While we just celebrated our Independence Day, July marks a month of tyranny for our outdoor plants. Japanese beetles are making their presence known in our rose gardens, vegetable patches, and on our shade trees. Japanese beetles arrived in this country sometime around 1916 from Japan. Their first port of call was New Jersey, and in the last ninety years they have spread north to Ontario, south to Georgia, and west to the Mississippi. The adult Japanese beetle is quite catholic in its tastes. It is known to feed on more than 200 kinds of trees, shrubs, vines, and herbaceous plants. Among its favorites are lindens, maples, apples, cherries, grapes, and, sadly, roses.
Japanese beetles feeding and frolicking.
With their sharp jaws, Japanese beetles munch the tender leaf tissue between the hardened veins, leaving behind only the skeleton of a leaf. Heavily damaged plants will appear scorched as the remaining tissue withers and dies. You may have noticed that Japanese beetles often attack one plant severely, leaving a neighbor relatively unscathed. Apparently, when a single or few beetles initiate an attack on a given tree, plant odors are released. These send a signal to other beetles something like "food's good, party over here!" Plant attractants are compounded when female beetles that are, well, on the prowl, release a chemical message called a sex pheromone. The sex pheromone says to the guy beetles “hey dude, how'd you like to spend a little time with me?” A raucous feeding/love fest ensues, and in the process your plant takes a beating.
In addition to chemical cues, Japanese beetles use a variety of visual cues to select their victims. Leaf and flower color seem to be particularly important. In a series of very clever experiments, researchers in Kentucky found that roses with large light colored blossoms, particularly yellow or white, were more attractive to beetles than roses with smaller, darker blossoms of red or orange. In the tree realm, researchers have noted that lindens with densely hairy leaves were less preferred than scantily haired varieties. Maples with purple or deep red leaves were preferred over those with green leaves.
White grub parasitized by a tiphiid wasp. Red arrow points to the ectoparasite.
One of the keys to managing the Japanese beetle is to reduce the number of the larval stages, known as white grubs. White grubs can be serious pests of lawns, especially during summers that follow years when rainfall was abundant such as last year and the one before. In a fascinating twist in a bug-eat-bug world, several species of parasitic wasps known as scoliids and tiphiids exploit the bounty of white grubs in the soil. The crafty females patrol the earth for odors that are released by the beetle grub as it feeds in the ground below. After detecting these odors, the wasp digs into the soil like a crazed terrier and locates the grub. With a potent sting she paralyzes the grub long enough to deposit an egg on the grub's skin. The egg hatches into a larva called an ectoparasite: ecto meaning on the outside, and parasite meaning an organism living in or on another. To grow, the tiphiid larva sucks the fluids from the body of its living host, ultimately killing its victim within the course of a week or two. The transformation from legless larva to winged wasp occurs in a silken pupa in a subterranean crypt. The wasp emerges in the following spring or summer and feeds on nectar and pollen prior to finding a mate and initiating its search and destroy mission of the white grubs in your lawn.
References
For more information on the biology and management of Japanese beetle visit the following web sites:
http://ohioline.osu.edu/hyg-fact/2000/2001.html
http://www.hgic.umd.edu/pubs/online/hg78.pdf