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About Applegater. (Jacksonville, OR) 2008-current | View Entire Issue (Sept. 1, 2011)
Applegater Fall 2011 9 Notes from a Rogue entomologist: A report from inside an insect outbreak BY riChard J. hiLton If you have noticed large webs in with insecticides. This year I have madrones, black walnuts and other observed the webworm in a number of trees around the area, then you have orchard and ornamental trees such as observed the fall webworm (Hyphantria apples, pears, peaches, plums, walnuts, cunea) in action. Unprecedented filberts, almonds, birch, sweet gum, populations of the fall webworm have ash, katsura, and even rosebushes. The been observed in southern Oregon this worst infestations seem to be in black year. As an adult, walnuts—there are the insect is a white a number of black he fact that the webworm was moth, and the walnut trees that showing up in grapes, which caterpillars, which are considered to be a non-preferred have been almost f e e d t o g e t h e r host, was a clear indication that this entirely defoliated within the webs year was going to be different. and encased in that they create, webbing. are very hairy and are in the same group At our latitude, it seems that the as the woolly worms, which are also fall webworm has just one generation known as woolly bears. The scientific per year; farther to the south with name for this family of moths, Arctiidae, a longer growing season, multiple is derived from the Greek word for bear, generations can occur. The life cycle for arktos. As an aside, the word “arctic” this insect consists of the adult moth, has the same derivation and refers to which emerges in early to midsummer. the northern constellations, the great The moth then lays a very large mass and little bears, also known as the Big of eggs, and when the eggs hatch, and Little Dippers. the caterpillars create a web wherein The fall webworm is native to they feed and skeletonize leaves. As North America and occurs across the the caterpillars molt and grow, they continent. Here in southern Oregon continue to feed on more foliage and it is most commonly found in our enlarge their web until an entire shoot madrones up in the hills, but this insect or branch will be enveloped. Finally, can live on a wide array of deciduous the fully grown caterpillars will disperse or broadleaf trees. In searching the from the web often for a considerable literature, the number of hosts that distance to find a crack or crevice in are attacked is quite large and the the bark or soil where they pupate over references range from “over 80” to winter, emerging the following summer “more than 200.” Clearly, this insect to repeat the cycle. is not a specialist feeding on just one In the 25 years that I have lived or a few plant species, but is rather an in southern Oregon, the first time I extreme generalist. The fall webworm can positively remember seeing the fall is considered to be an occasional webworm in an orchard was last year. In orchard pest, but it is easily controlled 2010, the fall webworm was observed T Rick Hilton examines webworm outbreak on a filbert tree. in a number of orchards, both organic and conventional, as well as in the OSU research orchard on Hanley Road. If you were observant you may have also seen an A fall webworm, native to North America, is found across the continent. occasional web in some landscape trees around wasps) and diseases. The unusual spring the Rogue Valley. At the time, the weather may have disrupted one or prevalence of the fall webworm around more of these biological control agents the valley was considered to be an and allowed the fall webworm to build unusual population spike, curious and up to our current unprecedented levels. notable, but nothing to get alarmed While this explanation may sound about. Then this summer, the white convincing (or not), without more adult moths appeared in June and July detailed study it is simply an educated in rather large numbers, followed by guess. Unfortunately, population the egg masses showing up. I had three ecology is a lot like economics in that it is much easier grape growers learly, this insect is not a to come up with bring in moths or specialist feeding on just one an explanation as egg masses during to why something a one-week period. or a few plant species… happened than to Considering how much time is spent training the shoots accurately predict what will happen. and manipulating the foliage, it is not That being said, can we predict whether too surprising that vineyardists would the fall webworm population next be among the first to find these egg year will continue to increase or will masses. However, the fact that the it collapse? The short answer is no. To webworm was showing up in grapes, quote Yoda: “Difficult to see, the future which are considered to be a non- is always in motion.” However, we will preferred host, was a clear indication monitor the emergence of the adult that this year was going to be different. moths next summer to give an early John Yungen, agronomist emeritus indication of overwintering survival at the Research Center, moved here and then, probably depending on the in 1955 and cannot recall ever seeing weather pattern, we will see how the fall webworm in webworm turns. the valley before Richard J. Hilton the past couple of 541-772-5165 ext. 227 years. So how can Senior Research Assistant / we account for the Entolomologist huge population Oregon State University increase of fall Southern Oregon Research and webworm over the Extension Center last two years? Well, Photos by Lynn Ketchum our best hypothesis is that the unusual cool and wet spring Advertisers ~ Call: conditions that we had in both 2010 and 2011 had some Jackson County effect that resulted Sally Buttshaw in much higher than normal survival of 541-646-8418 fall webworm. The population size of Josephine County plant-feeding insects is often regulated by Jill Wright mortality caused by 541-324-0950 predators, parasites Webworm nest as seen throughout the Rogue Valley. (especially parasitic C
