Japanese beetles readily consume hydrangeas, often making these popular flowering shrubs a preferred target in the garden. This invasive species feeds on over 300 types of plants, and hydrangea foliage is high on their list of favorites. The adult beetles emerge during the summer months to feed aggressively, leaving behind a distinct pattern of damage. This article focuses on identifying this pest, understanding its active season, and implementing strategies for immediate removal and long-term prevention.
Confirming Japanese Beetle Damage on Hydrangeas
The presence of the adult Japanese beetle (Popillia japonica) is confirmed by the distinct damage pattern they inflict on leaves. These beetles engage in skeletonizing, consuming the soft tissue between the veins and leaving behind a lace-like, webbed framework. This damage significantly reduces the plant’s ability to photosynthesize, and they also consume flower petals, spoiling the plant’s aesthetic appeal.
The culprit is a small, oval-shaped insect, about a half-inch long, with an iridescent, metallic-green body and copper-bronze wing covers. A distinguishing feature is the presence of six small tufts of white hair along the sides and back of the abdomen, visible just below the wing margins. These beetles are most active during the day, often congregating in large groups, which intensifies the damage on a single plant.
Understanding the Japanese Beetle Active Season
Destructive feeding on hydrangeas is confined to the adult stage of the Japanese beetle’s life cycle, beginning in early summer. Adults usually emerge from the soil starting in late May to early June, with peak activity occurring from late June through July. This feeding period is relatively short, lasting approximately six to eight weeks, during which the females feed, mate, and lay eggs.
The adult beetles feed aggressively during this window to fuel their reproductive cycle, making this the period of most visible plant damage. Understanding this timing dictates when immediate control measures must be applied to be effective. After this period, the adults die off, but the life cycle continues underground.
The pest spends about ten months of the year in the soil as larvae (grubs), feeding on grass roots. In the autumn, the grubs burrow deeper to overwinter, reemerging in spring to feed before pupating and transforming into new adults. This predictable annual cycle provides two distinct opportunities for intervention: targeting the adults on the hydrangea and managing the grubs in the lawn.
Immediate Control and Removal Strategies
For an active infestation of adult beetles, the most direct approach is physical removal. Hand-picking the beetles is highly effective, especially when performed early in the morning when cooler temperatures make the insects less active. The collected beetles should be dropped into a container of soapy water, which quickly kills them.
Immediate chemical intervention can be considered for severe infestations or on large shrubs where hand-picking is impractical. Products like insecticidal soap or neem oil can be sprayed directly onto the foliage to deter feeding and reduce the population. Neem oil disrupts the beetle’s feeding and reproductive processes, while insecticidal soaps kill on contact.
For a quick knockdown, sprays containing pyrethrins (derived from chrysanthemums) are available, though they must be applied with caution. When using any insecticide, apply it late in the day to minimize harm to beneficial insects, like pollinators, who are less active in the evening. Only treat the affected foliage, ensuring the product label specifies it is safe for use on the particular hydrangea variety.
Long-Term Prevention and Garden Health
Long-term management requires addressing the pest at its source: the grub stage in the lawn. Applying biological controls to the turf is an effective preventative measure that reduces the number of beetles that emerge the following year. Beneficial nematodes, such as Heterorhabditis bacteriophora, are microscopic organisms that parasitize and kill the grubs in the soil.
Another biological option is milky spore, a naturally occurring bacterium (Paenibacillus popilliae) that causes disease in Japanese beetle grubs. While these products are highly specific to the grubs and safe for other garden life, they can take one to two seasons to establish and provide meaningful control. Grub control treatments are most effective when applied in late summer or early fall when the newly hatched larvae are actively feeding near the soil surface.
Gardeners can also reduce future damage by making informed plant selections, as Japanese beetles prefer certain hydrangea types. Bigleaf hydrangeas (Hydrangea macrophylla) are often favored targets, while Panicle hydrangeas (Hydrangea paniculata) may show more resistance. Maintaining overall plant vigor through proper watering, soil health, and appropriate fertilization allows shrubs to better tolerate and quickly recover from feeding damage.