Last year gardeners were caught off guard with outbreaks of scale insects on their trees and shrubs. University of Illinois Master Gardeners received many calls of Magnolia foliage turning black and sticky residue on lawn furniture, yard ornaments and if you stood there for even a minute, all over you.
According to our Extension State Entomologist, Dr. Phil Nixon, scale populations will greatly increase every few years to very high populations and then crash as their natural predators catch up or an environmental stressor impacts the scales presence in the landscape.
Scale insects feed by inserting their siphoning mouthparts into leaf tissue in the younger nymph (immature) stages and into twig and bark tissue as older nymphs and adults. As a result of the intake of the plants sap, there is a constant flow of exudate leaving their bodies and dripping downwards onto the surface of leaves below and onto anything else.
Already this year, Extension is hearing from homeowners finding that sticky residue from maples impacted by lecanium scale females heavily feeding before laying eggs. These eggs will hatch into dozens of nymphs that will migrate to leaf tissue for the summer before returning to twigs and branches permanently for the winter. I suspect that future calls will be about Magnolia scale which appears later in the summer.
Scale insects can be managed with contact materials in the crawler stage or the use of a systemic insecticide if the tree is too large to treat otherwise. Good coverage is critical for good control, making sure the undersides and twigs and branches are covered. The key to treating scale insects is to manage them at their most vulnerable stages while the nymphs are crawling or feeding.
Systemic insecticides work best while the plant is actively growing and can absorb and move the product up into the canopy. Spring applications are suggested over summer applications. A third option, but one that has passed for 2016 is the use of a dormant oil or plant oil while the plants are still dormant in Early Spring. If you are battling scale insects on valuable ornamentals, a dormant spray in 2017 could be helpful to catch any scales missed this summer.
There are many scale insects in addition to Magnolia Scale and Lecanium Scale (on a variety of plants). Oystershell, Euonymus, Lilac and Cottony Maple scales are also frequently found. Even our evergreens can have scale, like Spruce Bud Scale and Pine Needle Scale. Cottony Maple scale is controlled naturally by the Twice Stabbed Lady Beetle in most years. Proper Identification will guide you to the proper product and time to treat. Not all of the products out there can be used on any plant, so please read the labels carefully before you buy them.
About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.