Japanese Beetle Adults Emerging Now

June 13, 2024
ICM News

Japanese beetle development seems to be a bit ahead of schedule this year, which is not surprising with the warm temperatures this spring. Japanese beetle adults begin emergence when approximately 1,030 growing degree days (GDD; base 50°F) have accumulated since January 1 and will continue emerging until 2,150 GDD have accumulated. Japanese beetle adults likely began emerging in the southern portion of the state last week, and emergence will likely occur within the next two weeks in northern Iowa (Figure 1). Entomologists in Missouri and Illinois have already confirmed emergence of Japanese beetle adults in their states.

Growing degree days map.
Figure 1. Growing degree days accumulated (base 50°F) in Iowa (January 1 to June 12, 2024). Adults begin emerging after 1,030 degree days. Map courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy.

Japanese beetles have a wide host range that includes many species of fruit and vegetable crops, ornamentals, and field crops. Feeding will look a bit different depending on the host plant, but since beetles have chewing mouthparts, they consume plant tissue and cause defoliation. In field crops, adults tend to be denser at the field edge. Additionally, a combination of sex pheromones, aggregation pheromones, and feeding-induced plant volatiles lead to clusters of Japanese beetle adults feeding in one spot. Since feeding can look severe near the edge of fields, it is important to scout the entire field to get a representative estimate of injury.

On soybean, adults prefer to feed between the leaf veins and can ultimately consume most of the leaf, leaving a skeletonized appearance (Photo 1). If adults are still present in the field, the treatment threshold for Japanese beetle in soybean is 30% defoliation before flowering (R1) and 20% after R1. It is important to note that defoliation can be caused by several pest species with chewing mouthparts. Estimations of defoliation should be made for the entire field and plant canopy. Humans tend to overestimate defoliation; use the Crop Protection Network’s Insect Defoliation Tool to train your “defoliation eye.”

Japanese beetle adults defoliate soybean and skeletonize leaves.
Photo 1. Japanese beetle adults defoliate soybean and skeletonize leaves, meaning they leave behind the leaf veins. Photo by Mark Licht.

Japanese beetles do not typically feed on corn leaves, but adults have a long emergence period and remain active throughout the summer. Once corn reaches VT or R1, consider scouting for Japanese beetle. Adults can feed on both the tassels and silks. Silk clipping can interfere with pollination (Photo 2). Consider a foliar insecticide during tasseling and silking if three criteria are met: there are 3 or more beetles per ear, silks have been clipped to less than ½ inch, AND pollination is less than 50% complete.

Japanese beetle adult on corn.
Caption: Japanese beetle adults aggregating and feeding on corn silks near the field edge. (Photo by Ashley Dean.)

Foliar insecticides are generally effective on Japanese beetles, but adults are highly mobile and could reinfest a field within a few days. For more information on Japanese beetle identification, sampling, and management in corn and soybean, read our encyclopedia article or review this article in the Journal of Integrated Pest Management.

Japanese beetles are known to occur throughout most of Iowa, but there are 22 counties that have never reported Japanese beetle. If you see Japanese beetle in a county not highlighted in this map, let us know by emailing bugtraps@iastate.edu so we can update the distribution record.

Be aware of look-alikes! The adult sand chafer is often confused with the Japanese beetle and typically emerges around the same time. Sand chafers are dull in comparison to the Japanese beetle: they have brown bodies and brown hairs on their abdomen, while Japanese beetles are bright green and coppery brown with white hairs on the abdomen. See this video for more information: https://www.youtube.com/watch?v=W3EzCXAUGTw.


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 13, 2024. The information contained within may not be the most current and accurate depending on when it is accessed.


Ashley Dean Agriculture and Natural Resources Extension Specialist II

Ashley is an education extension specialist for field crop entomology at Iowa State University. She coordinates the Iowa Moth Trapping Network, the Regional Corn Rootworm Monitoring Network, and the Iowa Pest Alert Network. She also develops educational resources for field crop pests in Iowa and ...

Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology, now the Department of Plant Pathology, Entomology, and Microbiology, at Iowa State University in 2009. She is a professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated...