The invasive species, which is actually native to Asia, has already become established near Milan, Italy. Switzerland is doing everything it can to stop these beetles from entering the country. In 2017, Japanese beetles were caught for the first time in precautionary traps installed along the border with Italy. What does this invasive species look like? Which host plants does it prefer? And what measures can be taken to prevent its arrival?
Popillia japonica, the Japanese beetle, is a scarab beetle native to Asia (Japan, northern China, Russian Far East) that found its way to North America early in the 20th century, where it caused considerable damage. In Europe, the beetle was first recorded in the 1970s in the Azores, where it succeeded in establishing itself in spite of the immediate measures taken to eradicate it. In 2014, this invasive species was found in Italy’s Parco del Ticino, near Milan. The beetles were probably unwittingly introduced by aeroplanes landing at Milan’s Malpensa Airport.
They then spread towards Switzerland and in 2017 were caught for the first time, in precautionary traps set up in Stabio, Ticino on the Swiss border (24 specimens caught in 2017 and 135 in 2018). In Switzerland, the species is considered a quarantine organism, meaning that infestations must be reported and combated.
Adult Japanese beetles are 8 bis 12 mm long and 5–7 mm wide, and they resemble native garden chafers (see Fig. 6). However, Japanese beetles have five tufts of white hair on each side of the abdomen and two tufts of white hair on the last abdominal segment (see Fig. 2), as well as a striking, shimmering metallic green pronotum. The elytra are brown and do not fully cover the abdomen. One way of distinguishing males from females is by their protibia (forelegs), which are. Moreover, females are slightly larger (see Figs. 3–9 in the EPPO-document).
The beetle’s eggs are around 1.5 mm long and range from translucent to cream coloured. They are laid in the ground, usually in groups of two to four, at depths of up to 10 cm.
The cream-coloured larvae of Japanese beetles are grubs clearly distinguished from those of other species by the spines arranged in a V shape on the last abdominal segment (see Fig. 2 in the EPPO-document).
The larvae also have a typical C shape, a dark head capsule with strong mouthparts, and thoracic legs. They go through three stages of development:
- L1 white, The larvae are 1.5 mm long, have legs, are typically C-shaped, and but often subsequently darken in colour; they have long brown hairs and spines, typically arranged in a V shape.
- L2 The head capsule is 1.9 mm wide and 2.2 mm long
- L3 The head capsule is 3.1 mm wide and 2.1 mm long.
Pupae (14 mm long, 7 mm wide) are initially light-coloured but darken when metamorphosing into beetles.
One peculiarity of this beetle is its typical response to danger: when it senses an approaching predator, it freezes and splays its legs.
Japanese beetles generally take one year to develop from eggs into adults, but in cooler regions the process sometimes takes two years.
The young larvae hatch two weeks after the eggs are laid in moist soil. At first, they are not very mobile and feed on plant roots. In late autumn, having reached the third larval stage, they burrow quite deep in the soil (25–30 cm and frost-free) to hibernate. In spring, as soon as the ground temperature rises above 10°C, the larvae migrate back to the upper layers of soil (2.510 cm) and again feed on roots.
After four to six weeks, the larvae pupate, hatching into adults between May and June, and immediately begin mating. Japanese beetles’ main flight period is between mid-May and mid-August. Females lay between 40 and 60 eggs, and they mate multiple times before oviposition, which ideally occurs in moist to very moist soil. The adult beetles live for about 30 to 45 days.
Japanese beetles mainly spread by being unwittingly transported but sometimes also when individuals fly off in search of suitable new habitats. At the same time, the beetles occasionally swarm. The natural range of the flight-capable beetles extends at a rate of between 3 and 24 km a year, giving them a daily radius of around 500 m. They are attracted for example by plant exudates.
The host plants of the Japanese beetle include over 300 species.
Larvae mainly prefer feeding on grass roots in wet meadows but also eat the roots of agriculturalcrops like maize and soya.
Adult beetles, by contrast, can be found eating the leaves, flowers and fruit of a range of forest trees and agricultural crops. Favoured trees include maple, chestnut, birch, hazel, plane, poplar, willow, lime and elm. Preferred crops include: apples, stone fruit species, vines, maize, soya, strawberries, blackberries, asparagus, rhubarb and roses.
Root damage caused under ground by grubs:
Grubs feed on the roots of grasses, maize, soya, tomatoes and strawberries, among other plants, damaging crops, grassy areas and lawns.
Damage caused by beetles above ground to leaves, fruit and flowers: Beetles often gather in large groups on their hostplants, eating their tops first. In so doing, they often completely devour some plants while leaving neighbouring ones virtually untouched.
On leaves, they mainly eat the tissue between the veins (skeletal feeding), causing the leaves to turn brown and drop off. Feeding traces on petals and fruit are uncommon.
Effective measures to combat Japanese beetles hinge on a sufficiently early discovery of their presence. Once the species has become established, it is very difficult to eradicate.
- In high-risk areas, pheromone traps (sex and plant attractants) can be installed. Visual inspections of host plants and the soil should also be carried out, with any suspicious signs or beetle sightings reported immediately to the respective canton’s Plant Protection Service (PPS). If the PPS confirms that there is an outbreak, an infestation area and a buffer zone are delineated. Isolated populations can be trapped en masse in pheromone traps. Such traps, however, are not suitable for combating large-scale infestations.
- In the case of smaller outbreaks, the beetles can be collected by hand.
- Unlike in the USA, in Switzerland there are currently no approved insecticides for combating larvae or beetles.
- Some biological control agents currently look promising: parasitic nematodes, entomopathogenic fungi and bacteria are used to combat Japanese beetle larvae in the soil.
In an initial laboratory experiment, Agroscope showed that fungi already used against other scarab beetles, such as cockchafers, summer chafers and garden chafers, are also effective against Japanese beetles. Further investigations under way with Switzerland’s Federal Office for Agriculture, the Canton of Ticino and Italian colleagues will hopefully determine the practicality of this method.
- Sprinklers could be banned on football pitches, lawns and golf courses during the main flight period (making these areas less attractive for oviposition, for which wet soil is preferred).
- Allowing grass to grow taller could also counteract the beetles’ spread and proliferation.
- Mechanical tillagein early autumn could drastically reduce the survival chances of larvae feeding near the soil surface.
Birds, ground beetles, shrews and moles are natural adversaries that could also help keep these invasive pests in check in Switzerland. However, they cannot fully eradicate the beetles.
Help us keep a look out for Japanese beetles!
What to do if you think you have found one:
- In the event of a suspected find, catch the beetle and keep it in a closed container, e.g. a screw-top jar, and immediately inform the cantonal PPS.
- Before returning to Switzerland from trips to Italy’s Malpensa or Parco del Ticino regions, please carefully search your luggage (rucksacks, bags, suitcases, etc.) and vehicle for ‘stowaways’ and check any accompanying pets for beetles that may have attached themselves using the small hooks on their legs.
Japanese beetles can be confused with native species, usually summer chafers or garden chafers (Phyllopertha horticola), but these species do not have white tufts of hair.