I have euonymus scale on my large euonymus sarcoxi. It is supported by a lattice and the lower parts including all of the largest branches are covered with scale. The upper leaves don't seem affected. Leaves on lower branches are covered with the insects as well. Do I need to remove the whole bank of 5 large plants or can I spray with horticultural oil? If I can spray, when can the spray be most effective. Any time ? late July and August? I know it needs to be done in the evening when the temperatures are a little cooler. I would like to avoid the scale from infecting other euonymus in the yard. Thank you for any help you can give me.
Larimer County Colorado
The University of Minnesota has a great Fact Sheet on euonymous scale: http://www.entomology.umn.edu/cues/Web/124EuonymusScale.pdf
It is best to control this insect when the crawlers (young larvae) are hatching--generally early June (but our season is off) and possibly a second generation will hatch in late July/August. If the scale has a hard outer shell, target spraying will do very little. Horticultural oils are effective on crawlers, but you would need to be attentive as to when these are active. You'll need a hand lens or a magnifying glass, as the crawlers are very small. But they will look like tiny little insects moving around the branches. Another option would be to take a piece of tape and gently tap the branch and then look at the sticky part--you'll see if you've napped any crawlers.
You are correct in that horticultural oils can be great controls...use summer oils (will be labeled as such) and as you mentioned, apply during a cool part of the day. If the crawlers are active now, I would avoid spraying the oils until the weather has cooled off--next week looks much better. Though the oils are effective at controlling pests, they can be phytotoxic on plants and cause burning of leaf tissue.
Here is additional information from the U of Wisconsin: http://www.entomology.wisc.edu/diaglab/pdfs/landscape/Euonymus%20Scale.pdf
Thank you very much for your help.