Lawn grubs are one of the most destructive pests to turfgrass, feeding on grassroots and leaving behind weak, brown patches. While many homeowners turn to quick chemical fixes, long-term and sustainable protection is possible with milky spore grub control.
According to Dr. Michael Klein, a leading entomologist in biological control of Japanese Beetles in turf and nursery environments, “milky disease” is a naturally occurring condition in white grubs from soils that contain specific bacterium spores. For larvae to become infected at a level that provides effective control, however, very specific conditions must be present.
What is Milky Spore?
Milky spore is caused by the bacterium Paenibacillus popilliae, which targets Japanese beetle larvae (and, in some cases, other beetle grubs). The spores do not cause disease in people, pets, or beneficial wildlife, which makes milky spore an excellent fit for The Mill’s “5B” Principles of Lawn Care.
When grubs consume the spores while feeding, they become infected with milky disease. Infected grubs die and release billions of new spores into the soil, gradually increasing spore levels and extending protection naturally over time.
Key Conditions for Milky Spore Grub Control
Dr. Klein notes that specific soil and feeding conditions must exist for spores to infect larvae:
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Target Species: Primarily affects Japanese beetle grubs.
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Soil Temperature: Must be above 65°F for spores to infect larvae.
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Feeding Stage: Grubs must be actively feeding and ingest spores.
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Soil Spore Levels: Adequate spore levels take up to 3 years to establish after the first application.
💡 Misconception Alert: Just having spores in the soil is not enough. Grubs must consume them under the right conditions to become infected.
Why Timing Matters
There’s often an urge to control grubs in the spring when they become noticeable, but spring feeding is short-lived, and soil conditions are not ideal for milky spore infection.
The best time to apply milky spore is late summer into early fall (August–October). Here’s why:
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Soil temperatures are above 65°F.
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Newly hatched grubs are voraciously feeding before winter.
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Turf is stressed from heat/drought and more vulnerable to grub damage.
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Grubs stay near the surface longer in fall, maximizing spore contact.
Application & Longevity
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Apply Milky Spore to affected areas.
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Water thoroughly to help spores penetrate the soil.
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Once established, spores persist for decades. Researchers have found active spores in soils treated decades earlier.
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Results build over time, typically reaching effective levels within 2–3 years.
Grubs, Beetles, and the Life Cycle
Understanding the Japanese beetle life cycle is critical:
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Spring: Overwintered grubs nibble roots briefly before pupating into adults.
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Summer: Adult beetles feed on foliage and lay eggs.
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Late Summer/Fall: Eggs hatch, grubs feed aggressively on roots (this is the ideal time for milky spore application).
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Winter: Grubs burrow deep to survive cold conditions.
Controlling both grubs and adult beetles is key. Milky spore reduces grub populations, while traps or selective insecticides can reduce adult beetle feeding and egg-laying.
Compatibility with Other Controls
It’s important to note: Do not use chemical grub insecticides when applying milky spore. These products kill grubs before they ingest spores, reducing the long-term effectiveness of the biological control.
Instead, think of milky spore as a long-term investment. Although it doesn’t provide instant kill like insecticides, it offers sustainable, organic control that strengthens year after year.
Identifying Infected Grubs
How can you tell if grubs are infected?
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All grubs look white, so infection is not obvious at first glance.
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By clipping off a leg, you can check fluid:
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Healthy grub fluid = clear
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Infected grub fluid = milky white
Infected grubs are actually beneficial to your lawn ecosystem, since they release more spores into the soil.
FAQ: Milky Spore Grub Control
Q: How long does it take to see results?
Expect 2–3 years for full effectiveness as spores build up in the soil.
Q: Is milky spore safe for pets and pollinators?
Yes. It only affects Japanese beetle larvae and does not harm pets, people, or beneficial insects.
Q: Does milky spore kill all grubs?
No. It specifically targets Japanese beetle grubs. Other grub species may require different controls.
Q: Can I apply milky spore in spring?
Spring conditions are too cool for effective infection. Apply late summer–early fall when soil temps are warm and grubs are actively feeding.
Q: Where can I buy milky spore?
👉 Shop Milky Spore Lawn Treatment at The Mill
Conclusion
Milky spore grub control is one of the most effective and sustainable options for long-term lawn protection. By applying at the right time and allowing spores to build in the soil, you’ll create a natural defense system that protects your lawn for decades.
Want to compare other approaches? Check out our Grub Control blog.
