Fall webworm is most often discovered when the unsightly, light gray, silken webs on the trees in late summer and early fall are observed. Although not real common, webs can cover the entire tree and consume all leaves in the process. To make matters worse, while these caterpillars defoliate trees they leave behind a massive amounts of black fecal pellets. Fortunately, most trees can survive a single defoliation. Controls are readily available for home and woodlot owners. Effective use of these controls requires a good understanding of the biology of this pest.
This pest is native to North America and is common from Canada into Mexico. It is one of the few American insect pests that has been introduced into Europe and Asia.
It was accidentally introduced from North America to Europe in 1940s. Since then it has spread to the west and east in Europe and now it occupies its entire range in Europe from France to the Kaspian Sea in the east.
Fall webworm larvae have been known to feed on over 85 species of trees in the United States. Pecan, walnut, American elm, hickory, fruit trees, persimmon, birch, sweetgum and some maples are the preferred trees.
This pest usually eats leaves late in the season and the nests are generally concentrated to limited areas.
Though the webs are very unsightly, damage to most trees is considered to be insignificant.
Healthy deciduous trees can easily survive losing all their leaves during the month of August.
From the tree's perspective, the leaves were busily providing energy for the three from May through July. By August the leaves have already earned their keep. Losing them in August rather than October will have a minimal impact on tree health. Repeated defoliation by fall webworm (loss of over half the trees leaves) over the course of several years can weaken trees. Trees that suffer other stresses caused by drought, or recent planting or construction, are less likely to survive.
Fall webworms have two generations each year. Wintering in cocoons in sheltered areas, the adult is a white moth that glues large clusters eggs on to the undersides of leaves in June.
These eggs are almost always laid on leaves near the tips of branches. Eggs hatch into caterpillars that feed in groups. They protect themselves from predators by keeping themselves covered with webs. In mid-July, these caterpillars make cocoons in the webs. In late July, a new generation of moths come from these cocoons and lay even more eggs on the trees. By the time the end of August rolls around trees are just covered with webs. Feeding will continue and webs will continue to grow until mid- September.
The trick to managing this pest is to target the first generation of caterpillars. Look out for early signs of webbing and kill the caterpillars before they engulf the tree in the second generation. Small webs can simply be pruned off and destroyed. When webbing covers too much of the tree, pruning away the affected area may simply not be practical- especially if you ruin plant structure or remove most of the tree. In this case you should apply an insecticide.
Homeowners can get very effective control with a new biorational material called spinosad. It is marketed under a number of labels including Fertilome BorerCaterpillar Leaf miner and Tent Caterpillar Killer, and Bulls-Eye Bioinsecticide. This material provides the added benefit of not killing many of the beneficial insects. A second biorational, the microbial insecticide Bacillus thuringiensis (e.g. Dipel, or Thuricide) is effective on younger caterpillars. Traditional broad spectrum insecticides that can provide good control include cyfluthrin (Bayer Advanced Product), permethrin (Eight), carbaryl (Sevin), and acephate (Orthene).
You can contact the local Purdue Extension Office by calling 829-5020 ext. 14 in Owen County or 448-9041 in Clay County for more information or publication copies regarding this week's column topic or to RSVP for upcoming events. It is always best to call first to assure items are ready when you arrive and to RSVP for programs. While most publications are free, some do have a fee. All times listed are Eastern Time.
Aug. 8-19 Indiana State Fair
Aug. 16 Owen Co. Ag Economic Development Initiative, 8 a.m.
Aug. 18 Indiana Aquaculture Assoc. Program, Floyd County
Aug. 21 SWCD Conservation Expo, Clay County
Aug. 20 I-69 Planning Meeting, Eastern Greene HS, 7 p.m.
Aug. 21 I-69 Planning Meeting, Morgan Co. Admin Bldg, 1 p.m.
Aug. 21 I-69 Planning Meeting, Bloomington North HS, 6 p.m.
Aug. 23 Poison Prevention Workshop, Methodist Hospital, 12:30 p.m.
Aug. 24 Alternative Sewer System Workshop, Vermillion Co.; 10 a.m.
Aug. 28 Clay Co. Ag Economic Development Initiative, 10 a.m.
Sept. 5 Farm Bureau Drainage School, Indy, 9 a.m.
Sept. 5 Purdue Forage Management Workshop, West Lafayette
Sept. 13 Ag Outlook Breakfast, Brazil, 8 a.m.
September 18 Riverwatch Training, Bloomington
September 19 Poison Prevention Workshop, Methodist Hospital, 12:30 p.m.
September 27-29 State Master Gardener Conference, Evansville