Invasive Insect Pests: The Japanese beetle

The Japanese beetle Popillia Japonica Newman (Coleoptera: Scarabaeidae) (Figure 1) has successfully settled in the USA since its introduction from Japan at the beginning of the 20th century and caused considerable damage (EPPO 2016). In addition to bioclimatic suitability, the lack of natural enemies, the spread of grassland and pasture land and a large number of host plants are factors for the high damage potential (Bragard et al. 2018). After a first discovery in Europe on the Azores in the 70s, the Japanese beetle could establish itself in Italy near Milan in 2014 (Pavesi 2014). The beetle has now reached Switzerland and was first detected in June 2017 near the border to Italy in Stabio (EPPO 2017). The Japanese beetle is a quarantine organism in Switzerland and any infestation must be reported and controlled.

The Japanese beetle is about 1 cm long and has a metallic green neck shield and two white tufts of hair on each abdomen. (Shanovich et al. 2019).

Japankäfer — Figure 1. Adult specimens of the Japanese beetle.
© Servizio fitosanitario Ticino (C. Marazzi)

The Japanese beetle can infest over 300 plant species (Fleming 1972). The larvae live mainly from grass roots in damp meadows. In doing so, they damage grasslands. The beetles feed on leaves, fruits and flowers. Affected are forest trees such as maple, beech or oak, but also crops such as apple, stone fruit, vines, corn, soya or blackberries. The development from egg to adult beetle takes one year, in rare cases even two years. The beetle hibernates as the last larval stage (Potter and Held 2002). When soil temperatures rise above 10°C in spring, the larvae migrate upwards and start feeding at the roots. The larvae pupate after 4-6 weeks. The main flying season is between mid-May and mid-August. The perfect soil for laying eggs has a medium to high soil moisture. The knowledge about the life-history was used to simulate the potential geographical distribution of the Japanese beetle under current climatic conditions (Allsopp 1996; Zhang et al. 2002; Zhu et al. 2017; Kistner-Thomas 2019).

The potential distribution and the seasonal occurrence of the Japanese beetle under present and future climatic conditions in Switzerland were estimated bioclimatically using the CLIMEX model (Kriticos et al. 2015). The simulations are based on the model parameters that were tested specifically for the Japanese beetle (Kistner-Thomas 2019) and on the climate scenarios for Switzerland. Until 2019, only in the Canton Ticino between Stabio and Chiasso were Japanese beetles caught in the traps. The simulations for today's climate conditions show that these sites have an optimal bioclimatic suitability (Figure 2). In the future, the potential distribution area could double. By the end of the century, the Swiss Central Plateau should be highly to very highly suitable for the longer-term survival of the Japanese beetle.

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Figure 2: Potential distribution of the Japanese beetle under current (1981-2010) and future (2070-2099) climate conditions. The maps show how favourable the climate at a site is for the insect's long-term survival. Results of simulations with model CLIMEX. The map on the right is based on the local climate scenario A2 for Switzerland.

This estimate shows the importance of preventing the spread of the Japanese beetle to northern Switzerland. A detailed analysis showed that cold and moisture stress could decrease in the future. In Switzerland, therefore, no decrease in suitability is visible as in Southern Europe, South America or Africa. (Kistner-Thomas 2019). An extended period of activity can be expected in all areas in the future.

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