Paenibacillus popilliae
A soil bacterium, not an insect, sold as a powder or granules for Japanese beetle grubs. Grubs that swallow the spores as they feed can develop milky disease: the bacteria multiply in the blood until it turns milky-white, and the grub dies. The commercial strain affects Japanese beetle and nothing else. Chafer, May and June beetle grubs are untouched. The evidence for it is weak. In multi-year trials on Kentucky golf courses, commercial spore powder neither raised milky disease levels nor reduced grub numbers, and University of Minnesota says modern university trials have not shown any benefit.
At a glance
- Mode
- Bacterial spores — must be eaten by the grub
- Label rate
- 1 tsp every 4 ft on a grid; 10 oz per 2,500 sq ft
- Active
- 0.02% spores, 1 × 10⁸ per gram (powder label)
- Host range
- Japanese beetle grubs only
- Speed
- Grub turns milky 14–21 days after infection
- Evidence
- Poor in Kentucky trials; UMN: no benefit shown
How it works
Milky spore is the bacterium Paenibacillus popilliae, formerly Bacillus popilliae, and the product is a dust or granule carrying its spores. A Japanese beetle grub swallows spores with the soil and roots it eats. They germinate in the gut within about two days and invade the blood, where large numbers of cells build up by day 5 to 10. By 14 to 21 days the blood is milky-white with spores, and a single dead grub holds about 5 billion of them. That is how the disease is meant to spread: each grub that dies seeds the soil around it. The label says as much. Uniform, highly efficient control cannot be expected until the entire area has been inoculated.
It has a real history. Between 1939 and 1953 a government programme spread spore powder across the eastern United States, and Cornell records grub numbers falling 10- to 20-fold. In some regions disease incidence later collapsed, from 41.5% of field grubs in 1946 to 0.2% in a later survey. Modern trials have not reproduced the old results. Redmond and Potter's multi-year Kentucky trials, published in 1995, found that commercial spore powder applied the conventional way neither caused milky disease nor reduced grub populations. The Potter and Held review concludes it is probably best regarded as one of many factors that help suppress Japanese beetle, not as an effective microbial insecticide. Minnesota Extension and the Northeastern IPM Center say the same, and Purdue says commercial products have not proven effective as a stand-alone, single application.
Applying it
Milky spore is spotted onto turf, not sprayed, and the rate is set by the product label. The figures below come from the St. Gabriel Organics Milky Spore Powder (EPA Reg. 63191-1) and Milky Spore Granular (EPA Reg. 63191-16) labels.
One teaspoon, 4-ft grid
Shake to fluff the powder. Put 1 level tsp on the turf every 4 ft (1.2 m) along rows 4 ft apart. A drop spreader is allowed at double the rate.
10 oz per 2,500 sq ft
1 lb per 4,000 sq ft; 10 lb per acre. The granular product goes on at 1 lb per 350 sq ft.
Water in lightly
Water lightly straight after, to soak the spores into the soil where grubs feed.
Any time the ground is not frozen
Granules: spring, summer and autumn for two successive years, per the label.
CAUTION label: avoid the dust
The label says it is harmful if inhaled or absorbed through skin and causes moderate eye irritation. Avoid breathing the dust, wash after handling, and keep it out of storm drains, gutters and surface water.
When to choose something else
Milky spore targets one grub species and has a weak record in modern trials. Choose something else when:
- You have not counted the grubs — a few per square foot are normal, and healthy turf outgrows them
- The grubs are not Japanese beetle — check the raster; masked chafer, European chafer and May beetle grubs are unaffected
- You need the lawn protected this season — the label itself says control waits until the whole area is inoculated
- The damage is to roses or grapes — controlling grubs in the lawn does not protect plants from adults flying in
- You want a biological option with better field evidence — Potter and Held found the nematode Heterorhabditis bacteriophora can be effective against Japanese beetle grubs, though commercial nematodes have performed inconsistently
Where to buy
These are the registered products whose labels we checked. They are not ranked. helpfulbugs takes no commission and does not sell either one.
- St. Gabriel Organics Milky Spore Powder (EPA Reg. 63191-1)10 oz, 1 lb, 2.5 lb, 10 lbUS · HOME
- St. Gabriel Organics Milky Spore Granular (EPA Reg. 63191-16)20 lb, 50 lbUS · HOME
Paenibacillus popilliae by target
Rate and timing differ by what you are releasing it against. Each page below is the program for one pest.