University of Maryland Extension

Japanese Beetles: On their way back in Maryland (2014)

Stanton Gill, Extension Specialist in IPM for Landscapes, Nurseries and Greenhouses
Japanese beetle adult on zinnia flower


We have been pretty much Japanese beetle free for the past 6 years and it has been great. Something happened in 2013 that is changing this blissful period. The drought periods in the summers of 2007 - 2012 kept the Japanese beetle populations suppressed and we saw only isolated cases of Japanese beetle damage. It started raining on a regular basis during the egg laying time for adult Japanese beetles in 2013 and there was a higher survival of Japanese beetle grubs in the soil which means more Japanese beetles in 2014. The cold temperatures of -7 F in January for a couple of days did nothing to reduce this grub populations. They moved deep into the soil to overwinter and did just fine.

Meanwhile, people have been planting ideal food in landscapes for the adult beetle. The bush type roses such as Knockout and Double Knockout roses have made a big hit in commercial planting sites and have been used extensively in landscapes over the last couple of years. These roses just add to the food sources for Japanese beetles in 2014. Little leaf linden trees, crabapples, rose of Sharon, hibiscus, and cherry trees are all favored hosts for Japanese beetle adults. If you are growing fruit bearing sweet and sour cherry, blueberry plants, apple trees expect a visit from Japanese beetles this year

We Learn From Past Experiences

Back in 2005 we saw a brutal onslaught of Japanese beetles that left a path of devastation in the Washington/ Baltimore corridor. The beetle populations had been building over the last 4 years, but in 2005 we received reports of record setting levels of damage in the landscapes. One landscape manager reported that within 5 days after they saw the first Japanese beetle emerge on June 24th, they were finding 14 -16 ft tall little leaf lindens completely defoliated. Nurseries visited in Frederick and Carroll Counties on June 30 had so many adult beetles on susceptible trees species that when we shook the branches the sky was clouded by swarms of escaping beetles. The population levels of Japanese beetles were at epidemic proportions on the East Coast in the 1940s - 1960s before settling into generally low levels for many years. This pest activity fell into a persistent, but almost “low incidence” pest status in many communities.

We are still seeing low populations in the oldest, established communities, but high levels in the newly developed neighborhoods. In the south and mid-west of the United States, however, the Japanese beetle is still a relatively recent pest, where expanding populations are wrecking havoc in many landscapes. 
This beetle was one of the early invasive species, but we did not call them this term then. Native to Japan, the Japanese beetle was first observed in the United States in New Jersey in 1916 by two Canadian entomologists visting in New Jersey who described them as a “curious southern species of beetle”. Little did these two Canadian entomologists realize how wide spread the Japanese beetle would be over the next century. Japanese beetle populations are entrenched and damaging plant material from Iowa and to Illinois, Kentucky, Alabama, northern Georgia to South Carolina. The range of the beetles continues to expand with localized infestations in many other states including Colorado. Colorado now has thousand cankers disease killing walnuts, emerald ash borer killing ash trees and Japanese beetles feeding on many of the remaining tree and shrub species growing in this arid land. You thought you had it tough, try growing anything other than rocks in Colorada lately.

Aggressive programs to eliminate this introduced pest in these isolated outcroppings have been effective but expensive. Constant vigilance and early interdiction will be a continuing process to keep Japanese beetles from spreading to new areas in the United States.

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