Has anyone seen research on the length of time that onyx will remain effective for the prevention of elm bark beetle damage. I have a client with six, 80 plus year old American Elms and I would like to limit the amount of pesticide that we apply. We are able to apply arbortech and we currently use merit on an annual basis along with protective sprays of onyx. I have read research stating that onyx can be applied in November to combat the first generation of bark beetle emerging in spring, but what about the second generation that can spread DED by chewing on the twigs? Should onyx be applied twice per year? All the research I have read is inconclusive.