Milky spore and Japanese beetles: a long-term tool with limits

This article is part of the series ‘Biological control in Missouri: Right tool, right place, right conditions.’

KANSAS CITY, Mo. – Many Missouri gardeners are all too familiar with Japanese beetles.

Milky spore (Paenibacillus popilliae), a naturally occurring bacterium, is often promoted for managing Japanese beetle grubs. But its effectiveness depends heavily on timing, weather conditions and realistic expectations, says University of Missouri Extension horticulturist Tamra Reall.

Milky spore works only when grubs ingest it. Infected grubs die and release new spores into the soil. Over time, this can build a resident population of the bacterium. It controls only Japanese beetle grubs, not other common species such as masked chafers.

Establishing milky spore is a slow process, says Reall. It might take several years before you see any measurable change. Missouri’s fluctuating temperatures and cold winters often limit its ability to build up in the soil, and even under ideal conditions, reductions in grub numbers tend to be modest.

Gardeners looking for faster results may want to consider other biological options.

Beneficial nematodes, particularly Heterorhabditß, seek out and infect grubs in the soil. They might provide quicker results, but they need adequate soil moisture and appropriate temperatures. A newer strain of Bacillus thuringiensis (galleriae) is labeled for scarab beetles and is most effective when applied to small, actively feeding grubs.

Keep in mind that reducing grubs in one lawn does not prevent adult beetles from arriving from surrounding areas, says Reall. Adult Japanese beetles are strong fliers and readily move across neighborhoods.

Early-feeding beetles trigger the release of plant-stress volatiles, and beetles produce pheromones. Both of these attract more beetles, quickly increasing damage if not addressed. In late June, check plants early in the morning when beetles are sluggish and flick them into a bucket of soapy water. Removing these first “scouts” prevents them from putting out the welcome mat for more beetles.

Natural enemies in Missouri include tiphia wasps, which attack grubs, and some birds, such as starlings, grackles and crows. Organisms such as the tiphia wasp contribute to gradual population suppression over time and are supported by minimizing broad-spectrum insecticide use.

Traps are often marketed for control, but unless you have multiple acres and can place traps at least 30 feet away from your garden, traps will likely lure more beetles onto your property than they catch.

Do this for success

• Don’t expect milky spore to provide quick control.
• Don’t use a single trap near garden plants; instead, use traps at an appropriate scale and distance.
• Do not assume that grub control prevents adult damage.
• Use milky spore as a long-term strategy with realistic expectations.
• Consider nematodes or Bt (galleriae) for more immediate grub suppression.
• Remove beetles early to reduce aggregation.
• Support natural enemies by reducing broad-spectrum insecticide use.
• Prioritize protection of high-value plants.

 “Biological control works best when we use the right tool, in the right place, under the right conditions,” says Reall.

More articles in this series.

Photo

Japanese beetle grubs. Photo by Tamra Reall.