Wednesday, June 29, 2011

Japanese Beetles Out in Full Force This Season

Robber fly eating Japanese beetle (Photo D. Stephan) 
Barbara Shew, our clinic director, said that she observed the cardinals in her yard putting on quite a show a few days ago.  They were knocking Japanese beetles from her crape myrtle flowers and then chasing the beetles as they tried to fly away.  I guess sometimes cardinals delight in chomping down some beetles in addition to their usual diet of seeds.  Cardinals are not the only animals that enjoy the occasional beetle treat.  My cat, Slinky, loves eating Japanese beetles.  She catches them, knocks them out with her paws, and then carries them to the hood of the car.  After letting them cook all day in the sun, she goes back and eats them! Predatory insects, like robber flies, assassin bugs, and others will feed on the adults.  Unfortunately, humans do not get the same satisfaction from finding these beetles in their yards. 

Japanese beetles emerge once per year to feast on a variety of ornamental and crop plants.  Their favorite ornamental hosts include roses, rosaceous trees, shrubs such as crab apple, crape myrtle and linden, grape, and fruit trees. 

Typically, Japanese beetles feed on the upper leaf surface eating the tissue between leaf veins.  This gives the leaves a lacy appearance, a type of damage called skeletonizing.  

Japanese beetle and skeletonizing damage (Photo: B. Shew)

They generally consume entire petals of roses and other flowers. Beetles aggregate on plants in response to chemicals (odors) released by damaged plants and pheromones released by female beetles.  The resulting hungry masses of beetles can rapidly devour trees, shrubs, vines, and flowers.

Japanese beetle damage (Photo: B. Shew)

 After they have fed for a while, the female beetles burrow into soil to lay their eggs.  Grubs (larvae) hatch from the eggs and feed underground through the rest of the summer and fall. Female beetles and the grubs prefer moist soils, so a damp summer means more beetles the following year.

The beetle grubs may be serious pests of the roots of grasses and shrubs, especially on turf farms and golf courses.  They burrow through the soil feeding on roots.  This can result in areas of dead grass.  The grubs overwinter deeper in the soil, and in the spring, they move just below ground level, complete feeding, and pupate.  New beetles emerge from pupae in late spring to start the cycle again.

Japanese beetles can fly long distances, so beetle traps do not offer any protection to landscape plants.  They may actually attract more beetles to your yard! Likewise, treating a lawn for Japanese beetle grubs will not reduce damage from incoming beetles.

Long-term protection for landscape and nursery plants can be achieved through the use of neonicotinoid insecticides such as imidacloprid (e.g., Merit, Marathon II) or acetamiprid (Tri-Star).  Acelepryn (chlorantraniliprole) is a new product with extremely low vertebrate toxicity but good efficacy against a number of pests, including Japanese beetles.

Japanese beetle on flower (Photo: B. Shew)

For more information on the biology and management of adult Japanese beetles in nurseries and landscapes, consult Ornamentals and Turf Insect Information note No. 146 at:

Special thanks to Steve Bambara and Dave Stephan for their help with this post!