Japanese Beetles Make an Impression

Japanese beetles have been a pervasive pest in the East Coast States for many years. Although populations wax and wane with weather conditions, they have made a phenomenal comeback recently. The population levels of Japanese beetles were at epidemic proportions on the East Coast in the 1940- 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. In the South and Midwest, however, the Japanese beetle is still a relatively recent pest, where expanding populations are wreaking havoc in many cut flower operations.    
Native to Japan, the beetle was first observed in the United States in New Jersey in 1916 by two visiting Canadian entomologists who described them as a “curious Southern species of beetle.” Little did these two scientists realize how widespread the Japanese beetle would become over the next century. Japanese beetles are now well established in most of the Northeastern and Mid-Atlantic states.  They are now invading and entrenching in the South and parts of the Midwest. Populations are entrenched and damaging plant material from Iowa and Illinois to Alabama, northern Georgia and South Carolina.  The range of the beetles continues to expand with localized infestations in many other states including Colorado.
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.

Know the Enemy –  Damage and Life Cycle of Japanese beetles

Japanese beetles directly damage cut flower plants as adults while the larvae (grubs) damage turfgrass.  The life cycle is one of the more completely documented turf and ornamental pests.
Adult beetles are metallic green with bronze wing covers. A row of white hair brushes is on the sides of the abdomen. Adult beetle activity commonly peaks during mid July through the first two weeks of August.  Optimal adult beetle flight occurs at 21C and 60% RH (Ontario Ag FS, 1997).  Adults feed during the day, consuming both flowers and foliage of host plants (the damage is called skeletonization), favoring hot weather and plants growing in full sun.  It has been noted that adults will feed on shorter plants initially after emergence (e.g. weeds), moving to taller plants later in the season (Ontario Ag FS, 1997).
After mating, adult females live 30-45 days.  They feed and lay eggs throughout the summer, ultimately laying 40-60 eggs in the soil.  Eggs are rather flattened, slightly wrinkled and oval; laid only 1 to 3 inches in the soil, a relatively shallow depth.  As soon as eggs are laid, they start to absorb moisture from the adjacent soil and increase in size quickly as long as moisture levels are adequate.   One to four eggs are laid at a time, with additional egg laying occurring every few days for over a month in midsummer (Potter, 1998).  
The key for egg survival is adequate soil moisture.  High egg survival leads to high percentage of successful hatch and result white grubs to develop. If the soil is dry, the eggs that are laid shallowly in the soil dry out and die before they can develop.  Therefore, grub mortality increases under conditions of extended drought.  Conversely, grub survival increases under conditions of rain/irrigation.
Grubs hatch in 10-12 days and feed on fine turfgrass roots until the fall. By late October to November, when soil temperatures drop, grubs cease feeding and move downward 15 – 30 cm (6 to 12 inches) into the soil to overwinter.
When soil temperatures warm up in the spring, grubs move up toward the soil surface and continue feeding on grass roots. Grubs mature from late May through June and molt to pupae in the soil. One generation occurs each year.

Cut Flower Plants that Attract Japanese Beetles

Japanese beetle adults begin their annual activity by mid-June (approximately), with peak activity in mid-July.  Adults prefer ornamental plants in full sun, and typically feed in groups. Certain plants in the landscape are magnets for Japanese adults. For example, if dahlia, rose or crape myrtle are in your cut flower fields, expect Japanese beetle adults  to be frequent visitors who will consume generous amounts of foliage.
The Japanese beetle Top Five preferred herbaceous plants include 1. hollyhock (Alcea rosea); 2. dahlia (Dahlia spp.); 3. hibiscus (Hibiscus spp.); 4. Common mallow (Malva rotundiflora); and 5. Evening primrose (Oenothera biennis).  Adults also feed on annual flowers, including zinnia (Zinnia elegans), common four-o’clock (Mirabilis jalapa)  and French marigold (Tagetes patula). (Hawley & Metzger, 1940).   
Many weeds, including grape, multiflora rose, sassafras, smartweed, and Virginia creeper (Potter, 1998) are also hosts to Japanese beetles; therefore grassy or weedy areas surrounding desirable cut flowers may act as potential breeding grounds for this pest.  Thus, maintaining good weed control helps to eliminate potential food sources.

Controlling Adult Beetles

The key to controlling adult Japanese beetles is to use a material that either repels the adult beetles from feeding or kills them quickly before they can inflict much damage to the foliage. Once the adults start damaging foliage the wounded plant tissue releases volatiles that additional beetles will detect, attracting other adults to feed on the plant. If a slow-killing pesticide is used, adults can cause a fair amount of damage and increase the feeding aggregation of other adult beetles on the plant.
Registered products that give very good control include Sevin (carbaryl), Astro (permethrin), DeltaGard (deltamethrin), Talstar (bifenthrin), and/or Tempo (cyfluthrin).  Wettable powder formulations may produce longer-lasting protection.  Imidacloprid (Merit), applied early in the season as a soil drench, will provide season-long systemic control of adults on herbaceous plants and small woody trees.  
Some research has shown good results using Merit (imidacloprid).  However, this depends upon the type of application.  Soil drenches, for example, should be applied about 30 days prior to Japanese beetle activity.  Merit foliar sprays will provide rapid plant protection, as it seems to cause adult beetles to stop feeding (we repeat: stop feeding.  The beetles do feed a little – causing a minor bit of damage — before they stop feeding.  This must be expected when using Merit, and not interpreted as an application failure).

 The ideal spray timing targets adults when they first appear and before damage occurs. Repeat applications are often desirable weekly on high value plants, particularly if this ideal spray window was missed.  Since larvae develop in turf, treatment of turf areas is also recommended as a dual control.  
Japanese beetle traps containing floral and sex attractant lures that attract adult beetles are used as a monitoring tool.   Traps have been misused by the public who mistakenly believe they control beetles, but beetles have been shown to often land and feed on plants close to traps (Potter, D.,  1998).