By Jeff Iles
Iowa State University
While visiting the state of Pennsylvania several summers ago, I witnessed the ravenous feeding habits of Japanese beetles and wondered what a full blown invasion would look like in Iowa. Well, folks, after witnessing the scope and severity of damage to many landscape plants this summer, none of us have to wonder anymore because Japanese beetles are here. In fact, they’re here in droves, and like that unwelcome house guest, I don’t think they’ll be departing any time soon.
By now I’m sure you know the story. Japanese beetles were mistakenly introduced to the eastern United Stated in 1916 via infested nursery stock. And in the absence of natural enemies they flourished. Larvae (grubs) feed on the roots of turfgrass plants and the adults feed on a wide range of landscape plants, but find particular favor with members of the rose family (Rosaceae).
Controlling Japanese beetles once they’ve descended on a landscape can be a futile exercise. Yes, insecticides are available and have proven moderately effective, but are of little value once hundreds of adult beetles have chosen to hold a family picnic on your prized crabapple. So, how are we to coexist with these beetles? The best plan of attack is always host plant resistance. Just as we cope with biotic agents causing apple scab, bronze birch borer and Dutch elm disease, we can take advantage of the naturally occurring resistance to Japanese beetle possessed by a myriad of landscape plants.
Conifers like white fir, white pine, Norway spruce and Canadian hemlock have demonstrated excellent resistance to Japanese beetles. Likewise, many maples and oaks are rarely fed upon by the beetles. And as it turns out, crabapples, long thought to be hopelessly susceptible to Japanese beetle feeding, can’t be characterized in so general a fashion. For example, the cultivars 'Adirondack', 'Bob White', 'David', 'Louisa', and 'Red Jewel' have proven moderately resistant to beetle feeding. And even among lindens, long known to be a favorite appetizer of Japanese beetles, several cultivars of silver linden and 'Legend' American linden seem to be less susceptible.
Of course, the arrival of Japanese beetles to Iowa couldn’t have come at a more inopportune time. With emerald ash borer bearing down on us from Illinois, Missouri and even Wisconsin, the last thing we need to hear is how more of our favorite landscape plants will be under attack from an introduced pest. But with careful landscape planning and wise plant selection, we can and will learn to deal with this latest pest.