Several new counties reported the presence of Japanese beetles since our last report. This brings to 24 the number of counties in Iowa where this pest is now confirmed (see map below).
A frequently asked question about Japanese beetle is, "What can I apply to the lawn this fall to prevent the next hatch?"
Life cycle reminder: The adult Japanese beetle feeds on the flowers, foliage and fruits of over 300 different kinds of plants. The adults lay eggs in July and August in grassy areas. These eggs hatch into white grubs that feed on plant roots and organic matter in the soil. The grubs remain in the soil until the following June when they transform to new adults and emerge from the soil to start the cycle over.
The Japanese beetle adult females lay their eggs in grassy areas all over the county and not exclusively in your lawn. Therefore, treating your lawn one year as a mechanism for preventing adult beetles the following year will not work. By the way, this is true of almost all insect pests. Japanese beetles will survive and emerge from so many places surrounding you and your property, that what you do to the relatively small area of your lawn will not make any difference in the following year.
It is practical and prudent to only treat lawns for white grub infestations that will damage the sod yet this year. Anything else is an unnecessary waste of time and money and insecticide.
Deciding if and when to treat for white grubs is not easy. It is too late now for the preventive white grub treatments to work well. Please see "White Grub Control Alternatives" for further thoughts.
Reported distribution of the Japanese beetle in Iowa as of 8/22/07