What can I do about Japanese Beetles?

Hand Removal in the morning or evening is one of the best solutions in dealing with Japanese Beetles, but here are some further suggestions based on Plant Choices, Cultural Practices, Natural Predators and as well, information about treating with Nematodes.

Editor’s Note: The information in this article is a response to a gardener’s question posted on the Master Gardeners of Ontario Facebook Page

written by Halton Master Gardener,

Cathy Kavassalis

C- shaped Japanese Beetle larva

You have probably noticed that Japanese Beetles have favourite foods. Reducing the number of favourite host plants, (okay… it is a long list with about 350 of them) is a start. There are also many plants they don’t like (see lists below) to Welcome to your Garden. In addition, researchers have found that some cultivars of favourite foods are less attractive to the adult beetles. For instance, Crabapple ‘Jewelberry’ gets a 6% damage annually, whereas Crabapple ‘Radiant’ is 85% defoliated. If you have a passion for something in particular, you can look for less susceptible varieties. If you are into hybrid tea roses, this will be tough. You will need to try some other things.

Favourite Foods of Japanese Beetles Not-so-Favourite’ Foods of Japanese Beetles
Plants to Welcome!
Dahlia (Dahlia spp.) Begonia spp.
Grape (Vitis vinifera) California Poppy (Eschscholzia californica)
Hibiscus (Hibiscus spp.) Columbine (Aquilegia spp.)
Hollyhock (Alcea rosea) Coral Bells (Heuchera sanguinea)
Perennial Rye Grass  (Lolium perenne) Coreopsis spp.
Raspberry (Rubus idaeus) Delphinium spp.
Rose, (Rosa spp.) Dusty Miller (Centaurea)
Floss Flower (Ageratum spp)
Forget-me-not (Myosotis spp.)
Foxglove (Digitalis spp.)
Hosta spp.
Impatiens spp.
Lantana camera
Nasturium (Tropaeolum majus)
Pachysandra spp.
Poppy (Papaver spp.)
Rose Campion,Catchfly (Lychnis coronaria)
Rose Moss (Portulaca grandiflora)
Stonecrop (Sedum spectabile)
Violets (Viola spp.)

Life Cycle – June to July

Here in Ontario, adults emerge from the soil in late June through mid-July (dependent on weather conditions). They then feed, mate, and lay eggs.  JB (as I call them as I’ve squished enough) adults prefer full sun and will start at the top of plants and move downwards emitting pheromones as they feed. These odours and odours from damaged leaves attract more beetles and they begin to aggregate. The earlier you start hand removal, the lower your population will be. After about six to eight weeks the adults die off. Pheromone traps that are available, usually result in attracting more JBs to your yard. These traps are not a good solution for a typical yard. Some folks recommend companion plants as deterrents, but research here is mixed. Annual Geranium (Pelargonium × hortorum) is toxic to JBs. They are attracted to it, but it paralyzes them when they eat it. Studies are not conclusive about its effectiveness at reducing populations. Using a diversity of plants to encourage beneficial insects is advised.

Natural Predators

Starting in the 1920s, researchers began introducing natural enemies of JB. These included little parasitic wasps (Tiphia popilliavora, Tiphia vernalis) and parasitic flies (like Istocheta aldrichi, Hyperecteina aldrichi, Dexilla ventralis and Prosena siberite). Unfortunately, some of these have had a negative impact on our native beetles, but when a pest population is out of whack, they may be beneficial. In addition, there are many native generalist predators, like ants, rove beetles (staphylinids), and ground beetles (carabids) that feed on JB eggs and larvae. To the dismay of many a lawn-lover, moles, skunks, and racoons also find grubs (JB larvae) delicious. Birds, like the European starlings, crows, grackles, gulls, chickens and many others will feed on the grubs or adults. All of these creatures can assist you in reducing the JB population. You can invite them to your garden by providing habitat and shelter, then encourage them to feast.

Cultural Practices

After the beetles emerge, feed and mate, the females begin laying eggs. She burrows down in soil (about 5 to 8cm) and deposits an egg.

  • Reduce or eliminate your lawn/turf areas. Roots from Perennial Rye Grass are some of the favourite food of the JB’s larvae
  • Wood chips around the base of plants can deter her from laying as she prefers moist, soft soil near the favourite food of her babies like perennial ryegrass (Lolium perenne), but will also feed on the roots of various ornamental and food plants (like my strawberries!).
  • To discourage her from laying eggs, STOP watering your lawn during July and August. Let it go dormant (Start irrigating your lawn again in late summer and autumn to promote recovery).
  • JBs don’t like tall grass, so increase your lawn height to 8cm.
  • Fertilize in autumn to promote root growth and enhance tolerance.
  • Some research suggests a spring application of aluminum sulfate to acidify the soil may reduce grub populations.
  • There is also some evidence that using endophytic perennial ryegrass or fescues (grass seeds treated with fungal endophytes Neotyphodium spp.) can make the larvae more susceptible to entomopathogenic nematodes.

….AND, that brings me to the next deterrent …nematode treatments.

Nematodes – Cool but Gross Allies

Nematodes (Entomopathogenic nematodes, like Steinernema carpocapsae, Steinernema feltiae and Heterorhabditis bacteriophora) are a group of thread worms that feed on JB larvae. They drill inside the soft bodies of young grubs and release associated mutualistic bacterium that dissolve the innards of the grubs and the nematodes and bacteria feed on the liquefied guts … cool, but gross. These nematodes are sold in garden centres, but timing is critical.  For this, we need to return to the Japanese Beetle life cycle…

Timing Nematode Treatments to JB Life Cycle – July to September

Our female JBs begin laying her eggs in July. Eggs hatch within a week or two and tiny grubs emerge. This is called the first instar. Mid summer rains keep these alive (in dry years they will die off). Within the next 20 days, the first instar molts to form the second instar. By late September, the grubs will be almost full-sized (about 2.5cm long) and typically are a third instar. As fall arrives and soil cools to about 15°C, the grubs move deeper, perhaps around 20 to 25cm below the surface. In spring when temperatures rise above 15°C, they become active again and rise to the surface and feed again before pupating in late May and June. Then the cycle starts again.

  • Research suggests late summer or early fall (after extreme heat has passed) and when larvae are still small (1st and 2nd larval stages/instars) and near the surface is the best time to apply nematodes.
  • Lift a square foot of turf in late summer to determine if the number of larvae merit treatment and to verify the stage of development. Eight to ten beetles in a square foot is the standard threshold for treating grubs in the lawn. (Below that, lawns will recover, and above that they will decline)
  • Nematodes are sensitive to heat, soil moisture, exposure to sunlight, and many other soil factors. Apply nematodes in the evening and keep the lawn moist; essentially, follow the directions.

The longevity of nematodes in the soil is very difficult to predict, but research suggests that they generally don’t persist long, due to a variety of biotic and abiotic factors. In the study noted below, nematodes declined significantly after just four weeks from their application (likely due to an increase in arthropods, nematode pathogenic fungi, as well as sensitivity to changing soil conditions).

Cathy Kavassalis – Halton Master Gardener

Further Reading:

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