How Many Japanese Beetles Will You See This Year?

The Japanese beetle, Popillia japonica, has an annual life cycle.  That is, it takes one year to complete the development and growth from egg to adult, and there is only one generation per year.  Japanese beetles that were present last summer laid eggs in moist soil covered with grasses (turfgrass, waterways, roadsides, etc.).  By mid-August, the eggs are hatched, and the larvae began feeding on plant roots and grow quickly to their full-grown size of about 1 inch long.  Japanese beetle white grubs move deeper into the soil and spend the winter 4 to 6 inches deep underground.  Pupae form in the spring, and the new adults emerge during June and July.

We should be at the peak of Japanese beetle adult emergence but, so far, the phones, emails, and social media seem quieter than usual.  I'm guessing the population of Japanese beetles will be lower than average this year. However, there will still be beetles eating your grapes, roses, and linden trees, and there will be exceptions where "explosive" numbers of beetles damage foliage flowers and fruits or any of their 300 or more favorite host plants.  When forced to guess why the Japanese beetle population might be lower this year, I would point to the drought last summer and fall as a stress to egg-laying and larval survival.  The Japanese beetle population crashed (was very low) back in the summer of 2014 but had recovered to "one of the worst years on record" by 2017.  The hypothesis for the low population in 2014-2016 was the prolonged and deep frost in the soil the preceding winter.  That does not appear to have happened this past winter.

We recently revised our Japanese beetle control recommendations.  In short, your choices are to tolerate the damage (especially on plants already defoliated), screen small, valuable plants, handpick from small plants, or use insecticides ahead of severe damage.  Please see our online article for more details.  https://hortnews.extension.iastate.edu/japanese-beetle

Japanese beetles on prairie plants
Japanese beetle adults showing characteristic shiny-green and coopery-brown coloration, and skeletonizing leaf-feeding damage.  

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