Japanese Beetles: The Problem
The stout, half-inch long beetles are a metallic green and have copper colored wings with tufts of white hair emerging from the bottom of the wings. The adults feed in groups on lush foliage starting at the top of a plant and working their way down. A single beetle does not do much damage on its own, but when beetles congregate, drawn in by a potent scent released by other feeding beetles, the damage is significant. Chewing on the topside of the leaves, the beetles will skeletonize a plant rapidly. They will also feed on the flowers and ripened fruit. These are sun-loving insects that emerge at the beginning of June to feed voraciously during their 30-45 day adult lifespan. The eggs laid during this time hatch after a few weeks and the emerging white grubs feed on root systems for the following 10 months.
Milky Spore: The Solution
By 1930, Japanese beetles were inflicting significant damage on a wide range of North American crops, impacting both agriculture and ornamental plants. Researchers at the time observed that occasionally, the grubs would appear to be covered in a milky residue. Upon further investigation, they discovered that this residue was, in fact, the naturally occurring bacteria, Bacillus popilliae. With the isolation of the bacteria and the effectiveness it displayed under laboratory conditions, it became the first registered microbial pest control agent in the U.S. and has since been sold under various trade names. The most common one is “milky spore.”
Milky Spore Explained
There are a few important details to know when applying this product. Milky spore:
- Does not work on contact. The feeding grubs must ingest the product for the disease to take effect.
- Can’t be applied at any time of year. The soils must be above 65 degrees F, when the grubs are actively feeding. It’s best applied in the summer and early fall.
- Does not affect the beetle stage of this pest, only the grubs.
How to Apply Milky Spore
Milky spore is readily available online, and at many garden centers. It is very easy to apply using a commercial dust dispenser or a similar, homemade device. Apply one teaspoon of milky spore powder to your turf or garden every four feet in rows four feet apart. (It will look like a grid pattern when you’re finished.) Next, water in the spore so it can reach the grubs in the soil. Be sure not to mow the lawn until you’ve completed this step.
The gaps in between application lines will be inoculated naturally as the grubs ingest the spore and it reproduces inside the host body, killing it within 7-21 days. Once the host is dead, the spores contained inside are released as the body decomposes. The spore reproduces itself by several billion in a short period of time and the effectiveness can last up to 10 years. This means you usually only need to apply milky spore once.
For a more targeted approach, apply milky spore powder around the base of infected plants. Water in the spore so that the material penetrates the soil, working its way down to where the juvenile grubs are feeding on the root system. The grubs must ingest the spore for it to be effective.
Pros and Cons
Milky spore has many attributes that make it a suitable choice for Japanese beetle control.
- Extremely safe for use around children, pets, and non-target species.
- Compatible with other pest control measures, such as beneficial nematodes and commercial insecticides.
- Naturally occurring and therefore will continue to reproduce over time.
- Provides long term and ongoing control.
- Only affects the juvenile grub stage of the pest.
- Because milky spore works through ingestion, it can take time to work.
Management Strategies for Japanese Beetles
Milky spore is best used as part of a complete management strategy that employs more than one method of control. Used in combination with other biological control agents and mechanical tools, success will be observed readily.
- Apply beneficial nematodes when the soil temps are above 45 degrees F and ensure soil is kept moist for the weeks following.
- When the adults emerge in June, go out in the dark with a bucket of soapy water to areas of suspect invasion. Knock beetles off into bucket of water and allow them to drown. Do not dump bucket until the beetles are dead.
- Apply milky spore in August when grubs are close to the soil surface and actively feeding.
- Plant non-target species in your yard and garden.
|Common Host Plants||Non-Target Species|
|American chestnut, American elm, linden, American mountain ash, apple, birch, black cherry, echinacea, flowering crab-apple, grapes, hollyhock, horse chestnut, lawns/grass, Norway Maple, plum, roses, walnut.||American elder, white cedar, black oak, box elder, common lilac, euonymus, fir, green ash, hemlock, holly, juniper, pine, red maple, red oak, rhododendron, scarlet oak, silver maple, spruce, white ash, white oak, white poplar, yew.|
An Appropriate Choice
Invasive species like the Japanese beetle are the most difficult pests to control because native beneficial predators do not readily exist. It’s fortunate that extension agents made keen observations in the 1930’s, leading to milky spore’s development. Its effectiveness, ease of use, and availability makes it the most appropriate choice for homeowners and market gardeners to control the Japanese beetle.