Popillia japonica: A Brief Introduction to the Japanese Beetle
In 1916, Popillia japonica, an iridescent insect of the family Scarabaeidae, descended upon North America’s crops, fields, and forests (Shanovich et al., 2019). As its colloquial name suggests, the beetle likely arrived through Japanese imports: rhizomatous shipments of Iris ensata (Shanovich et al., 2019). The invasion was first detected in New Jersey, and since the discovery, P. japonica has quickly spread throughout the eastern States, its distribution extending into most Canadian provinces (Allsopp, 1996).
Measuring between 8 and 11 mm in length, P. japonica adults boast brightly coloured bodies (Shanovich et al., 2019). Tufts of setae border the beetle’s elytra: spots of white beneath coppery wings (Shanovich et al., 2019). Males and females are similar in appearance, each bearing lamellate club antennae (Jackson & Klein, 2006), which sprout from a brilliant green head (Britton, 1928). The sexes can be differentiated from each other by the tibial and tarsal shapes of the frontmost legs (Shanovich et al., 2019): male tibiae involve spurs, facilitating copulative postures (Timmerman et al., 2001), while the female tarsi are longer and more slender (Shanovich et al., 2019). Feeding habits also distinguish males from females, as female pioneers tend to initiate aggregations, consuming various plants, which release attractive volatiles (Shanovich et al., 2019). Accordingly, botanical compounds, as opposed to pheromones, are apparently responsible for congregations: the males join the females, leading to gregarious copulation and extensive foliar damage (Shanovich et al., 2019).
As a generalist herbivore, the Japanese beetle’s destructive reputation applies to 79 families and over 300 species of wild and cultivated plants (Shanovich et al., 2019). Roses, for example, are particularly susceptible to herbivory, constituting P. japonica’s most preferred hosts (Shanovich et al., 2019). Weedy perennials, such as Oenothera biennis, are often targeted by emerging adults; as summer progresses, P. japonica frustrates agricultural yields, skeletonising soybean leaves and compromising corn (Shanovich et al., 2019). Asiatic fruit trees—such as cherry, peach, and plum—are also at risk of infestation (Britton, 1928). The scarabs voraciously devour the canopy, decimating flesh and defoliating limbs (Britton, 1928). North American varieties of Acer, Prunus, and Betula can likewise experience the Japanese beetle’s brunt (Shanovich et al., 2019). The insect feeds from the uppermost foliage, proceeding towards the lower branches: it prioritises sunlit leaves, as the tissues are superior in sugar content to the understorey’s shaded options (Shanovich et al., 2019).
In northern Japan, P. japonica’s populations are relatively inconsequential (Shanovich et al., 2019). A combination of unsuitable terrain and coevolved enemies has rendered the scarab manageable: a minor agricultural pest (Shanovich et al., 2019). Without such ecological challenges, the species can relentlessly thrive: American soil and turfgrass provide ideal conditions for pupal development (Shanovich et al., 2019). A larval mortality rate of 100%, for instance, requires temperatures below −9.4℃ (Kistner-Thomas, 2019). In the coldest limits of P. japonica’s North American range, relevant soil layers rarely plummet to subzero extremes, as snowfall accumulates throughout the winter, producing insulation (Kistner-Thomas, 2019). Comparably, with respect to the scarab’s southernmost colonies, North American summers often average temperatures of >31℃ for up to 10 cm of subterranean depth (Kistner-Thomas, 2019). As optimal habitats assume temperatures between 27.5℃ and 32℃ for P. japonica’s embryonic, larval, and pupal growth, North America effectively accommodates the beetle’s proliferation (Kistner-Thomas, 2019).
Efforts to control the invader are consequently difficult (Shanovich et al., 2019). In recognition of native P. japonica’s interspecific relationships, attempts to thwart North American counterparts have largely explored biological controls (Kistner-Thomas, 2019). Tiphia vernalis, for example, a parasitoid wasp, was introduced from Korea in 1925 (Bartlett et al., 1978). The predator has since established permanent networks, preying on immature scarabs by following underground kairomones (Shanovich et al., 2019). Similarly, microbial interventions have hindered the beetle’s success: the Buibui strain of Bacillus thuringiensis, for instance, has demonstrated incredible toxicity against P. japonica larvae (Shanovich et al., 2019). Paenibacillus popilliae and Paenibacillus lentimorbus also contribute to Japanese-beetle suppression: the bacteria multiply over 2–4 years, precipitating pernicious milky disease among insatiable grubs (Shanovich et al., 2019).
Despite P. japonica’s herbivorous potency, certain plants can synthesise resistance to the threat (Shanovich et al., 2019). The neem tree, for example, Azadirachta indica, causes appreciable death among the beetle’s third instars (Shanovich et al., 2019). Extracts of the toxins can offer protection for other botanical hosts: when applied to leaves and petals, the compounds significantly reduce attacks, persisting with residual repellency for several days (Shanovich et al., 2019). In consideration of such phytochemical strength, natural insecticides comprise an important area of research for P. japonica mitigation. As the beetle is annually culpable for over $450 million in American turfgrass loss, preventing invasive spread remains a crucial focus of environmental initiatives (Shanovich et al., 2019). Nevertheless, with climate change exacerbating the scarab’s pervasiveness, P. japonica may soon occupy North America’s western coasts, becoming ubiquitous throughout the continent (Kistner-Thomas). As scientists continue to study the Japanese beetle’s ecology (Kistner-Thomas), perhaps new discoveries in plant metabolites will afford a balanced solution for P. japonica and its hosts: defensive molecules conducive to sustainable interactions.
References
Allsopp, P. G. (1996). Japanese beetle, Popillia japonica Newman (Coleoptera: Scarabaeidae): Rate of movement and potential distribution of an immigrant species. The Coleopterists Bulletin, 50(1), 81–95. http://www.jstor.org/stable/4009259
Bartlett, B. R., Clausen, C. P., DeBach, P., Goeden, R. D., Legner, E. F., McMurtry, J. A., Oatman, E. R., Bay, E. C., & Rosen, D. (1978). Introduced parasites and predators of arthropod pests and weeds: A world review (C. P. Clausen, Ed.). Agricultural Research Service: United States Department of Agriculture.
Britton, E. G. (1928). The Japanese beetle. Torreya, 28(6), 107–109. https://www.jstor.org/stable/40596582
Coniferconifer. (2014). Popillia japonica [Photograph]. Wikimedia Commons. https://upload.wikimedia.org/wikipedia/commons/b/b6/Japanese_Beetle_%2814903709119%29.jpg
This image is licensed under CC BY 2.0.
Jackson, T. A., & Klein, M. G. (2006). Scarabs as pests: A continuing problem. Coleopterists Society Monographs: Patricia Vaurie Series, 5, 102–119. http://www.jstor.org/stable/4153166
Kistner-Thomas, E. J. (2019). The potential global distribution and voltinism of the Japanese beetle (Coleoptera: Scarabaeidae) under current and future climates. Journal of Insect Science, 19(2), 1–13. https://doi.org/10.1093/jisesa/iez023
Shanovich, H. N., Dean, A. N., Koch, R. L., & Hodgson, E. W. (2019). Biology and management of Japanese beetle (Coleoptera: Scarabaeidae) in corn and soybean. Journal of Integrated Pest Management,10(1), 1–14. https://doi.org/10.1093/jipm/pmz009
