Japanese Beetles Make an Impression

Japanese beetles have been a pervasive pest in East Coast states for many years. Although populations wax and wane with weather conditions, they recently have made a phenomenal comeback. The population levels of Japanese beetles were at epidemic proportions on the East Coast in the 1940s to 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 landscapes.

Native to Japan, the Japanese 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 Canadian entomologist realize how wide-spreading the Japanese beetle would be over the next century. Japanese beetles are now well-established in most of the Northeastern and Midatlantic states. They are now invading and entrenching in the South and parts of the Midwest. Japanese beetle populations are entrenched and damaging plant material from Iowa to Illinois, Alabama, northern Georgia and South Carolina. The range of the beetles continues to expand with localized infestations in many other states, including Colorado.

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Aggressive programs to eliminate this introduced pest in these isolated outcroppings have been effective, but expensive. Constant vigilous and early interdiction will be a continuing process to keep Japanese beetles from spreading to new areas in the United States.

WHY THE RESURGE ON THE EAST COAST?

The country has experienced several years of low-interest-rate mortgages that have spawned a building boom in many urban communities. New housing units with large areas of newly established turfgrass, combined with planting of ornamental plant species highly susceptible to adult beetle feeding, have helped fuel this upsurge. Japanese beetles also thrive in new housing developments with virgin soil (soil free of entomopathogens). On top of this, two years of above-normal rainfall during the summer months when adult females are laying eggs has produced the “perfect storm” conditions that promoted record Japanese beetle populations in 2003 and 2004.

This combination of events has caused vast beetle population explosions that are causing widespread damage to ornamental landscapes. In some areas, populations are also wreaking havoc in nurseries and golf courses. Control methods targeting both the adult and grub stages are estimated to cost more than $460 million a year.

KNOW THE ENEMY

Japanese beetles directly damage landscape plants as adults while the larvae (grubs) damage turfgrass. The life cycle of the Japanese beetle 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 can be found 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 21°C (72°F) and 60% Relative Humidity. 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.

After mating, adult females live 30 to 45 days. They feed and lay eggs throughout the summer, ultimately laying 40 to 60 eggs in the soil. When females lay eggs, they are rather flattened, slightly wrinkled and oval. Eggs are 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 mid-summer.

The key for egg survival is adequate soil moisture. Increased egg survival leads to high percentage of successful hatch and results in white grubs development. If the soil is dry, the eggs that are laid shallow in the soil dry out and die before the egg can develop. Therefore, grub mortality increases under conditions of extended drought. Conversely, grub survival increases under conditions of rain/irrigation.

Grubs hatch in 10 to 12 days and feed upon fine turfgrass roots until the fall. By late October to November, when soil temperatures drop, grubs cease feeding and move downward 6 to 12 inches into the soil to overwinter.

Come spring, when soil temperatures warm up, 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.

TURF THAT ATTRACTS JAPANESE BEETLES

Ten months of the year, the Japanese beetle grub is hidden away beneath the turf areas of a landscape or nursery, silently cutting away the root system of the grass. Grubs prefer healthy turf in full sun that is well irrigated and fertilized. If grub populations are low or the turfgrass is growing vigorously, the damage to the turf may go undetected.

Japanese beetles feed on all cool-season grasses, but they seem to prefer perennial ryegrass and hard fescues. Kentucky bluegrass is nutritionally inferior as a food source, but if fed upon, does recover from Japanese beetle damage faster because of its spreading growth habit compared to perennial ryegrass.

Incidentally, perennial ryegrasses with endophytes do not seem to have a high enough level of toxin in their roots in order to prevent grub attack. Endophytes may, however, boost ryegrass recovery following grub attack.

LANDSCAPE 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 a little leaf linden, Japanese flowering plum, rose or crepe myrtle are in the landscape, expect Japanese beetle adults to be frequent visitors and to consume generous amounts of foliage.

The following list includes the top 10 favorite plant foods of Japanese beetles, according to APHIS: 1) American linden, 2) crabapple, 3) apple, 4) Japanese maple, 5) Norway maple, 6) rose, 7) crape myrtle, 8) pin oak, 9) birch, and 10) Prunus spp. (plum, apricot, cherry, peach). Secondary preferred host plants include Black walnut, willow, grape, horsechestnut, hibiscus, asparagus, blueberry, sassafras, Virginia creeper, and summersweet (Clethra). Notice that the list of secondary preferred plants includes some wild plants that might be found in nearby hedgerows.

The top 5 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).

Many weeds, including grape, multiflora rose, sassafras, smart weed and Virginia creeper are also hosts to Japanese beetles; therefore, grassy or weedy areas surrounding desirable ornamentals 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 Japanese beetle adults start damaging foliage, the wounded plant tissue releases volatiles that additional beetles will detect, attracting them to feed on the plant. If you use a slow-killing pesticide, 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) or Tempo (cyfluthrin). Wettable powder formulations may produce longer lasting protection. Merit (imidacloprid), applied early in the season as a soil drench, will provide season-long systemic control of adults on herbaceous plants and small woody trees. In field research projects in 2002 and 2003 at a Maryland nursery, we did not have great success controlling feeding adult Japanese beetles on littleleaf linden using imidacloprid soil drenches.

Some research has also 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 you have missed this ideal window of opportunity to spray. Because 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, because beetles have been shown to often land and feed on plants close to traps.

HOW ABOUT JAPANESE-RESISTANT PLANTS?

Many littleleaf lindens (Tilia cordata) and American lindens (Tilia americana) were completely defoliated in Midatlantic landscapes in 2004. By late July, only brown skeleton-like veins remained from the leaf petioles of numerous street trees, including the linden cultivars ‘Greenspire,’ ‘Olympic,’ ‘Redmond’ and ‘Prestige.’ However, Silverleaf lindens (Tilia tomentosa) growing in the same landscape had little, if any, Japanese beetle feeding injury. The foliage of silverleaf linden (and cultivars) is just a little thicker with small hairs on the foliage that apparently makes it unattractive to adult beetles.

Another resistant tree to try is the Japanese tree lilac, Syringa recticulata. The tree lilac is well-adapted to urban soils and blooms in mid-summer. The late lilac, Syringa villosa, grown as a shrub or trained as a small tree is also a good choice. Both species of Syringa are very resistant to Japanese beetle feeding.

Using species that are seldom attacked by the insect can reduce damage to nursery plants. The top 10 least-preferred plants are: 1) magnolia, 2) redbud 3) dogwood 4) red maple, 5) Northern red oak, 6) burning bush, 7) holly, 8) boxwood, 9) hemlock and 10) ash (white and green). Other least-preferred landscape plants include false cypress, yew, juniper, forsythia, clematis, red maple, euonymus, tuliptree, ornamental pears and most oaks (white, scarlet, red and black).

USING BLOOMING PLANTS TO PREDICT ADULT BEETLE EMERGENCE

Can you predict the next adult beetle wave? Adults emerge from the soil from mid to late June (typically) in the Midatlantic region; and into July further north. Using plant phenology, adult beetle emergence can be predicted by observing what plants are in bloom during this time. Adults emerge around the time that the following plants are in full bloom: hydrangea ‘grandiflora’; Canada thistle; Queen Anne's lace; bottlebrush buckeye; elderberry, and Yucca, as well as the bloom of Kousa dogwood, trumpet honeysuckle, red hot poker, dusty zenobia, daylily and water lotus.

Stanton Gill is a regional specialist in nursery and greenhouse IPM at Central Maryland Research and Education Center, University of Maryland Cooperative Extension (College Park, Md.), and is a professor with the landscape technology program at Montgomery College (Montgomery County, Md.). Deborah Smith-Fiola, is president/senior consultant for Landscape IPM Enterprises (Keedysville, Md.). Suzanne Klick is a technician at Central Maryland Research and Education Center, University of Maryland Cooperative Extension (College Park, Md.).

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