The primary method of managing Japanese beetle grubs in turfgrass is with the use of insecticides such as trichlorfon (Dylox), halofenozide (Mach 2), and imidacloprid (Merit). All these materials are highly effective in maintaining Japanese beetle grub numbers below damaging levels. However, there is growing interest in the use of alternative pest control materials such as beneficial nematodes and microbial-based insecticides. One of the first microbial insecticides ever registered in the United States was milky spore disease. This bacterium was first detected in New Jersey in 1933. It became available commercially for use on turfgrass in 1948.
Milky spore disease is a bacterium that was initially referred to as Bacillus popillae, but it is now known as Paenibacillus popillae. The bacterium is applied to turfgrass as a dust. It is effective only in controlling Japanese beetle grubs; it has virtually no activity on other grub species. However, all larval stages are susceptible to the bacteria. Japanese beetle grubs must ingest the bacteria because the primary mode of entry is through the mouth. The bacterial spores reproduce within the grub, eventually turning the hemolymph, the internal insect fluids, an opaque white. Infested grubs eventually die, and bacterial spores disperse into the surrounding soil. Ingestion of the bacteria does not always produce infection as spores may pass through the gut and be discharged with fecal matter.
Milky spore disease was produced initially by collecting thousands of Japanese beetle larvae from the wild each spring and fall and bringing them to a laboratory for processing. Milky spore disease does not germinate well on an artificial medium or in test tubes. Nor will the bacteria develop on a dead insect, so infection needs to be induced through artificial means. This is normally accomplished by injecting live grubs with bacterial spores.
In the 1980s, it turned out that a different but related bacteria was actually being produced, which had little activity on Japanese beetle grubs. As a result, products were withdrawn from the marketplace. An earlier, more efficient, and consistent formulation method is now being used to produce milky spore disease.
Successful use of milky spore disease requires attention to environmental conditions, including temperature, moisture, soil structure, pH, and soil type. Efficacy of the disease may vary depending on cold temperatures; the spores are very cold-sensitive. Ideal soil temperatures for spore development during grub attack are between 60° and 70 °F (19° and 21°C). In cooler climates, like the Northeast, milky spore may take longer to spread than in warmer climates. It is important to know the soil temperatures within the profile during the fall and early spring when grubs are active.
Milky spore does not spread unless grubs are present in the soil. The density of the grub population is important in the establishment and buildup of the bacteria in the soil. As a result, the higher the density of the grub population, the faster milky spore will become established. In addition, infected grubs may eventually spread through the soil profile, increasing the likelihood of spreading the bacteria.
Bacterial spores tend to be concentrated in areas with high grub numbers. Spores bind tightly to soil particles and are normally located near the soil surface where grubs are likely to be located. Persistence in the soil ranges from 2 to 10 years with persistence as long as 30 years if grub recycling continues. In fact, studies have demonstrated that milky spore can last 15 to 20 years in the soil. It has been thought that milky spore may remain in the soil in a dormant but viable state until new infestations of grubs are present
Milky spore disease is a population suppressant. The goal when using milky spore disease is to keep Japanese beetle levels below damaging thresholds (10 to 12 grubs per square foot). However, the use of milky spore disease is not without problems or concerns. First, it can be very difficult to determine results because other microorganisms (for example, predators and pathogens) in the soil may kill Japanese beetle grubs more rapidly than milky spore. Second, milky spore affects only one species of white grubs; and, in some situations, the Japanese beetle may not be the only or the predominant species. Third, results may take several seasons, 3 to 5 years in cooler climates, so several years may lapse before adequate control of grubs is achieved. Furthermore, populations of the pathogen may not increase quickly enough to retard the spread of an isolated colony of grubs. Finally, the use of milky spore may be cost prohibitive compared with other currently available pest control materials.
Birds and other wildlife that inadvertently move infected grubs when feeding naturally disperse the spores of milky spore disease. Milky spore is usually compatible with conventional insecticides. Research has determined that milky spore has no impact on beneficial organisms in the soil. In addition, it appears that the bacterium is compatible with Tiphia wasps, which are parasitoids of Japanese beetle grubs.