Japanese Beetles - Know Your Foe

Around mid-June is the time of year when you might spot those rather attractive metallic copper and green Japanese beetles.  Attractive, until you look at the plants on which they are feeding as adults; then they’re not so pretty anymore.  It seems that these voracious eaters of leaves and flowers will consume any and everything, but they do have their favorites.  According to the University of Maryland Extension, there are over 275 species of plants that these beetles feed on.  Besides our prized roses, Japanese beetles eat other plants in the Rosaceae family including apples, pears, peaches, cherries, almonds, raspberries, and strawberries.  The beetles are especially fond of grapevines, but there is also a host of many other trees, shrubs, and flowering herbaceous plant leaves that they will eat. Unfortunately, these beetles are attracted to plants that have already been damaged, sometimes resulting in a mass-feeding frenzy with only the skeletonized leaf veins remaining. 

As you can probably guess, Japanese beetles weren’t always in this area.  They are originally from, you guessed it, Japan, where they were not a significant pest due to an abundance of natural predators.  Introduced to New Jersey in 1916, they have been thriving throughout North America east of the Rockies ever since.  Why are they so persistent and plentiful here?  That is because, in pursuit of a beautiful lawn, we are nourishing their babies.  Our efforts to create expanses of lush, green, frequently watered turfgrass lawns provide them with the optimum conditions for the success of their offspring. Immature Japanese beetle grubs feed on grass roots, and where the young will thrive is where you’ll find the adults.

Japanese beetles have a one-year life cycle. We notice them when the new adults emerge from the ground in mid-June, with the population peaking in early July here in Maryland.  Those adult beetles will be busy eating, mating, and egg-laying for 4-6 weeks, tapering off during the first week in August.  The females will lay 40-60 eggs near their feeding site.  Those eggs will hatch in about ten days and begin feeding on grass roots.  That will be their source of food as they bulk up to survive the winter further down underground.  In early spring the grubs move back up in the soil and start munching away on your grassroots again until around mid-May.  At that point, they will go through their pupal stage and emerge as adults in early summer to begin the cycle anew.

Japanese beetles are much easier to control if you catch them early before their feeding damage attracts others.  For a light pest level, simply knock them into soapy water. Severe infestations may require more drastic measures that should be carefully considered before introducing any chemical toxins into your local ecosystem.  Insecticides will typically only provide temporary relief and will kill many non-target beneficial insects. The commercially available Japanese beetle traps are not recommended; they simply lure more beetles into your yard from your neighbors’ property.  Many foliage plants can tolerate moderate defoliation and some will even produce new growth after being browsed. Most of these beetles remain close to the site where they emerged from the ground, so the all-around best long-term solution to discourage Japanese beetles is to reduce the areas of turfgrass lawn.

For more information, refer to the University of Maryland Extension fact sheet on Japanese Beetles on Trees, Shrubs, and Flowers.  https://extension.umd.edu/resource/japanese-beetles-trees-shrubs-and-flowers