Q • I was admiring the lovely white blooms on the ‘Diana’ Rose of Sharon in my city garden this morning, and there sticking out like a sore thumb was a Japanese beetle chewing on the petals. I got out my trusty wide-mouth jar filled with an ammonia solution, knocked him in and watched him expire. I’ve been using this trick ever since I had my first taste of this pest. But they’re back again! Can you shed any additional light on this problem?
A • Japanese beetles are stocky ½-inch long beetles bearing a resemblance to the slightly larger June bugs commonly seen in area gardens every summer. They are much more colorful, however, having iridescent green bodies with coppery wing covers. They feed on the flowers, fruits and foliage of hundreds of plant species but have a special affinity for roses, grapes, birch and crabapples, as well as any plant in the Mallow family, to which the Rose of Sharon belongs.
There is no single best cure for adult Japanese beetles as they are very difficult to control. Hand collecting can be quite effective in the small garden where beetles are not too numerous. A simple jar such as the one you use works well. Hold the jar directly beneath the beetles and tap the stems they are on. When disturbed, they tend to drop like a rock straight off the plant into the container. In addition to your ammonia solution, rubbing alcohol, or a solution of water and dishwashing detergent works just as well.
Japanese beetles seem to have a “herd” mentality and appear to be attracted to areas where feeding damage has previously occurred, so controlling the first few beetles as soon as they appear may actually help reduce damage later in the season. Commercial traps are best used for monitoring adult presence rather than as control devices, as they tend to attract many more beetles into the area than can be caught. Insecticides that can be used to control feeding adults include materials such as carbaryl (sevin), rotenone and various pyrethrins (including Talstar, Tempo, and Astro). Repellants containing garlic products and neem extracts (Azadirachtin) can also be used, though not all gardeners find them to be very effective.
Beetles overwinter as partially grown grubs. Female beetles lay eggs in the soil in summer, with well-irrigated lawns being a preferred site. Young grubs hatch in late summer and feed extensively on grass roots until fall, then move downward in the soil to pass the winter months. Grubs pupate in late spring, and adult emergence is well under way by early summer. Peak numbers are typically heaviest during July and early August and by mid-August their presence will have dwindled considerably.
Grub control is best achieved in late summer after eggs hatch, using registered grub insecticides available at garden centers. Milky spore disease, a biological control agent, works only on Japanese beetle grubs, requires up to two to three years to become effective, and works best only at high grub population densities. Predatory nematodes are also a possible option for those gardeners who choose not to use pesticides.
Write to Chip Tynan of the Missouri Botanical Garden at firstname.lastname@example.org or Horticultural Answer Service, 4344 Shaw Boulevard, St. Louis, 63110.