Mature red oak dropping leaves - Ask Extension
Hello,
Our 28 year-old redi oak has been dropping stems of whole leaf clusters, just after it is fully leafed out. We've been having it treated for t...
Knowledgebase
Mature red oak dropping leaves #875491
Asked July 01, 2024, 12:43 PM EDT
Hello,
Our 28 year-old redi oak has been dropping stems of whole leaf clusters, just after it is fully leafed out. We've been having it treated for the last 2 years with some kind of injection, only to have the leaf dropping get worse. The rep from the tree service says it has some kind of bug, which must be invisible to the naked eye. This is a large tree that has always had a thick canopy. We are at a loss for what to do. Can you tell us what we should be looking for, and specifically what treatment we should be asking about? Thanks very much.
Denver County Colorado
Expert Response
The twig dieback on your red oak is caused by a scale insect, the kermes oak scale. Nymphs overwinter on branches clustered around the buds and following new growth in the spring, the insects feed on sap and cause the twigs to dieback at the feeding site. Associated with the scale is a bacterium and the two together produce a condition known as drippy blight.
There are many different type of scale insects and they can be difficult to control. Controls include spraying them with a dormant oil to smother them or using a systemic insecticide that is transported through the tree's plumbing so that when the insect pest feeds, it ingests the poison and dies (unfortunately, systemics kill every insect feeding on the tree - good and bad).
For Kermes scale specifically, after months of feeding, the insects mate and the females lay eggs. The eggs hatch in September/October and move to the overwintering sites on the branches. That "crawler" stage is the time they are most susceptible to sprays but because timing is difficult to predict, injections are commonly used for control.
I don't know why your infestation has worsened but like all insects, scale insects are susceptible to weather events and other conditions that affect population numbers. I'm going to link you to several authoritative articles about Kermes Oak scale and scale generally and perhaps after looking through them, you can have another discussion with your arborist about why the condition hasn't improved:
https://extension.entm.purdue.edu/publications/E-29/E-29.html
https://planttalk.colostate.edu/topics/insects-diseases/1400-11-kermes-scale-oak-trees/
https://edis.ifas.ufl.edu/publication/IN615
There are many different type of scale insects and they can be difficult to control. Controls include spraying them with a dormant oil to smother them or using a systemic insecticide that is transported through the tree's plumbing so that when the insect pest feeds, it ingests the poison and dies (unfortunately, systemics kill every insect feeding on the tree - good and bad).
For Kermes scale specifically, after months of feeding, the insects mate and the females lay eggs. The eggs hatch in September/October and move to the overwintering sites on the branches. That "crawler" stage is the time they are most susceptible to sprays but because timing is difficult to predict, injections are commonly used for control.
I don't know why your infestation has worsened but like all insects, scale insects are susceptible to weather events and other conditions that affect population numbers. I'm going to link you to several authoritative articles about Kermes Oak scale and scale generally and perhaps after looking through them, you can have another discussion with your arborist about why the condition hasn't improved:
https://extension.entm.purdue.edu/publications/E-29/E-29.html
https://planttalk.colostate.edu/topics/insects-diseases/1400-11-kermes-scale-oak-trees/
https://edis.ifas.ufl.edu/publication/IN615
