Biology and control of the invasive Japanese beetle

The Japanese beetle (Popillia japonica) belongs to the Scarabaeidae family (scarab beetles), and originally comes from Asia (Japan, northern China, the Russian Far East). It was introduced to North America at the beginning of the 20th century and causes considerable damage there (to the tune of several 100 million US dollars annually). In Europe, the beetle was first discovered in the Azores in the 1970s, and on the mainland for the first time in 2014, in Italy near Milan. It is thought to have arrived there by plane, and has ultimately been able to spread further from there.

• The Japanese beetle in Europe

Adult beetles are 8–12 mm long and 5–7 mm wide. They are similar in appearance to our native Carabus hortensis beetles. Unlike them, however, the Japanese beetle has five white tufts of hair on each side of the abdomen and two white tufts on the last abdominal segment, as well as a striking, metallic green-shimmering neck shield (pronotum, fig. 3). The elytra are brown and do not completely cover the abdomen.

The Japanese beetle is sometimes confused with native beetles - most commonly with the June beetle (also known as garden chafer or garden foliage beetle, Phyllopertha horticola). This beetle has no white tufts of hair, however.

The three larval instars:

The larvae pupate after four to six weeks.  The initially light-coloured pupa (14 mm long and 7 mm wide, fig. 9) darkens in colour as it metamorphoses into a beetle. The adult beetles hatch between May and June. They start mating immediately. Adult animals live for 30 to 45 days.

With over 300 known host plants, the Japanese beetle has the potential to threaten a wide range of plants and habitats. Adult beetles are to be found on a variety of forest trees, garden plants or agricultural crops, where they feed on the leaves, flowers and fruit. These include: maple, alder, chestnut, birch, hazel, beech, oak, plane, poplar, willow, lime, elm, larch and, among cultivated plants, especially apple, stone fruit, persimmon, grape vines, maize, soya, strawberries, blueberries, bilberries, blackberries, raspberries and strawberries, wisteria, hibiscus, asparagus, rhubarb or roses.

Early summer damage to trees such as alder and hazelnut can be found immediately after the beetles hatch, before they move into the vineyards to feed on leaves or to mate.

In autumn, the beetles are often found on common loosestrife (Lythrum salicaria) or red clover (Trifolium pratense) – plants with purple/wine red flowers. The beetles do not fly to neighbouring clover plants with white flowers, for example. These plants are found in the more moist areas next to the areas planted with vines and in the areas bordering on forests or rows of trees.

The beetles are mainly transported over long distances from infested areas by vehicle or means of transport (i.e on trains, postal vans, lorries, or with goods transport (e.g. rolled turf, soil transport, plants, fruit deliveries, green waste), as well as by commuters or tourists, who unwittingly carry the beetles as “stowaways”. They can be carried in/on cars or coaches, on clothing, or in camping equipment, rucksacks, camera bags or with imported plants - for example roses from infested areas. Anyone visiting an infested area can thus unknowingly contribute to the spread of the pest – and this must be prevented at all costs.

In Switzerland, Japanese beetles have been discovered in fruit shipments (berries, table grapes) from abroad. With the consignments of fruit, the beetles can quickly be transported to new, previously infestation-free areas. Berries that are harvested and dispatched during the beetles’ reproductive season pose a particular risk for the introduction of the pest. The table grape season tends to be at the end of the beetle's development period, so the risk of damage caused by released insects from consignments of grapes is considered less by international experts.

Where large irrigated areas are available, especially permanently irrigated areas, countless larvae are to be found. In non-irrigated cultivation areas, on the other hand, only a few larvae are found, as the development conditions there are not ideal for them. The females of the Japanese beetle select breeding sites with a high degree of soil moisture in which to lay their eggs, to ensure good starting conditions for the eggs and larvae.

Automatic irrigation of turf or lawns in stadiums, at outdoor swimming pools, in gardens, on football pitches or lawns in parks or on campsites is thus a major problem, as the beetles can develop there both easily and unnoticed. They only become noticeable when lawns die as a result of them feeding on the roots or when the beetles infest other plants in the vicinity.