The Japanese beetle, Popillia japonica, is a highly destructive and invasive garden pest that causes significant damage to ornamental plants across the United States. Hydrangeas are among the hundreds of plant species that adult Japanese beetles consume during their summer feeding period. Understanding this attraction and the beetle’s life cycle is the first step toward protecting your landscape plants from severe defoliation.
Why Japanese Beetles Target Hydrangeas
Japanese beetles are generalist feeders, but they prefer plants that offer specific textural and chemical cues. Hydrangeas, particularly varieties like smooth hydrangea (H. arborescens) and oakleaf hydrangea (H. quercifolia), are highly susceptible targets. Beetles are drawn to plants with tender, soft, and thin-leaved foliage, characteristic of many hydrangea cultivars.
They are also attracted to certain floral and fruity scents. When a beetle begins feeding, it releases an aggregation pheromone, a chemical signal that calls other beetles to join the feast. This behavior explains why a small initial infestation can rapidly escalate into a severe problem.
Identifying Signs of Infestation
The most distinctive visual evidence of Japanese beetle feeding is a pattern known as skeletonization. This damage occurs because the adult beetles consume the soft leaf tissue (parenchyma) found between the tougher leaf veins. The result is a lace-like, transparent leaf structure that resembles a skeleton, which limits the plant’s ability to photosynthesize.
Infestation is also confirmed by the direct presence of the adult beetles, recognizable by their metallic green bodies and coppery-brown wing covers. These pests often congregate in large groups on the upper surfaces of leaves and flowers during the warmest parts of the day. Damage to the flower petals, which turn brown and tattered, can also be an early sign of their arrival.
Immediate Removal Techniques
For an active, localized infestation, physical removal of the adult beetles is the most immediate non-chemical treatment method. The best time for this mechanical control is early morning, as cooler temperatures make the beetles sluggish and less likely to fly away. A simple method involves gently shaking the beetles from the hydrangea leaves directly into a container of soapy water.
Water mixed with dish soap will quickly drown the captured insects. Insecticidal soaps or horticultural oils can also be applied as a contact spray to kill adult beetles present at the time of application. Products containing neem oil can deter feeding. However, all contact sprays require thorough coverage and reapplication, especially after rainfall. These actions help reduce the number of adults feeding and mating, limiting the eggs laid for the next generation.
Long-Term Population Management
Effective, long-term management requires targeting the larval stage, or grubs, which live and feed underground for about ten months of the year. Adult beetles emerge from the soil in late June or early July, meaning control measures must be applied before or after this period. Applying grub control products in late summer (typically August and September) targets the newly hatched, smaller grubs, which are most susceptible to treatment. Biological controls offer a sustainable strategy for reducing the grub population over time.
Milky spore disease, caused by the bacterium Bacillus popilliae, is a slow-acting but persistent biological agent that infects and kills the grubs. Beneficial nematodes, specifically Heterorhabditis bacteriophora, are available for application to the soil, though their effectiveness can be inconsistent.
While commercial pheromone traps exist, they are often controversial because they can attract more beetles into the immediate area than they actually capture, potentially increasing damage to nearby hydrangeas.