Milky Spore: Is It Effective?

Are you having problems with Japanese beetles eating your plants? These adults burrow into the ground in the fall to lay eggs that eventually turn into grubs. And anyone with a lawn knows these grubs can devastate lawns and gardens.

One organic control developed to tackle the Japanese beetle problem is called milky spore powder. But what exactly is milky spore, and what is the best way to apply it? Let’s explore the pros and cons of this treatment and how to use it.

What Is Milky Spore Disease?

In the 1930s, Japanese beetles demolished ornamental plants and crops across the United States, so much so that a solution was needed on a national scale. Scientists studying the insect invasion discovered Japanese beetle grubs with white tails. They attributed the whiteness to what is commonly known as milky spore disease. 

They then isolated a bacterium called Paenibacillus popilliae, the pathogen that causes milky spore disease. 

A spore powder was developed from the bacteria that cause milky spore disease. This became commercially available in 1948 when landowners would apply milky spore en masse. Today, it’s used by homeowners to control infestations of the Japanese beetle. 

How Does Milky Spore Work?

This bacterium is specific to Japanese beetle larvae, grubs, and adult beetles. It is not effective against other damaging grubs. Other organic insecticides attack grubs, but they are not of this particular strain of bacterium. 

The spores of P. popilliae live in soil where yeast, casein, and other amino acids and sugars are available. As Japanese beetle larvae feed on grass roots and the roots of other garden flora, they ingest spore content.

The spores begin to proliferate within the gut of the Japanese beetle larvae at two days and reach peak production at three to five days. Eventually, the spores move into the hemolymph of the beetle larvae, a place rife with the sustenance this spore loves. 

As the larvae grow and develop, the milky spore continues to infect its body. At two to three weeks, the milky spore populates the Japanese beetle larvae enough for the back end of the grub to turn white. Eventually, the grubs die, spreading the spores throughout concentrated populations. 

Effects

Japanese beetles have experienced a significant drop in population since the 1940s when gardeners and landowners were advised to apply milky spore in powder form. That is attributable to the tons of work done by scientists who individually and artificially infected Japanese beetle grubs with the bacterium. 

In 1980, scientists discovered Japanese beetles had become resistant to the milky spore bacterium, and the product was pulled from the market. Today an adapted form of the control is approved for home use. 

Interestingly, milky spore works best in areas where high concentrations (about 10 to 12 per square foot) of Japanese beetles and their offspring live. This makes a lot of sense when you learn the overall goal of isolating the bacterium was population control, not elimination. In smaller numbers, Japanese beetle grubs don’t do enough damage to require organic controls. 

Furthermore, the spore can remain in the soil of a lawn or garden for two years, up to 10 years. Sometimes, it can remain for longer. Milky spore bacteria will proliferate within live grubs, but not dead ones. Those that remain in the soil go dormant as they bind to grass cells waiting for more larvae to feed on. 

Should You Use Milky Spore Powder?

Because this disease only infects Japanese beetles, it’s perfectly safe for all other animals (insects and humans included). Therefore, it is not useful in the garden or on lawns infested with other kinds of grub. 

Secondly, if there isn’t a large enough concentration of the larvae in your lawn or garden, the spore will not be able to proliferate. The treatment won’t last as long. This isn’t a huge deal if your plants aren’t in immediate danger. 

If the larvae are still under the soil and have yet to emerge, there may be no sign that your plants are in peril. As a result, it’s difficult to tell that soil treatment is necessary when there aren’t a huge amount of beetles around. 

More recent studies suggest the virulence of this disease is losing a foothold in lawns and gardens everywhere. One particular study showed there has been a huge drop in the effectiveness of the treatment in gardens where high concentrations of the Japanese beetle are not present. 

If you’re considering a lawn or garden treatment for Japanese beetles, know you’re likely in it for the long haul. Unless you treat multiple years in a row, your garden and lawn could still show a sign of infestation. 

And if you treat at the wrong time, your garden could still suffer. The grub that you want to eat the treatment could be a couple of feet underground. There may be better alternative treatments, as well. 

How To Apply

There are two forms of commercially available milky spore to treat grubs in lawn and garden areas: powder or granular. Both can be found at garden and hardware stores near you. Each has its own method of application. 

Milky spore granules come in a bag of 10 pounds or more and must be applied to the soil with a drop spreader. Fill your drop spreader, spread the granules uniformly over the treatment area, and apply them at a rate of one pound per 350 square foot area. 

Adjust the proportion as needed for larger or smaller areas. After you apply milky spore granules, water them in. 

It’s possible to couple applications of these granules with beneficial nematodes. If that’s your plan, spread nematodes at the same time you water. Apply the granules in spring, summer, and fall before grubs hatch for the best results. Most guides recommend reapplications in the lawn and garden for two consecutive years. The spore content will spread in the soil and control a Japanese beetle infestation for up to 10 years. 

Dust-like forms of this organic control come in boxes with measurements as low as 10 ounces up to 50 pounds. Begin by shaking the container to fluff the bacteria. Then apply milky spore powder in a grid pattern at four feet apart and water it in. Just 10 ounces will cover up to 2500 square feet.

If you plan to apply milky spore as a powder in conjunction with nematodes, lay out the powder grid and then apply nematodes with a sprayer. This will water both the dust and the nematodes into the lawn or garden. 

The great thing about this form of milky spore is it lasts much longer than granules and generally only requires one application per decade. Some applications last 20 years. 

Make sure to apply this treatment in early fall as opposed to other seasons because the grub you’re after is active during these times. 

Other Methods for Treatment

Nematodes (specifically Heterorhabditis bacteriophora) are often used in conjunction with milky spore. You may be able to skip the milky spore altogether and instead use nematodes, which are a better option if you’d like to control a host of other pests and Japanese beetles. 

Try growing plants that aren’t susceptible to attack by Japanese beetles. Plant those that are susceptible sparsely around your home so beetle populations can’t gain enough density to cause significant damage. Create an image of your garden plan, and include placements for at-risk plants where they’re spaced out from one another. 

There are commercially available traps for Japanese beetles on the market, too. Floating row covers and handpicking the beetles off your plants also work to prevent adults. A fermented fruit trap surrounded by water attracts and subsequently drowns beetles. Geraniums attract and trap them as well. However, be aware that all of these methods are intended to take out adult beetles. They do not handle the grubs themselves.

If you’ve tried milky spore treatments that haven’t worked out, make efforts to improve the health of your soil and lawn overall. Mow your lawn on the highest height setting. Apply nitrogen fertilizers annually. With improved health comes improved vigor, and your lawn may be able to muscle past the nibbling at the root system that grubs do.

Frequently Asked Questions