What are Japanese Beetles?
Japanese beetles (Popillia japonica) are among the most destructive garden pests in North America, devouring leaves, flowers, and fruit of over 300 plant species (Photo 1 and 2). But the damage starts even before you notice it below the surface, their white grubs feed on the roots of grasses (Photo 3) and plants, causing brown patches in lawns and weakened ornamental plants. While chemical treatments are available, many gardeners and landscape professionals are turning to a safer, more natural method of control: entomopathogenic nematodes.
Photo 1. Japanese beetles feeding on leaves and fruits
Photo 2. Japanese beetles feeding on flowers
Photo 3. Japanese beetle grubs feeding on grass roots
What Are Entomopathogenic Nematodes?
Entomopathogenic nematodes are microscopic, soil-dwelling roundworms (Photo 4) that seek out and kill insects. They’re harmless to humans, pets, and plants, but lethal to many soil-dwelling pests, including the larval stage of Japanese beetles.
These nematodes carry symbiotic bacteria (typically from the genus Xenorhabdus or Photorhabdus), which are introduced into the host insect once the nematode penetrates its body. The bacteria multiply, quickly killing the host, and providing a food source for the nematodes to reproduce before moving on to find new victims.
Photo 4. Entomopathogenic Heterorhabditis bacteriophora Nematodes
Best Nematode Species for Japanese Beetle Control
Not all nematodes are effective against Japanese beetles. The most commonly recommended species are:
Heterorhabditis bacteriophora (Hb): Especially effective against beetle grubs (Photo 4). This species actively hunts for Japanese beetle grubs and is well-suited for use in turf and garden soils.
Steinernema glaseri: Another promising candidate, though less widely available, with good results against root-feeding grubs.
Among these, H. bacteriophora is the most accessible and widely used in commercial and residential applications.
How to Apply Nematodes
Applying nematodes is simple, but timing and conditions are critical for success:
Photo 5. Watering can for application of nematodes
Timing: Apply nematodes with watering can (Photo 5) in late summer or early fall, when Japanese beetle grubs are young and near the soil surface. A spring application can help reduce second-year populations.
Soil Conditions: Nematodes need moist soil to survive and move. Water your lawn or garden before and after application to ensure good penetration.
Application Method: Mix nematodes with water and apply using a hose-end sprayer, watering can, or nematode-specific sprayer.
Avoid UV and Heat: Apply in the early morning or evening to protect nematodes from UV light and heat, both of which can kill them quickly.
Advantages of Using Nematodes
Eco-Friendly: Nematodes are a biological control agent with no harmful residues.
Selective Targeting: They primarily attack soil-dwelling insect larvae, minimizing harm to beneficial insects.
Safe for All: Non-toxic to people, pets, and pollinators.
Long-Term Control: Once established, some nematode species can persist in the soil, offering ongoing protection.
Limitations and Considerations
Cost: Nematodes can be more expensive than chemical insecticides, though their long-term benefits often outweigh the initial cost.
Storage and Handling: They’re living organisms and must be stored cool and used promptly.
Patience Required: Results may take several weeks, and repeated applications may be needed over seasons.
Conclusion
If you're battling Japanese beetles in your garden or lawn, entomopathogenic nematodes offer a sustainable, biological solution that works beneath the surface. By incorporating these microscopic allies into your pest management plan, you're not only reducing the beetle population but also contributing to a healthier, chemical-free environment.