Japanese beetles plagued some parts of rural Missouri in 2016. What’s their outlook?

Published: November 21, 2016

In 1934 Japanese beetles made it to St. Louis, after being accidently introduced in the U.S. in 1916.  They have migrated across much of the state with some areas having extremely high numbers. These beetles are scarce in north Missouri but heavy in the southwest and central areas of the state (note Table One). Kansas City is typical of north Missouri, and for some reason in the Southeast, despite the weather being favorable, they don’t seem to have become as troublesome.

If you don’t have them now, you may expect them in the future. They feed on quite a variety of vegetables, such as beans, asparagus stems, the foliage and silk of corn and the foliage of okra.  Heavy clipping of corn silks will keep the corn from setting kernels. They also feed on rhubarb, grape, raspberry, elderberry and blackberry, some tree fruits, and hundreds of ornamental plants and trees. Commercial traps are available and will help alert you to their presence when they are first coming into your area. Grapes are such a preferred food that beetles feeding on wild grapes might be an early indicator of their presence. Grubs, the larvae of the beetles, feed on the roots of corn, beet, beans, asparagus, tomato and onion, as well as many grasses. Japanese beetles overwinter as a partially grown grub in the soil below the frost line. After pupating in the soil the adult emerges in early summer. Feeding activity lasts four to six weeks. They mate after emerging and lay eggs in the soil, usually in grass, where they hatch in mid to late summer. Larvae feed on roots until the soil cools in the fall.

* Source of data is University of Missouri’s Integrated Pest Management program, from the ‘Pest Monitoring Network’. https://ipm.missouri.edu/pestMonitoring/

After several years of extremely high numbers, the populations may drop and stabilize at lower numbers due to natural controls (note in Table One the St. Louis counts). Climatic occurrences that harm natural controls will cause the population to rebound. Beetle populations have been lower in the years following a severe drought but will rebound (note in table, drop from 2012 to 2013, and increase in following years). A number of insecticides are labeled for their control but control is made difficult because the continued emergence over several weeks. Neem oil has been reported to have some repellent activity. Always follow the label when using pesticides. Most have pre-harvest restrictions. Trapping may attract more than it kills so unless you are doing mass trapping to reduce numbers, place the trap away from produce and landscaping areas.

For more information, see The Japanese Beetle fact sheet at http://www.lincolnu.edu/web/programs-and-projects/ipm

Editor’s note: I posed this question to some of my colleagues to the east, where Japanese beetles have been around for a couple of decades. “We are experiencing some initial years of infestation. Folks out in the country got a bit distressed…..maybe they were just really bad this year. But this was typical of some calls: The Japanese beetles have almost formed a swarm and ate most of the upper leaves of a large elm tree. What is going on with them this year and will it kill my tree? Can anything be done?” Here was an excellent reply: “The St Louis Metro East experienced the worst of the Japanese Beetle front in the early 2000’s…particularly in Clinton County where we had a lot of dairy. You would be driving along and see this black cloud-like haze, which turned out to be a large group of Japanese beetles…like being inside a large popcorn popper when driving through. I have not really even thought about Japanese beetles for at least the last 8 years…just have not been at levels that have been problematic. Levels have gradually leveled out to where they can be found but not at devastating levels.” Dr. Elizabeth Wahle, Ph.D. University of Illinois Extension- Horticulture