Extension Educator, Horticulture
"Timing is everything." You've heard this from your mom, your doctor, your golf pro, your spouse. A bouquet of flowers the day after a birthday just doesn't have the same sentiment as the day before. A pre-birthday bouquet says: "I'm thinking of you. Please share your life with me". A post-birthday bouquet says: "I'm an idiot. Please don't hate me".
Timing is also everything with insect pest control. Insecticides as a part of a pest management program are best applied when the insect is most vulnerable. Applied at the wrong time the same insecticide may provide little, if any, control. For example bagworms are a common pest on spruce, juniper and arborvitae. Bagworm insecticide sprays are best applied when the caterpillars are small and feeding usually in late June. Once the caterpillars are snug in their bags insecticides are ineffective.
Timing is everything with controlling scale insects. The tricky part about scale is they often go unnoticed until they reach high populations. Scale insects have no discernable head or legs, live under a waxy covering and suck plant juices. Despite their resemblance to helmets, their feeding can weaken and even kill trees and shrubs.
Adult scales are difficult to control. They are most susceptible to insecticides in the young crawler stage. Scale start out as eggs. The eggs hatch into crawlers that move (albeit slowly) along stems or leaves. You can't expect much speed when the most mobile life stage is called a "crawler".
Once the crawlers settle on the plant, they form a waxy coating then use their piercing mouthparts to suck out plant juices. Feeding causes leaf yellowing, plant stunting, and possible death of branches or entire plant.
Adult scales look like odd bumps on plant stems or leaves. To determine if it's really scale, take out your trusty horticultural tool, your thumbnail, and scratch the suspect bump. If it pops off, it may be scale. If it doesn't easily pop off, it may be a normal part of the plant.
Several different scale species are possible. Oystershell scale attacks many plants including ash, redbud, birch, dogwood, elm, hemlock, lilac, maple, poplar, privet, walnut, and willow. It looks like; you guessed it, tiny grey or brown oystershells.
Euonymus scale is common on variegated euonymus and wintercreeper. Pine needle scale makes Scotch and mugho pine look like white flocked Christmas trees.
Accurate identification of which scale is present is crucial to timing insecticide applications to when young crawlers are present.
To scout for scale crawlers wrap a piece of black electrician's tape around a branch with the sticky side out during the predicted crawler stage. Crawlers will get stuck on the tape. Pine needle scale crawlers are dark red, euonymous scale crawlers are yellow, and oystershell crawlers are whitish. All of these colors can easily be seen on black tape.
The Vanhoutte spirea (Spiraea x vanhouttei) shrub can be a good indicator as to when to scout for oystershell, pine needle or euonymus scale crawlers. Drive down any older neighborhood now and you will see the arching branches of these large spireas covered in white flowers. For example for pine needle scale spray in late May as Vanhoutte spirea blooms and spray second generation pine needle scale in early August as Queen Anne's lace is blooming.
Several pesticides are labeled for scale control such as acephate (Orthene), insecticidal soap, and horticultural summer oil. Generally multiple sprays ten to twelve days apart are needed. Dormant season oil sprays can also be effective except for oystershell scale. Be sure to read and follow all pesticide label directions.
Check with your local UI Extension for pest identification and management. Keep plants healthy with proper watering and fertilizing. Prune out branches heavily infested with scale to quickly reduce populations.