The Japanese Beetle Menace

 by Julie Barnes

What's been skeletonizing my beautiful hardy hibiscus? It looks like a two toned insect pest: measuring about a half inch long; metallic green abdomen surrounded by small white tufts of hairs; and coppery brown wings. Throughout Eastern United States the adult causes widespread damage devouring ornamental trees, shrubs and flowers on nearly 300 different host plants such as: roses, Rosa spp.; flowering cherry, Prunus spp.; flowering Crabapple, Malus spp.; zinnias, Zinnia spp.; Boston Ivy, Parthenocissus tricuspidata; birch, Betula spp.; canna, Canna spp.; Elm, Ulmus spp.; or marigold, Tagetes spp. Strategies for managing the larval and adult stages in this country are estimated to cost more than $460 million a year.

The Japanese beetle as its name implies is native to Japan and was unfortunately brought to New Jersey through a Japanese iris bulb shipment in 1912. With no natural enemies, diseases, or competition this pest was able to become established here quite easily. Now that Japanese beetles are here some knowledge may help on how to best deal with them.

Japanese beetles spend most of their one-year lifecycle underground as white, c-shaped grubs. They feed on grass roots and are particularly damaging to turf when their populations are high. Patches of dead or dying grass will be most visible during spring (April & May) and fall (September & October) when they are most active. Sometimes, the sod can even be rolled back like a piece of carpet. Skunks, possums and other wildlife find these grubs to be quite irresistible and may dig up lawn areas to harvest them. In winter the grubs become dormant moving deep underground. Once the weather warms, they return again to the surface, feeding on plant roots before emerging from the ground as adult beetles. In our Pennsylvania area this is around late June. The beetles are most active thru July and the first two weeks of August. Flying to potential host plants they feed during the day when it is hot and mainly where the plants receive full sun exposure. With sharp chewing mouthparts the beetles will eat leaves, flowers, and fruit. Leaves take on a skeletonized appearance as they eat the soft tissue between the leaf veins. Flowers will appear finely shredded while fruits will be chewed into and hollowed out. While feeding, they often send pheromones to attract more Japanese beetles to congregate on certain plants. Mating occurs during this time. Each female beetle lives 30-45 days laying 40-60 eggs in the soil that hatch into root feeding grubs in 10-12 days. By Mid-August most adult Japanese beetles are gone for the year.

So, what can you do about them? Since Japanese beetle eggs and grubs need moisture to thrive, consider withholding water and allow your lawn to go naturally dormant when drought- like conditions appear in July and August to affect their demise. Hand-picking beetles is an effective method of control on low-growing trees and plants by either plucking or knocking them into a container of soapy water. The best time to do this is early in the morning or in the evening when they are less active. Be wary about using Japanese beetle traps as they will bring beetles in from areas well outside of your yard causing heavier feeding on plants you actually want to protect. There are a number of insecticides to use but if you are trying to maintain a pollinator-friendly garden they are best not to be used on flowering plants or trees that will attract bees and other pollinators. Organic insecticides may be effective deterrents for a few days or when infestations are light. Any insecticide must be used with caution by following the directions and safety precautions carefully. Supposedly, Geraniums contain a substance that temporarily paralyzes Japanese beetles within 30 minutes of consuming its flower petals. The beetles typically recover within 24 hours, that is, if they are not spotted by predators while helpless. Planting Geraniums near plants you want to safeguard is maybe something to think about.