Scarab beetles (family Scarabaeidae), including species like the Japanese beetle and European chafer, are known for their destructive feeding habits in both adult and larval forms. Managing an infestation requires a targeted approach that addresses both the above-ground adults and the subterranean grubs. This guide provides practical steps to manage and remove these persistent garden pests.
Understanding the Scarab Life Cycle and Damage
Effective scarab beetle management requires understanding their life cycle, which dictates the optimal timing for intervention. Scarabs undergo complete metamorphosis, cycling through four stages: egg, larva (grub), pupa, and adult. Treatments must align with when the pests are most vulnerable.
Adult beetles emerge from the soil in late spring or early summer, mating and feeding on ornamental plants and turf grass. Females lay eggs in the soil, often preferring moist turf. Grubs hatch in early to mid-summer. The C-shaped larvae, known as white grubs, are the most damaging stage, feeding voraciously on plant roots just beneath the soil line.
Damage varies by life stage. Adult scarabs cause visible damage to foliage, flowers, and fruit, often resulting in a skeletonized appearance on leaves as they consume the tissue between the veins. Grub root-feeding activity leads to patches of grass that turn brown, wilt, and exhibit drought-like symptoms. In severe infestations, the turf loses its root structure and can be lifted and rolled back like carpet, revealing the actively feeding grubs beneath. This intense feeding lasts from late summer through the fall until the grubs burrow deeper to overwinter.
Non-Chemical and Preventative Control Methods
Before using direct elimination methods, cultural and mechanical controls can significantly reduce scarab populations. Modifying lawn care practices makes the environment less attractive for egg-laying adults. Since scarab eggs require moisture to hatch, reducing watering during the peak egg-laying period in mid-summer decreases the subsequent grub population.
Mowing grass at a higher setting (three inches or more) encourages deeper, more extensive root systems. This deeper growth allows turf to tolerate grub feeding without showing significant damage. For vulnerable garden plants, physical barriers like lightweight floating row covers prevent adult beetles from accessing foliage and laying eggs. These covers allow light and water to pass through but physically block the insects.
Hand-picking remains a highly effective mechanical control for localized adult beetle infestations. Adult scarabs often drop off the plant when disturbed. Gardeners can exploit this by placing a container of soapy water beneath the plant and gently shaking the foliage, causing the beetles to fall and drown. Pheromone traps are available, but they are generally more effective as monitoring tools than control methods, as the attractant can draw more beetles to the area than the trap catches, potentially increasing localized plant damage.
Targeted Insecticide and Biological Treatments
When scarab populations reach damaging levels, targeted treatments are necessary, and timing is paramount. Chemical control is categorized as preventative or curative, based on when the product is applied relative to the grub life cycle. Preventative insecticides (e.g., chlorantraniliprole) are applied earlier (mid-June to early August) to kill small, newly hatched grubs. These products offer a long window of protection with low toxicity.
Curative treatments, such as those containing trichlorfon or carbaryl, are used in late summer and fall when damage is visible and grubs are larger. These products quickly reduce the active grub population but have a shorter residual effect, requiring precise timing. For a biological approach, beneficial nematodes (Heterorhabditis bacteriophora) are applied to the soil to parasitize and kill the grubs. Nematodes must be applied to moist soil and protected from direct sunlight to remain viable.
Another biological option is milky spore disease, a naturally occurring bacterium (Paenibacillus popilliae) specific to Japanese beetle grubs. While milky spore provides long-term control by building up in the soil, its effectiveness is slow to establish and limited to specific species. A more broadly effective microbial product contains Bacillus thuringiensis galleriae (Btg), which acts as a stomach poison to a wider range of white grubs, offering both preventative and curative control.