There were few Japanese beetles this year in most of Delaware. The following are some reasons why.
Many people in Delaware were very happy about the reduced numbers of Japanese beetles that they saw this summer. So what did happen? Most likely, we are seeing fewer Japanese beetles in areas where the soil was dry last year in July-August. Remember that we had a very dry period (5 weeks without rain) in 2008 during mid summer. Japanese beetle grubs do not survive well in dry soils. If it was very dry last July and August, the adult beetles would try to avoid laying eggs in the dry turf and soil, and if it was moist when they layed eggs, then became very dry later, the grubs did not survive. If most turf areas were dry last year, then few beetles would have emerged this summer.
Another thing that happened this summer is the cool weather during July. At cool temperatures, beetles are not as active as during warm temperatures. So, even if we had the same number of beetles, you would see less activity during a cool summer.
Because Japanese beetle grubs survive best in moist soil, it should mean that they will survive well this summer and fall, so more beetles will probably emerge next year than what we saw this summer.
Adapted for Delaware from an article by Dave Smitley in the Michigan Landscape Crop Advisory Team Alert website http://ipmnews.msu.edu/landscape/