Popillia japonica - an invasive species coming to Europe

IPM-Popillia aims to counteract the spread of one of the most important recently introduced pests. The Japanese beetle, Popillia japonica, is a quarantine pest to Europe (Council Directive 2000/29/EC) and a candidate to be listed as a high priority pest in the EU. The new EU Plant health Law foresees eradication or containment of such pests. Given the impossibility of eradication, the overall objective of IPM-Popillia is to provide environmentally safe containment of P. japonica.
In Europe, P. japonica has so far only occupied a very small spot within a large area with a suitable climate (Pavesi 2014). Pest risk mapping based on climate indicated that the area at risk for P. japonica invasion ranges from the Atlantic to the Black Sea, and from the Mediterranean to Northern Germany, Great Britain, and Southern Scandinavia (Kistner-Thomas 2019). Furthermore, there is evidence that high human activity may facilitate P. japonica establishment in regions with per se unsuitable climate (Zhu et al. 2017). This provides space for the virtually unlimited spread of P. japonica in Europe within the next years. The impressive expansion history of the Japanese beetle in the US underpins this threat (Fleming 1972, USDA/ APHIS 2015).

How to identify a Japanese beetle

Like typical beetles, Japanese beetles have six legs and two antennae. They have wings to fly but are clumsy flyers and often bonk into objects. Their oval body is of a length of approximately 10 mm. The head and thorax (middle part) of their body are shimmering in metallic green, and their wing coverings are copper-brown. Toward their back end, a row of five patches of white hairs covers each side of their body, and there are two white patches on the tip of their back end. The white patches easily distinguish the adult Japanese beetle from other beetles that resemble it. The larvae that develop in the ground can hardly be identified without the use of a binocular.

From Asia to North America and Europe

As its name implies the Japanese beetle is originally from northern Japan and the far east of Russia. There, it is a minor pest as it has several natural enemies. Additionally, the local climate and unsuitable terrain for the development of the larvae help prevent population growth. In 1916, the Japanese beetle was first discovered in the US probably introduced with plant material from Japan. After this first discovery outside of its home range, the beetle moved to several other countries, always causing severe damage. It was first spotted in Italy in 2014, and ever since its population there has exploded. It has started to move northwards with its first discoveries at the southern border of Switzerland in 2017.

During its invasion to California, starting in 2010, the potential damage of P. japonica to fruit production was estimated to exceed 6.2 billion dollars, based on the gross production value of fruit at risk in this country (CDFA, 2016). Due to its recent appearance and so far limited distribution, data on economic losses caused by the P. japonica invasion into Europe are not available so far. However, the annual costs to control the pest in the US exceed $460 million (USDA/APHIS 2015). 

Damage from adult beetles

Adult beetles generally feed on leaves, chewing out the tissue between the veins, only leaving the leaf skeleton. They prey on more than 300 plant species, including vegetable crops, flowering plants, trees, and shrubs. They show a preference for leaves of grapes, fruit-bearing trees, and soy. However, they also feed on ripe fruit for example of varieties of plums, raspberries, or apples, and show a preference for rose blossoms. Depending on food supply, they can also feed on the leaves of forest trees including oaks or linden. Because of this wide host range, the Japanese beetle is a severe threat to many sectors in agricultural production. Winegrowers in the infested zone in northern Italy experienced severe damage caused by the invasive pest in 2019, with an average of 200 (!) beetles per plant causing a 30% reduction of the leaf surface of vines within a feeding period of 10 days only (Vignaioli Piemontesi, pers. comm).

Damage from grubs

The damage of Japanese beetles is not limited to their adult form. Their larvae feed underground just below the surface, where they consume roots and thereby reduce the vitality and yield of their host plants. These grubs prefer roots of lush pasture and turfgrasses but can also feed on roots of maize, beans, tomatoes, strawberries, or nursery seedlings. Predators such as wild boars or crows use the grubs as a food source and cause secondary damage when digging for them. The digging can cause more disruption to the sward than the initial damage by the grubs. Measures for containment of this regulated pest complicate the trade of fruit crops, potted plants, or turfgrass (National Plant Board 2016).

Life cycle

Usually, there is one generation of Japanese beetles per year. The larvae overwinter in the soil. As soil temperatures rise in early spring, they move closer to the surface and start feeding on roots. After a few weeks of feeding, they pupate. The adult beetles emerge from Mid-May to Mid-July, depending on latitude. Females live 30-45 days and can lay up to 60 eggs in this period, preferably in moist, loamy soil covered with pasture grasses. After hatching, the young larvae feed on fine roots and organic matter and develop until the third larval stage. With declining soil temperatures in late autumn, the grubs move deeper into the soil to overwinter.

Environmental concerns

Besides the economic impact, the P. japonica invasion of Europe raises strong environmental concerns. There is evidence that increased pesticide use negatively affects biodiversity (REF), and this is likely to occur in the course of the Japanese beetle’s invasion. A simple estimate on pesticide use in viticulture, the agricultural sector most affected to date in Europe, may serve as an example: Vineyards with a heavy infestation of Japanese beetles in the infested zone in the Piedmont region are treated with two extra insecticide applications a year. Farmers use either pyrethroid-based or neonicotinoid-based products. Currently, the area of about 50 hectares receiving these additional treatments is still small. However, P. japonica has not infested the core area of Piedmont’s wine-producing region so far, which is only about 100 km away from the current infested zone and comprises about 45’000 hectares of vines in total. In a worst-case scenario with the whole region infested by the invasive pest, plant protection measures against Japanese beetles in Piedmont viticulture alone would sum up to an additional release of 30’000l to 90’000l of insecticides a year. Considering the total area under vine in Europe, which is more than 3’000’000 hectares, there is no doubt that the environmental impact of the Japanese beetle’s invasion will be huge. Other crops at risk may cover smaller areas (e.g. soft fruits) or maybe sufficiently protected with a single pesticide application (e.g. soybean), but will nevertheless contribute to an overall increase of pesticide use and, consequently, increased threat to the European biodiversity.

P. japonica adults are also able to feed on leaves, buds, and fruits of a variety of forestry plants. The economic impact on forestry is probably negligible but can be significant for woody nursery crops (Giroux et al. 2015). Nevertheless, nutritional resources in forests are an important factor facilitating the further spread of the pest. In addition, the Japanese beetle is likely to cause environmental loss when damaging plants in wild habitats. In the US, the expansion of this invasive pest conflicts with Monarch butterfly conservation measures. Japanese beetles aggregate and feed on flowers of Asclepias syriaca, the monarch’s most important larval food plant, which may reduce fruiting and seed set by more than 90% (Baker and Potter 2018).

Climate change impacts

There is evidence that climate change will exacerbate the threat by the Japanese beetle’s invasion into Europe. It is not only the distribution borders of P. japonica, which may be pushed north- and also southwards, but also its voltinism that will change as a consequence of climate change (Kistner-Thomas, 2019). Regions along the northern border of the Japanese beetle’s current distribution, like Hokkaido in Japan, or southern Ontario in Canada, have a lower risk of P. japonica outbreaks due to the insect’s bi-annual development in colder climates. A similar lower-risk scenario, with P. japonica mainly developing over two years, is anticipated for most of central and northern Europe under current climatic conditions. This may however change dramatically with an increase in cumulative growing degree-days in northern latitudes, leading to a reduced probability of bi-annual life cycles in P. japonica development. As a consequence, not only North America or Asia but also central and northern parts of Europe will face a higher risk of Japanese beetle outbreak populations.