A new study modeling potential future climate-change scenarios finds the range of suitable habitat for the Japanese beetle (Popillia japonica) would shift northward, while parts of the world below 40 degrees north latitude would see showed contractions in suitable habitat because of heat stress. (Photo credit: Joseph Berger, Bugwood.org)

By Andrew Porterfield

Japanese beetles (Popillia japonica) were never a major problem in their origin country of Japan, but they quickly became a major agriculture and home garden pest in North America after they were discovered in New Jersey in 1916.

Andrew Porterfield

Since then, warmer temperatures and a lack of natural predators that kept it in check in Japan have encouraged the beetle’s spread to every U.S. state east of the Mississippi River except Florida, plus some infestations in midwestern states and eastern Canada. A few occurrences have been found (and eradicated) in California and Nevada, and in 2014, P. japonica was found in mainland Europe for the first time, in the Ticino Valley in Italy.

The beetle feeds on at least 300 species of plants. The beetle typically produces one brood a year, and both larval and adult forms are considered destructive pests. So far, control has been limited to chemical treatment on isolated populations, and quarantines of shipments of plants from the eastern to the western United States (and nine provinces in Canada).

Climactic conditions also have been behind much of the success of the Japanese beetle. Thus, a question for researchers studying the beetle is what impact could climate change have on its infestations and survival?

Erica Kistner-Thomas, Ph.D.

To answer that, a research team led by Erica Kistner-Thomas, Ph.D., climate hub fellow at the U.S. Department of Agriculture’s Agricultural Research Service in Ames, Iowa, compared data on current niches occupied by the Japanese beetle with data models that project how different scenarios in climactic change over the next few decades could change the range of P. japonica. Their research is one of only a few studies to compare climate change and Japanese beetle ecology, and it was published in late March in the Journal of Insect Science.

Some earlier studies suggested that a projected 3.5 degree Celsius increase in temperature by 2050 in the midwestern United States could boost the feeding window for beetles by almost 300 percent. At the same time, models have predicted that the same climate change could reduce beetle populations in places like Arkansas, because of hotter and dryer conditions there. Kistner-Thomas’ team took the work a step further by looking at whether certain climate models could accurately predict current populations, and then they looked to see how those models determined future distribution of the beetle.

The researchers first detailed the known distribution of P. japonica, mapping 1,028 locations where the beetle is known to live, including 40 in Asia (Japan and Russia), six in Europe, 981 in North America, and one in Central America.

They then used CLIMEX software, a license-only modeling program that can help determine the effects of climate on distribution of non-warm-blooded organisms. CLIMEX software assumes that climate is what primarily determines the geographic distribution of a species, giving what it calls an Ecoclimatic Index, which scales from 1 to 100 the suitability of climate to a given species.

Current distribution was based on data gathered for the beetle from 1981 to 2010. Two model scenarios were used that represent variations in sensitivities of the Earth’s climate to a doubling of carbon dioxide by 2050. The team assumed that global greenhouse emissions were tracking along what climate scientists call the worst-case scenario of an average increase in global temperatures from 1.3 to 2 degrees Celsius by 2050.

A new study modeling potential future climate-change scenarios finds the range of suitable habitat for the Japanese beetle (Popillia japonica) would shift northward. Compared to it current range in North America (A), the southern U.S. showed range contractions in the two future scenarios (B and C), while ranges expanded significantly in British Columbia, Saskatchewan, and Manitoba in Canada as well as in New England and the northern Midwest in the U.S. (Image originally published in Kistner-Thomas 2019, Journal of Insect Science)

The climate model data of current distribution of P. japonica matched well to known existing populations of the beetle. For the future, the models showed that the range of suitable habitat shifted northward overall. Parts of the world, particularly areas below 40 degrees north latitude, showed contractions in suitable habitat because of heat stress (above 93.2 degrees Fahrenheit, or 34 degrees Celsius.) The southern U.S. showed range contractions of about 29 percent, while ranges expanded significantly in British Columbia, Saskatchewan, and Manitoba in Canada and also expanded in Europe to parts of Norway, Sweden, Finland, Ireland, and the United Kingdom. Possible expansion also pointed to more future beetles in New England and the northern Midwest in the U.S.

The model “fit” well enough to existing data to bolster the accuracy of the model’s predictive ability, Kirsten-Thomas says. “It is not surprising that projected rising temperatures in the northern latitudes enables the invasive Japanese beetle to expand its range northward, in these model situations,” she says. This has also been shown in other insects. “The southern pine beetle, a native forest pest previously restricted to the pine forests of the southeastern United States, has recently established in parts of the northeastern U.S. including New York and Connecticut.”

These studies, while still based on modeling and not empirical data for future populations, still underscore the need for awareness of the potential for the beetle to invade what will become suitable habitat, if it hasn’t been suitable already.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.