The Japanese beetle (Popillia japonica) first came to the United States in 1916 when a ship bearing crates of Japanese iris bulbs pulled up to a New Jersey port. It has since spread to 30 states and continues to expand west at a rate of 5 to 10 miles a year. If you have not yet gone to battle with this creature, be thankful. The adults feed on the leaves, flowers and fruit of nearly 300 plants. Asparagus, apples, stone fruits, rhubarb, cane berries, blueberries, limes, grapes and corn are particularly appealing. Meanwhile, the grubs thrive on the roots of turf and pasture grasses. The only factor, other than food source, that appears to limit these beetles is soil temperature. The mean temperature must be between 63 and 81 degrees and mean winter temperature must be above 15 degrees.
To control this beetle, it’s important to understand its life cycle. The female lays her eggs in July, between 2 and 4 inches below the soil surface. Roughly two weeks later, the eggs hatch and the young white grubs dig to the surface, to begin feeding. They primarily feed on turf roots through late summer and fall, but will also munch on the roots of vegetable plants, nursery seedlings and field crops. They overwinter 4 to 8 inches below the soil surface then return to the surface in the spring to resume feeding. Pupation occurs in May or June, and the adults emerge to spend the next six to eight weeks feeding and mating. The mated female deposits 40 to 60 eggs in the soil. The adults die. The cycle begins anew.
We have two phases to contend with: the grub phase and the adult phase. Because the adults are highly visible, they are often the primary control target. Many growers, myself included, have dutifully spent early morning hours dropping them into containers of soapy water (the soap breaks the water’s surface tension, ensuring their demise). Not a particularly stimulating job, but if you’re in the right frame of mind it does provide a good healthy dose of guilty pleasure. This must be a daily task once the first adults have emerged. Adults release two pheromones, one that attracts other adults to a food source and one that attracts males to virgin females. Left unchecked, they will amass and your plants’ leaves will be skeletonized.
Traps laden with pheromones and floral lures attract and capture adults, but their value is highly questionable. There is strong evidence that these traps not only attract adults from your growing area, but also from areas up to 1 mile away. They can actually increase the rate at which your plants are defoliated. Unless you have a small, isolated population of beetles, don’t use them. Other options for managing adults include covering plants with nets and planting natural repellents such as catnip, chives, garlic and tansy.
The grub phase lasts for about 10 months. If these beetles are infesting your operation, you must attempt to control them. There are a few biological control options. Bacillus popillae, or milky spore, was registered in 1948 specifically for Japanese beetle grub control in turfgrass. When a grub consumes milky spore, the spores germinate and multiply. The grub dies within seven to 21 days, releasing billions of spores. The spore population builds over the course of two to four years, killing successive grub generations. Milky spore will not harm your earthworm population, or anything else for that matter. The caveats: the grub population must be sufficiently high to sustain the bacteria population, and milky spore must be applied over a wide area to have an impact. It’s said an entire neighborhood must participate for a noticeable impact. (I’ll leave the interpretation of “neighborhood” up to you.) Even though milky spore was registered in 1948, entomologists still question its impact on grub populations. Rather than depend on it solely, consider it one of many tools in your pest control toolbox.
Entomopathogenic, or insecticidal, nematodes actively seek out grubs to complete their life cycle. The juvenile nematode penetrates a grub and inoculates it with a symbiotic bacteria. The juvenile then feeds on the reproducing bacteria while the grub slowly liquefies. Nematodes begin preying on grubs within 24 to 48 hours of being applied to the soil, a relatively quick response time compared to milky spore. Neither the nematode nor the bacteria will harm plants or animals. Three species—Steinernema glaseri, S. kushidai and Hetero-rhabditis bacteriophora—are considered effective against scarab beetles, the family to which Japanese beetles belong. H. bacteriophora is the most commercially available. Note: nematodes are living organisms with an assortment of habitat requirements, from soil type to temperature and moisture levels. Do your research before you invest in a billion of them. Also, our knowledge of nematodes as biological insecticides continues to evolve and kinks in their mass production are being worked out. It’s in your best interests as an organic grower to stay abreast of this exciting research.
The females of two wasp species, Tiphia vernalis and T. popilliavora, lay eggs in scarab beetle grubs. Both are Asian wasps introduced in the 1920s to control the Japanese beetle. They have since established themselves and are performing their job as assigned, although their rate of population growth and range expansion leaves much to be desired. To my knowledge, these wasps are not commercially available. I mention them because you can encourage their presence by maintaining weedy borders alongside your growing areas. The T. vernalis female feeds on aphid honeydew; the T. popilliavora female feeds on the nectar of wild carrot and other umbelliferous plants.
These are a few options for control. In the end, you have to determine what is right for your operation. Keep in mind, it’s unlikely you’ll ever eliminate them. What you will do is reduce their population to manageable levels over time. The best long-term solution is to maintain good, healthy soil. Good soils produce strong, healthy plants that can withstand such pesky infestations.
The author, a brand new contributor to Growing, is a biologist who lives and farms in Vermont’s Northeast Kingdom.