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Chicago Urban Gardening

The day to day experiences of a University of Illinois Extension Urban Horticulture Educator in Chicago, Illinois
Japanese Beetle

Japanese Beetles: Q and A

Posted by Ron Wolford -

Japanese beetles have extended their territory in 2011. Many can simply look outside at their linden, elm, roses, etc. to see that these small metallic green and bronze-colored beetles have ventured into virgin territory. Producers are also noting their advance in 2011 as the beetles chew on row crops and specialty crops which leads to some questions.

Question One: What does the lifecycle look like for Japanese beetles?

Answer: They go through a yearlong lifecycle. Adults tend to emerge around the latter part of June. They mate, lay an egg or two, and continue the process for just over a month. Adults can feed on several hundred different plant species. The beetles will devour leaves, portions of the tassel in corn, and silks. Resulting white grubs (the juvenile stage of the beetle) are sensitive to dry soils and devour grass roots until October temperatures bring them to a halt. Feeding resumes the following spring with the grubs completing development toward early/mid June. Grubs go through a total of three instars (growth stages). At that time, grubs change into teardrop-shaped pupae that are initially light brown and which later turn metallic green.

Question Two: Why do Japanese Beetles congregate in mass?

Answer: They congregate for a few reasons. First, females release a pheromone (mating attractant) as they feed, drawing in males, which also begin to feed. Secondly, the first beetles in an area release another set of chemicals that alert other beetles to the presence of a food source. Finally, the beetles are also attracted to damaged foliage. These characteristics have led the University to discourage homeowners and producers from using Japanese beetle traps. Traps draw in beetles via the use of a pheromone. Beetles begin to feed on surrounding plant material causing damage and releasing attractants that draw in additional beetles – both male and female. Females then release more pheromones, and a snowball effect results in which beetle numbers can literally skyrocket. Stated bluntly – producers and homeowners should "ditch" the traps.

Question Three: How far can Japanese beetle adults fly?

Answer: Japanese beetles can fly miles. Because of this, the U of I typically does not recommend grub management in the spring to manage adult beetles. Grub management with the intention of managing adults is simply a "teaspoon in the ocean." Grubs may be eliminated from your property, but adults will simply emerge and fly in from everyone else's untreated property.

Question Four: Are there any great non-chemical control options for Japanese beetles?

Answer: During the day, Japanese beetles violently buzz around. During the evening, they seem to calm down quite a bit. Around the home, this means that one can simply touch the beetle and have it drop into soapy water thus eliminating it. Unfortunately, producers raising row crops and specialty crops don't have any convenient non-chemical control options available to them.

Question Five: How important is scouting for Japanese beetles in the field?

Answer: Scouting fields is essential. Beans require 30 percent defoliation pre-bloom. That is not 30 percent in an isolated area – that is 30 percent of all the leaves removed from the entire field. Such field densities are often not reached even when significant damage is rampant on area ornamentals. In corn, fertilization progress is the key. Producers can check fertilization in corn by carefully shucking the ear and shaking it. Fertilized silks will fall from the ear. Once about three-fourths of the ear is established – the opportunity for the Japanese beetle to impact yield evaporates. As a result, field edges are often hit severely while the majority of the field's interior establishes the ear unhindered. If the majority of the field has established the ear, subsequent Japanese beetle pressure is a non-issue, but scouting is the only way to tell.

Source: Matt Montgomery, Extension Educator, Local Food Systems and Small Farms,

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