I have cherry laurels as a privacy screen (approx. 8.5 ft. tall). 2 have heavy infestation of White Prunicola Scale. My research indicates treatment with a soil drench of imidacloprid. My confusion comes from the directions on the 1 gal. container. It says use the height of the shrub (in rounded feet - up or down) times 3 to equal the amount of product to use (in oz). I have 7 of the cherry laurels in a row. Looking at just 1 @ 8 ft high (with heavy infestation) times 3 = 24 oz. (In a gallon of water). Here's the problem - in that same instruction for shrubs it states "DO NOT apply more than 9.4 oz a.i. imidacloprid per 1000 sq. ft " Given this restriction I would be 14.6 oz over the limit for just the one shrub. What is the correct amount of product/gallon to use? How do I address the other 6 cherry laurels in the same row (2 heavily infested and 1 partially)? I also have horticultural oil to soak the branches.
Harford County Maryland
Imidicloprid has been implicated as part of the problem causing colony collapse disorder, which is killing bees. It is no longer sold to homeowners. Supplies on hand at home should not be used on flowering plants which will be attracting pollinators. (Better to use it up on wind-pollinated plants, like hemlocks, if you need to use it.)
That said, there are other ingredients which will kill white prunicola scale when they are in their crawler stage (when they don't have a protective scale cover.) There are crawler in May, mid-July to mid-August and in September. Plenty of time to spray. You can use insecticidal oil (at the summer rate), insecticidal soap, neem oil, pyrethrins and more. You can also spray in June.
With oils and soap, read the label for air temperature limitations!
Besides pruning off heavily infested branches, you can also scrub the bark with a stiff (not wire) brush before you spray. This knocks the tops off the scales and exposes them to the spray, as well as the crawlers.
Here is our page on the scale. Read the management at the bottom carefully: https://extension.umd.edu/hgic/topics/armored-hard-scale-trees-and-shrubs#manage
And water these stressed plants during dry spells--be mindful that includes fall. Fall droughts kill many evergreens.