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Milky Spore is a product that contains the spores of a naturally occurring soil bacterium. When these spores are eaten by a Japanese beetle grub they cause a Disease called Milky disease that slowly kills the grub.
Milky disease has proven to be an effective biological control of Japanese beetles grubs. However, it has been used with mixed results - as expected with any biological control. Its had a successful history of reducing beetle populations in certain parts of the eastern United States, especially when used in community-wide treatment programs. On the other hand, there has not been enough research conducted to understand how well it works. The land grant universities that have run trials have come to the same conclusion - commercial applications of Bacillus popilliae spores offer very little benefit and are not worth the expense.
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It depends on where you live, if you actually have Japanese beetle grubs in your lawn and you are willing to gamble a few bucks - about $10 to $14 per 1000 ft2 - buying and applying the spores with the aim of protecting your lawn from Japanese beetle grubs.
Do you live where Japanese beetles are a problem? Japanese beetles have been a major problem in the eastern states and have been migrating into the Midwest.
Milky Spore is not effective in cooler northern climates - the spores need soil temperatures to be above 70° F for several months.
Also consider, Japanese beetle grubs rarely reach the damage threshold of 10 grubs per ft2. A healthy lawn will recover quickly from grub feeding without showing any signs of damage.
Milky only controls Japanese beetle grubs and not other white grubs that might be in your lawn at the same time. There are 10 species of white grubs that are lawn pests.
Adults are strong fliers and will travel 5 miles to find a good place to eat and reproduce. If you treat your lawn with the ambition of stopping Japanese beetle adults from eating your garden plants - you will be disappointed. They will just fly in from your neighbors yard!
A community-wide program has to be used to reduce the populations of adult beetles.
Tip: Those Japanese beetle pheromone traps will attract beetles to your property - more beetles than they will actually catch. Only use these if you have a lot of acreage and hang them on the outskirts - as far as you can get them from your garden and landscape plants.
The spores of a treated area are washed into the soil where they stick to soil particles, roots and organic matter near the soil surface and remain viable for several years. When a grub feeding in the area eats spores, the ingested spores germinate in its gut and the bacteria infect and kill the grub. As the dead grub decays, it releases a couple billion new spores back into the soil - spreading the disease. Other grubs feeding in the area are then infected and continue to spread the disease. Over time - 2 to 5 years - the spores have the potential to completely saturate a treated soil with the ability to suppress a grub population several years.
Mixed Results and Debate
As with all biological controls - sometimes it works and sometimes it doesn't. Biological controls are very dependent on environmental factors. The spores need to remain viable, be eaten by the grubs and then rely upon infected grubs moving to another area to spread more spores.
There is a lot of debate about how effective Milky Spore actually is. Environmental conditions, misapplications or just being impatient can be blamed for some of the failures. Grubs move slowly through the soil and the spores do not move by themselves, so it can take 2 to 5 years for the spores to be dispersed throughout the soil by infected grubs. There is also concerns that the Japanese beetle grubs might be developing resistance to Milky Spore disease.
Control Success Depends On:
Here's a good application demonstration that shows how simple it is to apply milky spore.