Japanese Beetle Adult Emergence Begins in Southern Iowa

June 21, 2022
ICM News

The cool spring has delayed development of many important crop pests across Iowa. However, with recent warm temperatures, Japanese beetle adult emergence has been on track with previous years. 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).

Growing degree days map.
Figure 1. Growing degree days accumulated (base 50°F) in Iowa (as of June 20, 2022). Adults begin emerging around 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.”

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 adults feeding on corn silks.
Photo 2. 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.

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 21, 2022. 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...