 4 egg masses. Larger trees with dbh of 19 cm, 26 cm or 35 cm would require around 5, 7, and 9 egg masses respectively to reach the aesthetic injury level of 25% defoliation.
Orangestripped oakworm eggs are typically laid on terminal ends of lower branches. Therefore, by looking up at the undersides of lower leaves you can get a decent count of the masses of yellow eggs. Soon after egg hatch you can estimate the number of egg masses that hatched by the number of damaged branches. Since young caterpillars feed in groups, one branch with one group of damaged leaves probably resulted from a single egg mass. Thus, based on the threshold, if you went out now and saw a 12 cm dbh tree with 4 separate areas of defoliation (4 egg masses hatched) you might expect near 100% defoliation later in the summer and consider some form of management now if the tree is a key component of the landscape or has experienced other stresses that could weaken its recovery.
Management of these caterpillars, and other gregarious species, can be as easy as pruning. Since the larvae all feed together you can prune out infested branches and nearly stop further defoliation. On large trees with large populations pruning may be impossible or unfeasible. In this case, registered insecticides may be used to reduce damage and frass. Again though, in late summer when larvae are full grown the damage is done so insecticide applications are probably of little value.
Orangestripped oakworms feed on most oak species and can also occasionally damage maple, birch, and hickory trees. Even though they look well-armored with black spines and frontal horns they do not sting and are safe to handle. We are lucky to have so much information on this pest to help with management decisions. Understanding the biology and aesthetic injury thresholds of other key pests would really improve IPM of urban landscape plants.
Coffelt, M.A. and P.B. Schultz. 1990. Development of an aesthetic injury level to decrease pesticide use against orangestriped oakworm (Lepidoptera: Saturniidae) in an urban pest management project. Journal of Economic Entomology 86: 1512-1515.
Coffelt, M.A. and P.B. Schultz. 1993. Quantification of an aesthetic injury level and threshold for an urban pest management program against orangestriped oakworm (Lepidoptera: Saturniidae). Journal of Economic Entomology 86: 1512-1515.
Coffelt, M.A., P.B. Schultz. and D.D. Wolf. 1993. Impact of late-season orangestriped oakworm (Lepidoptera: Saturniidae) defoliation on oak growth and vigor. Environmental Entomology 22: 1318-1324.