What Do Japanese Beetles Eat in Gardens and Lawns?

The two stages feed in different places and at different times. Adults come out of the soil in late June or early July and feed mainly in July and August, with some still active in September. The grubs hatch about two weeks after the eggs are laid, and most grub damage to lawns shows from late August through early October.

Adults fly up to several miles to feed, so they can arrive from lawns you do not own. Treating the grubs in your lawn is unlikely to reduce the adults on your roses, because beetles from untreated grass fly long distances to the plants they prefer, and picking adults off the roses does nothing for the roots under the lawn. Chewed leaves, flowers and fruit point to adults, and patches that peel up like a loose carpet point to grubs.

Adults eat the tissue between the major leaf veins and leave the veins behind, which gives the leaf a lace-like, skeletonized look. Damaged leaves turn brown and may fall off. The beetles also chew holes in flower buds and petals and in soft fruit, and they clip corn silks.

To confirm the beetle, look for these marks on the insect itself.

Look during the day on the sunny side of the plant, because the beetles prefer warm, sunny conditions and are especially drawn to ripening fruit and rosebuds. Start looking in late June or early July, when the first adults appear.

Roses are the best-known target. Healthy roses survive the feeding, but the blossoms are often ruined. Hibiscus, hollyhock and marigold are among the worst beetle magnets, and so are rose of Sharon and Virginia creeper.

Among woody plants, lindens, crabapples, apples, cherries, plums, birches, elms and Japanese and Norway maples are preferred. Susceptibility varies, and crabapple cultivars range from barely touched to severely damaged.

Young or unhealthy plants are the exception to "mostly appearance." Severe, persistent feeding can stunt, injure or even kill them, so a newly installed tree or shrub needs more protection than an established one of the same species.

Fruits, vegetables and herbs tolerate limited leaf feeding, but severe damage can slow growth and reduce the yield. Green beans are among the worst beetle magnets in the vegetable garden. So is basil, along with grapes, brambles, apples, cherries, plums, peaches and sweet corn silks.

On sweet corn, the beetles clip the silks. The damage shows once silks are clipped to less than half an inch, and the ears come out smaller, incomplete or gap-toothed. The clipping does not matter if pollination is complete or nearly so before the beetles arrive, which you can tell when the silks have turned brown or the tassels have stopped shedding pollen.

Fruit is the other thing at stake. The beetles prefer early-ripened or damaged fruit, and thin-skinned stone fruit shows gouged patches, so split or wounded fruit left on the plant keeps a feeding site open.

If you decide to spray a food crop, the plant must be listed on the product label, and the days between spraying and harvest on that label set how long you wait before picking.

Grubs chew grass roots, which cuts the grass's ability to take up water and nutrients. Severe feeding leaves dead patches that roll back like a loose carpet because no roots hold them. Moles, skunks, crows and other animals dig up the grubs and add a second kind of damage.

Healthy turf can typically tolerate up to 10 grubs per square foot in Minnesota. Other regions use numbers between 5 and 12, and the guide to how to identify lawn grubs shows how to sample and count. If no grubs turn up under a brown patch, look at the still-living turf at the edge of the damage.

Females lay their eggs in moist soil, and eggs and young grubs survive better in a lawn that gets frequent, shallow watering. Occasional deep watering suits the grass better and the beetles less, and letting turfgrass go dormant from late June to late July is another option.

Dish soap does not kill grubs. After the grubs are dealt with, repair bare patches by overseeding in early fall.

Beetle-damaged leaves give off odors that attract more beetles, so one early feeding site grows into a cluster while a neighboring plant that is just as attractive stays lightly touched.

The same odor is the reason early handpicking helps, because removing the beetles reduces the number of new ones the damaged leaves attract. Where the Winsome fly has become abundant, in the Minnesota metro region and eastern Minnesota, skip any beetle carrying white eggs, because the parasite kills the beetle within 5 to 6 days.

Traps tend to bring in more beetles than they catch. They bait beetles with a sex pheromone and a floral scent, and Penn State reports that they trap only about 75 percent of the beetles they attract, so the plants in your yard feed the rest.

Minnesota Extension advises against traps in home gardens, Iowa State says they bring more beetles to the garden, and Colorado State reports more feeding injury in yards with traps. Utah State is the exception, suggesting mass trapping for small, local populations with the traps placed near plants of lower value.

Adult feeding is mostly cosmetic. Healthy, mature trees and shrubs tolerate a lot of it, and in most cases the damage will not kill plants. For fruit crops, economic thresholds have not been established, so no percentage of missing leaf tissue tells you when to treat. The decision rests on what is at stake, such as a ruined rose bloom, scarred fruit or a young plant that cannot spare its leaves.

The beetles feed for six to eight weeks, so handpicking continues until their numbers drop, and most feeding is finished by mid to late August. Neem oil is effective for several days with repeat applications, though less effective against large numbers, and insecticidal soap does not work on adult beetles. Utah State does not recommend foliar insecticides on trees and shrubs, because they are expensive and generally ineffective in landscapes.

Plants that attract beneficial insects cover the helpers worth protecting. The guide to natural pest control covers gentler options.

Boxwood, clematis, chrysanthemum, conifers such as arborvitae, spruce and pine, daylily, geranium, ginkgo, forsythia, lilac, magnolia, red and silver maple, oak, redbud, rhododendron and yew are plants the beetles usually do not damage, and shrubs such as barberry, spiraea, weigela and winterberry holly join them. Crabapple appears on both the susceptible and the low-damage lists because cultivars differ.

Four o'clocks are sometimes said to poison the beetles, but Minnesota Extension found no evidence of it. A 2013 Colgate study saw no dead beetles when they were fed only four o'clock plants.

The site's guide to Japanese beetle-resistant plants lists replacement choices plant by plant.