Japanese beetles

One of the most serious garden pests, Japanese beetles, hits gardens in zone 5-6 in July and early August. Gardeners in warmer zones may deal with them earlier. Most gardeners have had to deal with this serious pest at one time or another. The first Japanese beetles were found in New Jersey in 1916 and since then have spread west and south. They are as the name suggests, native to Japan.

The Japanese beetle is a fat, oval, metallic bronze and green beetle, with a row of white spots along the sides of the body. The spots are clumps of white hairs. The immature stage is a tan-white grub that is found in the soil of grassy areas where it feeds on plant roots. It has a distinctive v shaped row of bristles on its rear end. Japanese beetles can fly a good distance. 

 Late June to early July is the time when Japanese beetle damage on plant parts above ground first becomes apparent and feeding continues for about a month. However, beetles emerging from the soil can continue into August and some beetles may persist all summer. 

Feeding on the roots of turf grasses is heaviest in the later part of summer and again in early spring. When Japanese beetles emerge from the soil they eat for a few days and then begin mating and the females begin laying eggs back in the soil. In the afternoon she leaves the plants she has been eating, goes to the ground and burrows up to 4 inches down and lays a few eggs. This continues for days until she has laid roughly 60 eggs. 

 In warm moist soil the eggs will hatch in about 10 days and the new grubs begin feeding on grass roots. Eggs and small grubs are very susceptible to drying out and not irrigating the lawn in summer may help control a grub population.

 Adult Japanese beetles will feed on over 300 species of plants, including most of our ornamentals, trees, and fruits. They eat the tissue from between leaf veins, turning them into ragged skeletons which brown and fall off. They may also eat soft fruit. The damage the grubs do in lawns and on golf courses appears as yellow spots, where the turf can be rolled back in patches like a rug. 

 As they feed and mate Japanese beetles release a scent that attracts other Japanese beetles and populations in an area may soon contain thousands of beetles. They can do serious defoliation damage to plants, although the plants will generally recover in late summer when feeding damage is less. It is estimated that damage and cost of controlling Japanese beetles to Americans is over 450 million dollars each year.

Japanese beetle damage on hibiscus

Control in home landscapes In July homeowners can apply grub control products to the lawn containing imidacloprid (Merit) and halofenozide. It won’t help with the beetles this year but will help prevent a big increase next year. In spring if damage is apparent use grub products containing trichlorfon (Dylox) and carbaryl (Sevin). Remember that these pesticides will harm many kinds of microorganisms in the soil and may have toxic effects on pets and children too. 

 New research has shown that keeping lawns 3 inches or more high in July and August helps prevent female beetles from laying eggs in the soil. If you can convince your neighbors to do this too, you may lower the grub population and then the Japanese beetle population.

For ornamental plants a general insecticide for ornamental plants can be used but once again these will kill many types of insects, good and bad. Systemic products may do some good - but the beetle has to eat part of the plant to be killed. They are best used early, before beetle populations get high. 

 Neem oil does not effectively kill Japanese beetles but using a spray on foliage and flowers does have a repellant effect. They will avoid feeding on those plants. Follow label directions for applying. 

 For fruit trees, including ornamental types of fruit trees, a multipurpose orchard spray should be used as directed on the label. For vegetables use an insecticide approved for edible plants. Vegetables that don’t require pollination can be covered with lightweight row cover material to exclude beetles. 

 Hand picking and using a hand vacuum to remove beetles as soon as they are seen is helpful. Some people have used lint rollers to capture the beetles. Once beetles get established, they attract more beetles. Japanese beetle traps are on the market but for them to work you must locate them far away from the plants you want to protect. 

 Research published in the Journal of Economic Entomology, 04/08/2009, found that a mixture of two essential oils, wintergreen and ginger oil, was quite effective at repelling Japanese beetles. Peppermint oil was also a strong repellant. (Coffee and citronella oils actually seemed to attract Japanese beetles). More research is being conducted to find the best way to use these oils as beetle repellants on crops. These oils could be applied to food crops safely and would be relatively benign to the environment.

 Other research conducted at the USDA Application Technology Research Unit in Wooster, Ohio found that geraniums could be part of the war against Japanese beetles. Entomologist Chris Ranger found that Japanese beetles that ate geranium leaves were paralyzed within 30 minutes and stayed immobile for up to 24 hours. While immobilized they are easy prey for birds and other predators or could be swept or vacuumed up. Interestingly enough the paralytic effect of geranium leaves on Japanese beetles has been known since the 1920’s, about the time the pest arrived in the United States. Ranger and a colleague have just begun to develop a natural pesticide using geraniums and have applied for a patent. 

Other natural controls 

 Japanese beetle traps that attract the beetles and then drown them have been around for years, but in small yards the placement of traps can be a problem. The traps attract more beetles than might normally be in the area and need to be placed a good distance – 100 feet or more, away from plants you want to protect. 

 A few years ago, milky spore disease was introduced into areas where Japanese beetles are found. This fungal disease attacks the Japanese beetle’s immature stage- the grub that resides in your lawn destroying grass roots. Milky spore disease is now found in stores and garden supply catalogs for you to sprinkle over affected areas. A new protozoan disease that also kills Japanese beetle is being released.

 These natural controls are safe for the environment and kill only Japanese beetles. Both the milky spore disease and protozoan controls take many years to begin to be effective in an area. But combining those controls with faster acting repellents made from essential oils or a geranium knock out spray could be the environmentally friendly solution homeowners have been waiting for. 

Japanese beetles can do a lot of damage to gardens and fruit plantings when their numbers are high. Keys to control are early interventions and treating lawns for grubs. Remember plants seldom die from the beetles feeding. 

Plants that attract Japanese beetles 

Japanese beetles will eat over 300 species of plants, but they are attracted to some plants more than others. Here’s a list of plants that Japanese beetles really like;

 Apple, crabapple, Malus spp Asparagus, Asparagus officinalis Cardinal flower, Labelia cardinalis Clematis, Clematis spp. Clethra, Summer-sweet, Clethra spp. Dahlia, Dahlia spp., Evening-primrose, Oenothera biennis Gladiolus, Gladiolus spp. Golden Glow, Rudbeckia lanciniata Hortensia Grape, Vitis spp. Hibiscus, Hibiscus moscheutos, Hollyhock, Alcea rosea, Japanese maple, Acer palmatum Mallow, Malva rotundiflora, Morning-glory, Ipomoea purpurea Red raspberry, Rubus idaeus Rhubarb, Rheum rhabarbum Rose, Rosa spp. Sunflower, Helianthus annuus Sweet corn, Zea mays Virginia creeper, Parthenocissus quinquefolia Zinnia, Zinnia spp. Japanese beetles are also attracted to almost any yellow flowers and foliage. They prefer plants in sunny locations too. 

 Plants Japanese beetles don’t like very well. 

Ageratum, Ageratum spp. Begonia, Begonia spp. Burning-bush, Euonymus spp. California poppy, Eschscholzia californica Columbine, Aquilegia spp. Coral-bells, Heuchera sanguinea Coreopsis, Coreopsis spp. Dusty-miller, Centaurea cineraria, Forsythia, Forsythia spp. Foxglove, Digitalis spp. Hosta, Hosta spp. Impatiens, Impatiens spp. Lantana, Lantana camara. Larkspur, Delphinium spp. Lilac, Syringa spp. Lily-of-the-valley, Convallaria majalis Lychnis coronaria Pachysandra, Pachysandra spp. Poppy, Papaver spp. Moss-rose Portulaca grandiflora Nasturtium, Tropaeolum majus Redbud, Cercis spp Sedum, Sedum spp. Violet, pansy Viola spp. 

 In addition, Japanese beetles rarely feed on evergreens like pines, spruce, junipers and arborvitae.