Since its arrival in North America in 1916, the Japanese beetle (Popillia japonica) has become established and expanded its distribution in the United States and Canada. It is now reaching Midwestern corn and soybean fields. In corn, adults feed on corn leaves but cause more damage by clipping off corn silks, which interferes with pollination. (Photo originally published in Shanovich et al 2019, Journal of Integrated Pest Management)

By Andrew Porterfield

Since its accidental arrival in New Jersey in 1916, the Japanese beetle (Popillia japonica) has successfully adapted to its new surroundings, expanding through all eastern U.S. states and crossing the Mississippi river.

Andrew Porterfield

More successful in the U.S. and Canada than in its home country (because of climate and natural predators), P. japonica has been eating (and thriving) more recently through more Midwestern states and Canadian provinces and is now poised to spread damage to the U.S. corn and soybean crop, centered in the upper Midwest. This poses significant economic issues for soy and corn farmers, since the area boasts the world’s largest production of both crops.

The Japanese beetle has taken well to the United States. The insect’s adult form is a strong flyer and can travel over large areas, and it feeds on as many as 300 host plants. Its larval form can burrow into turfgrass and other shallow roots, while the adult beetle can cause significant foliar damage. In a new guide published this month in the Journal of Integrated Pest Management, Hailey Shanovich, graduate student in entomology at the University of Minnesota, and colleagues at UM and Iowa State University trace the beetle’s current path and provide an overview of what to expect over the coming decades as the insect expands its territory. Climate models have begun to show that areas now planted with corn and soy are suitable for the beetle but have not been infested yet.

Since its arrival in North America in 1916, the Japanese beetle (Popillia japonica) has become established and expanded its distribution in the United States and Canada. The distribution shown here comes from data from the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (as of 2018 for the U.S.) and the Canadian Food Inspection Agency (as of 2016 for Canada). (Graphic by Hailey Shanovich, originally published in Shanovich et al 2019, Journal of Integrated Pest Management)

The Japanese beetle is a new arrival for corn and soy fields. “This pest has been in the Midwest for a couple decades but mostly in urban environments,” says Erin Hodgson, Ph.D., associate professor and extension entomologist at Iowa State University and senior author on the JIPM article. “These beetles are now more commonly found in field crops, like corn and soybean, in the Midwest, and we wanted to provide a resource for crop consultants and those working in agriculture.”

Erin Hodgson, Ph.D.

The Japanese beetle is a generalist herbivore, meaning it will feed on nearly any plant available, although it has shown some preference for certain plants. In addition, host-plant resistance has been found in soy and other plants that could play a role in fending off the pest. For corn, adults feed on corn leaves but cause more damage by clipping off corn silks, which interferes with pollination. For soy, defoliation is the chief concern, as the beetles feed on leaf tissue.

Monitoring and Treatment

The first step for an integrated pest management approach, however, starts with scouting for adults in a field and estimating leaf damage to corn and soybean, Hodgson warns.

A good representative field sample is essential to determine potential beetle damage, since the insects can be found at higher numbers along the edges of a field. Scouting also is difficult because the adult beetle is highly mobile. For treatment, Hodgson and her team also recommended foliar insecticides. This need can vary between corn and soy.

For corn, Hodgson and colleagues suggest protecting silks during the first five silking days and using an insecticide if there are more than three beetles per ear, silks are less than a half inch, and pollination is less than 50 percent.

For soybean, they recommend trying to estimate whole-field defoliation (infestations can be concentrated) and sampling the entire plant, not just the top.

In soybean, Japanese beetle (Popillia japonica) feeds on leaves, causing defoliation that can range from mild (shown here) to severe. (Photo credit: Erin Hodgson, Ph.D.)

Chemical controls include soil-applied insecticides that target larvae and adults over the winter. These chemicals include imidacloprid and halofenozide, which have been effective against early stage larvae. Other effective active ingredients include neonicotinoids, neonics plus abamectin, pyrethroids, organophosphates, pyrethroids plus organophosphates, and diamide seed treatments.

Some insecticide resistance has been reported, but mostly to pesticides that have been taken off the market. However, “future management practices should be aimed at utilizing an integrated pest management (IPM) strategy to delay development of insecticide resistance of the Japanese beetle in the Midwest,” the researchers write.

In corn, Japanese beetle (Popillia japonica) infestation and damage can range from minor (left) to severe (right). (Photos originally published in Shanovich et al 2019, Journal of Integrated Pest Management)

Other Control Methods

While Hodgson’s team recommends foliar insecticides, they also reviewed other existing (and potential) management methods:

• Cultural methods can include removal of popular host plants like smartweed, evening primrose, or buckwheat and ryegrass. Reducing irrigation during peak flight periods and adding more organic matter to the soil can also help with controlling beetle populations.
• Soybeans have started to show evidence of host-plant resistance to the beetle. Some quantitative loci have been identified to resist the beetle specifically, or as well as to nematodes. No such resistance has been detected in corn.
• Biological controls include nematodes, parasitoid wasps, and bacteria. These products are still quite expensive, and often, such as in the case of nematodes, must be applied at very high rates (e.g., 250,000 juveniles per square meter) to be effective.
• A form of Bacillus thuringiensis called Btj is very toxic to Japanese beetle larvae and has been isolated from Japanese soil. However, it has not yet been successfully commercialized for sale in the United States.

“This is a highly mobile pest and they like to aggregate for mating and feeding,” Hodgson says. These behaviors can cause significant defoliation for individual plants but may not be represented field wide. Managing an active pest can be frustrating because continuous invasions of new adults can be moving into fields.”

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.

(Hodgson portrait photo credit: Rachel Kennedy)