Many Missouri gardeners are familiar with the damage caused by Japanese beetles.
Milky spore, a naturally occurring bacterium known as Paenibacillus popilliae, is often promoted for managing Japanese beetle grubs. Its effectiveness, however, depends on timing, weather conditions and realistic expectations, according to 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, which can gradually build a population of the bacterium. It controls only Japanese beetle grubs and does not affect other common species such as masked chafers.
Establishing milky spore is a slow process, Reall said. It can take several years before measurable changes are seen. Missouri’s fluctuating temperatures and cold winters also can limit its ability to build up in the soil, and even under favorable conditions, reductions in grub numbers tend to be modest.
Gardeners looking for faster results may want to consider other biological options.
Beneficial nematodes, particularly Heterorhabditis, seek out and infect grubs in the soil. They may provide quicker results but require adequate soil moisture and appropriate temperatures. A newer strain of Bacillus thuringiensis galleriae also is labeled for scarab beetles and is most effective when applied to small, actively feeding grubs.
Reducing grubs in one lawn does not prevent adult beetles from arriving from surrounding areas, Reall said. Adult Japanese beetles are strong fliers and readily move across neighborhoods.
Early-feeding beetles can attract additional beetles by triggering the release of plant-stress volatiles and producing pheromones. Reall recommends checking plants early in the morning in late June, when beetles are sluggish, and knocking them into a bucket of soapy water. Removing the first beetles to arrive can help reduce additional damage.
Natural enemies in Missouri include tiphia wasps, which attack grubs, and birds such as starlings, grackles and crows. Reall said minimizing the use of broad-spectrum insecticides can help support those natural controls.
Traps also are commonly marketed for Japanese beetle control, but Reall said a single trap placed near garden plants may attract more beetles than it catches. Traps are more appropriate where several acres are available and they can be placed at least 30 feet from the plants being protected.
Gardeners using milky spore should view it as a long-term strategy rather than a quick fix. Nematodes or Bt galleriae may provide more immediate grub suppression, while early removal of adult beetles can help limit aggregation. Reall also recommends prioritizing protection of high-value plants and reducing broad-spectrum insecticide use to support natural enemies.
“Biological control works best when we use the right tool, in the right place, under the right conditions,” Reall said.
