Distribution. Japanese beetle was accidentally introduced to North America with nursery stock shipped to southern New Jersey, and it was first detected during the summer of 1916. Its origin, as its name suggests, was Japan. It spread rapidly in the eastern United States, attaining Pennsylvania in 1920, Delaware in 1926, New York in 1932, Maryland in 1933, and Connecticut in 1937. During the 1940s, it spread further in the Middle Atlantic region and New England. During the 1950s, it spread to the Great Lakes region, and as far south as South Carolina. By 1975 it is was found in most states east of the Mississippi River, and in Ontario. Japanese beetle can live where the mean soil temperature in the summer is between 17.5° and 27.5°C, the mean winter soil temperature is above — 9.4°C, and there is at least 25 cm of precipitation during the summer. Thus, its natural spread westward across the Great Plains is hampered by dry conditions, but it remains a threat to Pacific Coast areas, where it can thrive. Indeed, Japanese beetle has invaded California, but was successfully eradicated.
Host Plants. Japanese beetle has a very wide host range, often given as 400 species or more. The plant families Rosaceae, Malvaceae, Polygonaceae, Tilia-ceae, Ulmaceae, and Vitaceae are quite susceptible. Actually, about 50 species are avidly fed upon, and another 50 species are regularly attacked. Other plants are damaged occasionally, usually during the period of great insect abundance when favored foods are exhausted. Among the most preferred hosts are trees such as birches, black walnut, crabapple, crepe myrtle, elm, horsechestnut, linden, maples, and sassafras, and fruits such as apple, apricot, blueberry, cherries, grape, nectarine, peach, plum, and raspberry. Preferred vegetables include asparagus, corn, and rhubarb, with some feeding on beet, broccoli, lima bean, mustard, okra, potato, and snap bean. Other important plants often eaten are poison ivy and rose. Vegetable crops that usually are avoided by beetles are artichoke, Brussels sprouts, cabbage, cantaloupe, carrot, cauliflower, celery, cucumber, eggplant, endive, leek, lettuce, onion, pea, parsley, pepper, pumpkin, salsify, spinach, squash, sweet potato, turnip, and watermelon. Interestingly, Japanese beetle readily consumes, but it is poisoned, by some geranium spp. and perhaps other species.
Natural Enemies. Native natural enemies, though numerous, often are not adequate to keep Japanese beetle from becoming abundant. Among the impor tant bird predators are the grackle, Quiscalus quiscala; meadowlark, Sturnella magnam; and starling, Sturnis vulgaris. Mammals such as the common mole, Scalopus aquaticus; shrew, Blarina brevicauda; and skunk, Mephitis mephitis; similarly feed on beetles, especially the larval stage, and are often quite destructive to plants owing to their digging for the grubs. Native predatory and parasitic insects are generally lacking.
A significant effort was undertaken to introduce from Asia exotic beneficial organisms that might suppress Japanese beetle. Several flies (Diptera: Tachini-dae) were introduced, including the adult parasitoid Hyperecteina aldrici Mesnil and the larval parasitoids Prosena siberita (Fabricius) and Dexilla ventralis (Aldrich). Among the wasps introduced, only the larval parasitoids Tiphia popilliavora Rohwer and Tiphia vernalis Rohwer (both Hymenoptera: Scoliidae) are important, with the latter species causing over 60% parasitism in some locations. However, the entomo-pathogenic nematode Steinernema glaseri (Nematoda: Steinernematidae) and the milky disease bacteria Bacillus popilliae and B. lentimorbus have proved most effective in population reduction. An excellent summary of biological control efforts was given by Fleming (1968). The effectiveness of B. popilliae seems to vary somewhat over time, perhaps explaining the periodic increases in Japanese beetle abundance (Dunbar and Beard, 1975). Other pathogens of Japanese beetle also are known. In a survey conducted in Connecticut, for example, gregarines infected 0-100% of grubs, the microsporidian, Ovavesicula popilliae, was found in 494% of grubs, and the rickettsial disease Rickettsiella popilliae and the fungus Metarhizium anisopliae were noted (Hanula and Andreadis, 1988). In this study, infection rates by milky disease ranged from 0-100%
Life Cycle and Description. There normally is one generation per year, but in northern areas some individuals require two years to complete their development. Adults generally emerge in May-June, remain abundant through the summer months, and then perish in September. The eggs are deposited in June-August, with the first instar developing in late summer or autumn, and larvae overwintering as second or third instars. The third instar normally is completed in the spring. Total development time is about 200 days when reared at 20°C, but only about 140 when reared at 25°C.
Egg. The eggs are usually deposited in the proximity of favored adult food plants. Grasses are the favored oviposition site, and moist soil is preferred over dry. The female digs in the soil to a depth of 5-10 cm to oviposit. She normally deposits about 3-4 eggs at a location, and repeats the digging and ovipo-
sition processes over a period of weeks until 40-60 eggs are produced. The maximum number of eggs produced is about 130. The eggs are unusual as they vary slightly in shape, ranging from spheres initially measuring 1.5 mm in diameter, to ellipsoids measuring 1.5 mm long and 1.0 mm wide. The eggs are white to yellowish white. They enlarge as the embryo matures, as is common among scarab beetle eggs, until they are about twice the original size. Enlargement is due to water absorption, and lack of water inhibits egg development or causes death of eggs. During dry periods females selectively oviposit in low, poorly drained areas and irrigated fields. Duration of the egg stage is about 21 days at 20°C, but only 8 days at 30°C. No egg hatch occurs below 15°C or above 34°C, and 30°C seems to be about optimal.
Larva. There are three instars during the larval stage. Upon hatching the young grub measures about 1.5 mm long, bears three pairs of thoracic legs, and has 10 abdominal segments. Although the larva is completely white initially, the head darkens to yellowish brown; after feeding commences the tip of the abdomen becomes grayish brown. Numerous short, dark spines occur on the dorsal surface of the abdomen. On the ventral surface of the tip of the abdomen there are several rows of spines, the configuration of which forms a "V"; this configuration serves to distinguish this grub from other soil-dwelling beetle larvae. When dug from the soil the larva typically assumes a curled posture, sometimes referred to as C-shaped. By completion of the first instar the larva usually attains a length of about 10 mm. The second and third instars are similar in appearance, except in size. Body lengths attained by larvae are about 18.5 and 32.0 mm for instars two and three, respectively. Head capsule widths for the instars are about 1.2, 1.9, and 3.1mm, respectively. Larvae develop at temperatures from 17.5° to 30°C. Duration of the instars is about 30, 56, and 105 days for instars 1-3, respectively at 20°C. In contrast, duration of the instars is about 17, 18, and 102 days at 25°C. Survival of grubs is favored by adequate moisture in the summer, snow cover or warm winter temperatures, and acidic soil.
The larvae move considerable distances in the soil. They will be deeper in dry than moist soil, but temperature is probably a more important determinant of location. During the summer nearly all larvae are within the top 5 cm of soil. During winter the grubs move to depths of 5-15 cm, and then return to the surface in spring. In habitats with suitable food, such as sod, larvae rarely move laterally more than 1 m during a season, whereas in fallow ground they may move several meters.
They sometimes grapple with one another as they attempt to mate, forming balls of beetles that consist of 25-200 males surrounding a female! Males locate virgin females by her production of sex pheromone (Tumlinson et al., 1977). (See color figure 125.)
The most complete treatments of Japanese beetle were provided by Fleming (1972, 1976), but there are numerous lengthy reports on biology or management. Sim (1934) gave an account of characters to distinguish Japanese beetle larvae from similar grubs.
This is a very important insect in the northeastern United States, but less so on vegetables than some other plants. Their overwhelming abundance and the tendency of adults to aggregate are significant elements in their damage potential. Adults feed on the upper surface of foliage, eating the tissue between the veins and leaving a lace-like skeleton. Such leaves invariably perish. They also feed on the stems of succulent asparagus, and on the young silk of corn. They attack fruit, and have the curious habit of feeding readily on the fruit of peach but avoiding its foliage. Adults consume foliage at the rate of 30-40 sq mm per hour. Grubs feed on plant roots; initially the rootlets are attacked, but larger roots are consumed as the larva matures. Although the roots of grasses are preferred, roots of many other plants, including numerous vegetables, are consumed.
Biological Control. Milky spore disease formulations consisting of Bacillus popilliae and B. lentimorbus are commercially available. This product usually is recommended for control of grubs on turf, where most larvae develop, but this materially benefits vegetable production by reducing the numbers of adults present as defoliators. In recent years there has been considerable interest in the use of entomopathogenic nema-todes as biological insecticides for larval control. Steinernema and Heterorhabditis spp. (Nematoda: Stei-nernematidae and Heterorhabditidae) can be applied to turfgrass successfully, especially if applications are followed by watering. High levels of suppression can be attained, though there are significant differences in nematode species and strain effectiveness (Klein and Georgis, 1992; Selvan et al., 1994). Some work indicates that the nematodes may be more effective than insecticides (Cowles and Villani, 1994). Research on nematodes has been largely restricted to turf, though the aforementioned findings are likely applicable to crops also.
Traps. Several studies have been conducted to assess the potential of trapping to eliminate or suppress beetles, or to protect plants. In general, though large numbers of beetles have been captured in such efforts, the results are disappointing. For example, large numbers of traps containing food-based lures were placed on Nantucket Island, Massachusetts, for a three-year period. Although densities were reduced by about 50%, the beetles persisted (Hamilton et al., 1971). In Kentucky, placement of 2-7 traps adjacent to host plants not only failed to protect susceptible plants, they seemed to increase damage levels.
Cultural Practices. Nonhost plants act as an impediment to movement of Japanese beetle. Sorghum, for example, which is not eaten by Japanese beetle, reduces the rate of dispersion of adults from soybean patch to soybean patch (Bohlen and Barrett, 1990). In contrast, strip cropping of corn and soybean has no effect on adult distribution (Tonhasca and Stinner, 1991), probably because both plants are suitable hosts.
Screening and row covers can often be used to prevent adults from gaining access to vegetable plants. Unfortunately, corn is probably the most highly preferred vegetable, and this plant is too large to cover conveniently.
Injury to corn can be prevented or reduced by modifying the time of planting. Early planting, in particular, is beneficial in culture of corn because the corn ears are pollinated before the adults become abundant and consume the corn silks; once the ear is pollinated the silks have no value. Where the growing season permits, planting of late-season varieties also is beneficial, because the silks are not produced until after most adults have perished.
There are numerous reports that certain plants, if eaten, are toxic to Japanese beetles. Plants reported to be toxic are geranium, Pelargonium domesticum; bot-tlebrush buckeye, Aesculus parviflora; and castorbean, Ricinus communis. There is evidence that the first two species are toxic, but there are no data to support reports of toxicity associated with castor bean. Castor bean is known to cause mammalian toxicity, which may account for this erroneous information concerning Japanese beetle. Unfortunately, though geranium and bottlebrush buckeye are, in fact, somewhat poisonous to Japanese beetle, they are rarely eaten by this insect, so impart little suppressive value.
Soil preparation can affect Japanese beetle grub populations. Because beetles favor acidic soil, application of lime to acidic soil to attain a neutral pH can result in lower grub populations (Polivka, 1960). Tillage can destroy grubs, and repeated tillage or roto-tilling is more disruptive than a single cultivation.
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