In August and September, I observed bronzing due to mites feeding on some young swamp white oaks. The tops of the leaves were very bronzed along the main veins, while the undersides of the leaves remained unaffected. When looking at the leaves with my hand lens and under the microscope, I saw a very heavy infestation of mites. Mites suck plant juices from the cells of the leaf.
Mites have overlapping generations – eggs, nymphs and adult forms may all be present at one time. Heavy infestations can be sprayed with a dormant oil, insecticidal soap, or a miticide (pesticide specific to mites) but many pesticides are effective only on nymphs or adults so a second application may be necessary seven to ten days after the first. Be sure to read the label to determine if a second application is necessary.
Written by: Linda Williams, forest health specialist, Woodruff, (Linda.Williams@wisconsin.gov), 715-356-5211 x232.