Which Plants Do Japanese Beetles Avoid?

Scented herbs look like a natural shield for a rose, but the Kentucky entomologists who wrote the university's Japanese beetle fact sheet rate companion planting as generally ineffective. The same sentence puts insecticidal soap and extracts of garlic, hot pepper or orange peel in that group.

Odors from damaged leaves seem to be an important reason beetles gather on particular plants. The result is often a large cluster on one plant while neighboring, equally attractive plants are only lightly infested.

Basil and marigold are on Minnesota Extension's list of plants Japanese beetles prefer, so planting either one beside a rose gives the beetles one more thing to eat.

The approaches the extension pages do support are choosing plants the beetles seldom damage and dealing with the beetles on the plants you keep.

Check the beetle before you decide anything. An adult is 1/3 to 1/2 inch long, with a metallic green head and thorax, copper-brown wing covers, and a row of small white hair patches along each side of the abdomen.

Look at the sunniest upper foliage first. The beetles feed in groups, starting at the top of a plant and working down, and they are most active on warm, sunny days.

On leaves they chew the tissue between the veins and leave a lacelike skeleton, and damaged leaves turn brown and may fall off. They also eat flowers and fruit. A single beetle eats little, but damaged leaves call in more of them, and it is the group feeding that does the severe damage.

Most beetle damage is cosmetic, and in most cases it will not kill the plant. Whether a chewed plant needs help therefore depends on its age and on what you grow it for.

On vegetables, fruit and herbs, regular harvesting in July and August also decreases feeding on the edible parts. The natural pest control guide explains how monitoring and thresholds fit into a garden-wide plan.

Minnesota Extension lists these as plants Japanese beetles usually do not damage. They are boxwood, clematis, chrysanthemum, conifers such as arborvitae, spruce, fir and pine, daylily, geranium, ginkgo, Japanese tree lilac, forsythia, common lilac, magnolia, red and silver maple, oak, redbud, rhododendron and yew.

The list says "usually," and beetle numbers swing from year to year, so a listed plant carries a lower risk without being immune. Cultivars of one species can differ too, as with flowering crabapples, where many varieties are severely attacked and some cultivars are resistant.

Adults feed on more than 300 plant species, so these lists only show where to look first. Minnesota and Kentucky extension name the following as favorites or especially prone to damage.

The Japanese beetle diet guide covers what they eat in more detail.

Adults come out of the soil from mid-June to early July in Pennsylvania and from late June or early July in Minnesota, so that is when to start checking roses, grapes and fruit.

Start when the first beetles show up, because damaged leaves attract more beetles and a plant that already has large numbers is harder to manage. Feeding lasts six to eight weeks and is mostly over by mid to late August.

Handpicking is practical and effective on a small planting or a few plants, especially while beetle numbers are low, and removing beetles also reduces how many new ones arrive. Go out in the early morning or evening, when the beetles are still cool and sluggish and drop off the plant instead of flying away.

For a plant you cannot keep picking, fine netting or cheesecloth over it during peak beetle activity keeps the beetles off. Do not cover plants in bloom that need pollination, such as fruit. Handpick until bloom ends and fruit begins to set, then cover.

In Minnesota, skip any beetle that has distinctive white eggs on it. They are eggs of the Winsome fly, a parasite, and a parasitized beetle dies within 5 to 6 days.

Traps look like the obvious answer, yet Minnesota Extension says not to use them. A trap carries a scent that mimics virgin female beetles, which is highly attractive to males, and a sweet-smelling food-type lure that attracts both sexes, and together they can draw in thousands of beetles a day.

University of Kentucky research found that many more beetles fly toward a trap than end up in it, so susceptible plants near the trap and along the flight path are likely to suffer much more damage than if no trap were used. Penn State, which also advises against traps, gives a smaller gap, saying a trap catches only 75 percent of the beetles it attracts, and the plants in your yard feed the rest.

A spray comes in when handpicking and netting are not practical or you want to add to them, and the plant is one you want protected. Insecticidal soap does not work on adult beetles, and neem oil deters feeding for several days before it has to be repeated.

Products that last longer mean fewer sprays, but pyrethroids and carbaryl are toxic to bees, so a plant in bloom points you to neem oil or Bacillus thuringiensis galleriae, which Minnesota lists as low risk or not toxic to bees.

Read the label before you buy. The plant has to be listed on it, and on a food crop the label gives the number of days to wait between the last application and harvest.

For a tree, consider a professional applicator, and leave trunk injections to a certified tree care professional. The neem mixing guide covers reading the label rate if you choose neem.

Killing the grubs under your lawn will not protect the plants above it. Adults fly long distances from untreated grass to the plants they prefer, so grub control is unlikely to reduce the number of adults on landscape plants. Treat grubs only to protect the lawn itself.

Grub damage shows up on the lawn in a few ways.

Japanese beetle grubs look like other white grubs, and only the pattern of spines and hairs on the underside of the abdomen tip tells them apart. The lawn grub identification guide walks through sampling before you treat.

Females lay their eggs in moist grassy areas, and mid-summer moisture keeps eggs and newly hatched grubs from drying out, which is why irrigated lawns often carry high grub numbers in dry summers. Egg and young grub survival are also higher in a lawn that gets frequent, shallow watering, so occasional deep watering is better for the grass and less appealing to the beetles.