Japanese beetles, an invasive pest that has become increasingly noticeable in the Chicago area, typically emerge around late June, according to experts at The Morton Arboretum in Lisle. Adult beetles, identifiable by their metallic green bodies and coppery brown wing covers, feed on more than 300 species of ornamental plants, with a preference for roses, grapevines, and crabapple, cherry, and linden trees. While their feeding causes extensive leaf damage by consuming the tissue between veins, it rarely results in plant death.
The pest poses additional challenges during its larval stage, when white grubs live underground, primarily in turfgrass. These grubs feed on grass roots, potentially causing significant damage to lawns by allowing patches of grass to be peeled away like a carpet. Both eggs and grubs thrive in moist soil conditions, making irrigation practices a key factor in managing their populations.
Sharon Yiesla, a plant knowledge specialist at The Morton Arboretum’s Plant Clinic, advises homeowners to avoid overwatering lawns as a preventative measure. “Water only when the soil is actually dry,” she said, emphasizing that this approach can deter grubs and conserve water simultaneously. Manual inspection of soil moisture before activating automatic watering systems is recommended, particularly during July and August when female beetles lay eggs in sunny, moist lawns.
Traditional Japanese beetle traps are discouraged, as they attract more beetles from the surrounding area than they capture, potentially increasing local infestations. Instead, handpicking beetles early in the season is advised to reduce their numbers before they release pheromones that attract more beetles. Collected beetles can be disposed of by shaking them into containers filled with soapy water.
Long-term control of Japanese beetles requires disrupting their life cycle by targeting grubs in the soil. While insecticides can be effective, caution is necessary due to potential harm to beneficial soil organisms and legal restrictions, particularly regarding the use of neonicotinoid pesticides on certain trees such as lindens. The Plant Clinic recommends consulting experts before applying chemical treatments.
Biological controls, including beneficial nematodes and bacterial agents like Bacillus thuringiensis galleriae (Btg), offer alternative options for grub management. However, these methods require precise application timing and may take several years to become fully effective. Some biological treatments, such as milky spore bacteria, are not suited for the Chicago climate as they cannot survive harsh winters.
Planting additional trees and shrubs to increase shaded areas is another suggested strategy, as female Japanese beetles prefer laying eggs in sunny, moist soil and are less likely to do so in shaded locations.
For further guidance on managing Japanese beetles and other garden pests, The Morton Arboretum’s Plant Clinic is available to provide expert advice through multiple channels, including phone and email.