Japanese beetles: what damage to look out for & how to get rid of them

The Japanese beetle, beautiful yet destructive, can decimate your garden. Continue reading to find out more about this interesting insect.

Japanese beetles, Popillia japonica, can be quite damaging in your garden. They have voracious appetites, both as larvae and as adults. This article details the information you need to spot their damage, and prevent them from becoming a problem.

Japanese beetle: origins & why they are harmful

Popillia japonica is a member of the chafer or scarab family of beetles, Scarabaeidae, commonly known as the Japanese beetle. They are native to Japan, where population levels are controlled by natural predators. In other parts of the world, such as the USA and parts of Europe, however, they spread quickly and have become highly destructive invasive pests.

Japanese beetles have been found to be significant pests on turf and forest trees, as well as ornamental plants and crops. They are known to be polyphagous, feeding on around 300 known host plant species from 80 different plant families. Some of the favoured hosts found in the UK include Acer (maples), Althaea rosea (hollyhock), Asparagus officinalis, Betula (birch), Fagus (beech), Fragaria (strawberry), Larix decidua (larch), Malus (apples), Populus (poplars), Prunus (stone fruit), Quercus (oak), Rhododendron, Rosa (roses), Rubus idaeus (raspberry), Tilia (limes), Trifolium (clover), Ulmus (elms)., Vaccinium (blueberry), Viburnum, and Vitis (grapevine)., and Zea mays (maize), as well as many turf grass species.

Japanese beetles were first introduced in the United States in the early 1900’s in the state of New Jersey. They quickly moved throughout the eastern states and have spread west throughout most of the country, and even throughout the southern parts of Canada. In the 1970’s they were introduced to the Azores and were first discovered in mainland Europe in 2014. In Europe, Japanese beetles are regulated as quarantine plant pests by the European Union.

Tip: organisms that cause significant economic and environmental damage must be restricted and their entry to new areas prevented. These organisms are classified as quarantine pests.

Life cycle of the Japanese beetle

Proper identification of Japanese beetles goes along with knowing and understanding their life cycle. In July through September, adult females lay their eggs in soil near plants upon which they are feeding. Adult females go through several cycles of mating and egg laying. They can lay 1 to 5 eggs per cycle before returning to feed on plants and mate again and will lay over 40 eggs throughout their lifespan. Within 2 weeks, the eggs hatch and develop into larvae. In warmer climates the larvae hibernate over winter to develop into pupae, whereas in colder climates, larvae will hibernate over two winters in order to fully develop into pupae. The pupae emerge from the soil in the spring as mature adults.

Eggs: vary in their size and shape. They can be round, elliptical, and even cylindrical and measure up to 1.5mm in length. Eggs can be translucent to creamy-white in colour with hexagonal surface patterns. They are either placed individually or in groups under the soil surface.

Larvae: are C-shaped, white to cream in colour with well-developed legs and a distinct tan-coloured head capsule. They can only be distinguished from other white larvae through the pattern of spines and hairs on the tip of the underside of their abdomen.

Pupae: resemble adults and are 7mm wide and 14mm long. Their wings, legs, and antennae are functionless and held close to their bodies. Their colours change from cream to tan as they mature and eventually emerge from the soil as mature adults.

Adult: Japanese beetles are distinct in their emerald, metallic-green bodies with copper-brown elytra or wing cases. They look quite similar to other chafer beetles with the exception of their unique white tufts that extend out from the sides of their abdomen under their elytra. Females are larger than males and can be up to 14mm long and 7mm wide.

Damage: what to look out for

Usually damage to plants is the first visible sign of a Japanese beetle infestation. After noticing plant damage, a closer inspection should take place to track population cycles and numbers.

Typical damage caused Japanese beetle by larvae

Japanese beetle larvae are soil-bound and feed on the roots of plants, especially turf and lawn grasses. The damage is noticed as large brown, dead or dying patches as the larvae feed on the roots. There are many species of insect pests whose larvae cause similar damage. In order to be sure which species is causing the damage, dig up a brown patch to uncover the larvae causing the damage. As was mentioned earlier, the larvae are identified by their unique pattern of spines and hairs on the tip of the underside of their abdomens.

Typical damage caused by adult beetles

Plant damage caused by adult Japanese beetles is easy to identify. While they generally prefer foliage, they also frequently feed on flowers and fruit. Their feeding on foliage results in lace like leaves as they skeletonize the leaves, feeding on the softer tissues and leaving the vascular tissue of the veins and midribs. Other insects cause similar damage to plants, so always take a closer look at the plants and surroundings. The adult Japanese beetles will always be found close to the plants they are feeding on. Always check for the tell-tale white tufts on their abdomens to identify the emerald-green and copper-brown beetles.

Tip: Japanese beetles (Popillia japonica) typically feed from the tips of the plants down so it is usually quite easy to spot their damage.

How to get rid of Japanese beetles

There are many ways to rid your garden of Japanese beetles. The most ecological methods involve manually removing them or using biological controls or bio-insecticides. Manually removing and disposing of the adults can be quite effective and is best done in the mornings when the insects are slow-moving. Using a bucket of soapy water, pluck the beetles off the plant or knock the plant causing the beetles to fall into the bucket where they will drown in the soapy water. This will have to be repeated multiple times throughout the growing season as adults are mobile and can fly from area to area. For high-value or individually isolated plants, horticultural mesh and netting barriers can be effective at keeping Japanese beetles from causing damage. One product to avoid are pheromone traps. Studies in the USA have shown that these attract more adult Japanese beetles to an area than they trap, thereby increasing localised damage. These traps are designed to be used in special circumstances to only monitor population levels.

Tip: the first step to managing an outbreak of Japanese beetles is to capture a specimen, properly identify it, and report it to the relative authority.

The larvae are harder to control manually but there are biological control agents that can manage populations. Two specific and effective biological controls are the Heterorhabditis bacteriophora (HB) parasitic nematodes and milky spore (Paenibacillus popilliae). Both organisms will attack and destroy the larvae, preventing them from causing further damage to turf and maturing into adults to cause more damage and breed. Our Plantura HB Beneficial Nematodes are a highly effective biological control agent for Japanese beetles and does not cause any damage to the soil or surrounding plants. For best results, apply our nematodes in the late summer or early autumn while soil temperatures are optimal.

Ways to prevent Japanese beetles

As a bit of prevention is worth more than a cure, it is good to be proactive against possible future outbreaks.

• Autumn soil cultivation in your garden is a great way to monitor for Japanese beetle larvae and destroy those that are found.
• During outbreaks of Japanese beetles, avoid excess irrigation, as damp, moist soil is preferred for egg laying and egg viability.
• Promote biodiversity. While Japanese beetles are not known to be palatable by birds, other beetles are. Biodiverse gardens are known to have fewer issues with pests when compared to monocultures.
• Protect susceptible specimens and high-value crops with horticultural fleece and fine netting.
• In new plantings, choose species that are less palatable to Japanese beetles. Some examples include box (Buxus sp.), Clematis sp., mums (chrysanthemum sp.), daylilies (Hemerocallis sp.), geraniums (Geranium sp.), ginkgo (Ginkgobioloba), and lilacs (Syringa sp.), among others.

Have large brown patches in your lawn that could be from possible beetle larvae damage? Read our in-depth article on lawn grubs, common culprits & how to combat them.