Extension Educator, Horticulture
What has no discernable head or legs, lives under a waxy helmet and sucks plant juices? Seems like there should be a good punch line there somewhere. The answer is the insect scale.
Scale feeding can weaken and even kill plants. The tricky part about scale is they often go unnoticed until they reach high populations. In addition there are several different scale species with different life cycles.
Scale are not going to win any insect races. After all you can't expect much speed when the most mobile stage of a scale's life is called a crawler.
The scale eggs hatch into crawlers that move along stems or leaves. Once the crawlers locate a place to settle, they use their piercing mouthparts to suck out plant juices. Feeding causes leaf yellowing, plant stunting, and possible death of stems or the entire plant.
Adult scale don't move for the most part and therefore just look like part of the plant. To find out if your plant really has scale, take out your trusty horticulture tool, your thumbnail, and scratch the suspect bump on the plant stem or leaf. If it pops off then it is probably scale. If it doesn't easily pop off, then it's a lenticel or some other normal part of the plant.
A common scale is Euonymus scale. It is usually found on, you guessed it, euonymus. Evergreen wintercreeper types of euonymus are most commonly attacked instead of burning bush or winged euonymus. Variegated cultivars are more susceptible than green leafed cultivars. Euonymus scale is also found on pachysandra groundcover.
In euonymus scale the crawlers resemble tiny yellow spots that move around on leaves or stems. Euonymus scale overwinters as a mated female on plant stems. Eggs develop beneath the scale and then hatch over a two- to three-week period in early to mid June. Second generation occurs now in late July to early August. Crawlers may also blow in the wind to infest nearby plants. Leaves become spotted with yellow or white areas.
Scale control requires a certain amount of vigilance. As with many insect problems, stressed plants are more susceptible to attack than healthy plants. Therefore keeping plants healthy with proper watering and fertilizing will help. Prune out branches heavily infested with scale to quickly reduce the population. Many of the scales are susceptible to a variety of parasitoids and predators, including lady beetles, green lacewing, and predatory mites. However, beneficials generally can't control a large infestation.
Accurate identification of which scale is present is crucial in control using insecticides. The young crawlers are most susceptible to insecticide applications. By knowing the species, then a prediction can be made as to when the young crawlers will be present or when scouting for crawlers should occur. Inappropriate insecticide sprays may kill beneficials helping to control the scale or other insects. This can create population explosions of other insect pests.
One method of scouting for scale crawlers is to wrap a piece of black electrician's tape around a branch with the sticky side out. Crawlers will get stuck on the tape as they try to crawl across it.
Although there are several pesticides labeled for scale control, insecticidal soaps and horticulture summer and dormant oils are effective in controlling scale plus they are safer for you and beneficial insects. Generally multiple sprays ten to twelve days apart are needed. Dormant season oil sprays are effective for most scale except oystershell scale and only partially effective for pine needle scale. Be sure to read and follow all label directions.
Check out the laminated cards Bad Guys II: Landscape Pests to help in identifying scale. $8 from your local U of I Extension office or order PH: 800-345-6087.