Circulifer tenellus Baker Homoptera Cicadellidae

Natural History

Distribution. Beet leafhopper originated in the Mediterranean region and apparently was introduced to the western hemisphere by Spanish explorers. It spread throughout Central and South America, the Caribbean area, and to Hawaii and Australia. In North America the beet leafhopper is common and is damaging throughout the western United States from southwest Texas to Washington. It also occurs in low numbers in the eastern states where it is not considered to be a pest. Beet leafhopper can remain fairly local, feeding throughout the year on crops and weeds in a small geographic area. However, it commonly is quite dispersive, moving north to British Columbia and east to the Great Plains area. Even within the generally infested area west of the Rocky Mountains, there is considerable annual movement from Arizona to Utah and Colorado.

This species is well-adapted for life in the desert. It accepts many plant species, develops rapidly, and disperses readily to find a new food source. Although more than one generation may develop on desert annuals, adults of the first generation display a strong tendency to disperse even in the presence of succulent hosts. The major breeding areas are the San Joaquin Valley of California; the lower Colorado River area of southern California, southwestern Arizona, southern Nevada, and southern Utah; the Rio Grand River area of New Mexico and Texas; the lower Snake River plains of Idaho and Oregon; the Columbia River area of Oregon and Washington; and some small, scattered areas in western Colorado, northern Utah, and northern Nevada. In most cases, heat and drought eventually force the leafhoppers to disperse to cooler and moister areas, where they tend to populate long-lived summer hosts. However, in some areas where irrigation is practiced, particularly in southern California, a succession of crops and weeds allows good survival without the need for dispersal.

Host Plants. Beet leafhopper often has complex host-plant requirements, and they vary regionally. During the spring, rapid growth of annual weeds that serve as overwintering host plants allows the population of leafhoppers to increase rapidly. They then disperse to summer hosts, which may support them until autumn when they return to their winter-spring annual hosts. In some cases perennial "hold-over" plants are fed upon after the summer hosts until winter annuals become available; they persist on the latter until spring. Many publications have addressed host associations and damage, but an excellent overview of host relations was given by Cook (1967).

The overwintering and spring plants are mostly plants in the family Cruciferae, and include bittercress, Erysimum repandum; pepperweed, Lepidium spp.; tan-symustard, Descurainia pinnata; flixweed, D. sophia; hedgemustard, Sisymbrium irio; and tumblemustard, S. altissimum; but also plants from other families including filaree, Erodium spp.; plantain, Plantago spp., and others. Summer hosts are often members of the family Chenopodiaceae such as Russian thistle, Salsola kali; halogeton, Halogeton gomeratus; lambsquar-ters, Chenopodium album; nettleleaf goosefoot, Chenopo-dium murale; and sagebrush, Atriplex spp.; but include others such as perennial pepperweed, Lepidium alys-soides. Hold-over hosts include perennial saltbush, Atriplex spp.; sagebrush, Artemisia spp.; rabbitbrush, Chrysothamnus spp.; snakeweed, Gutierrezia spp.; and perennial pepperweed, Lepidium alyssoides.

Crop plants damaged by beet leafhopper are bean, beet, cantaloupe, cucumber, pepper, spinach, sugar-beet, Swiss chard, squash, tomato, and watermelon. Sugarbeet, beet, spinach, and Swiss chard are most favorable for beet leafhopper growth; the other crops are fed upon but are poor hosts. All of the aforementioned crops can be injured, but the cucurbit crops are least susceptible.

The effects of host plants on beet leafhopper biology are variable. Harries and Douglass (1948) compared nymphal development time on several common host plants including tumblemustard, filaree, Russian thistle, flixweed, bittercress, pepperweed, and sugarbeet, and reported similar development times, 14.5-17.1 days when reared at 32°C. In contrast, however, sugar-

beet was a much superior plant with respect to ovi-position.

Natural Enemies. The principal natural enemies are little-known natural parasitoids including big-headed flies (Diptera: Pipunculidae), wasp egg parasi-toids (Hymenoptera: Mymaridae and Trichogramma-tidae), wasp nymphal parasitoids (Hymenoptera: Dryinidae), and twisted-wing parasites (Strepsiptera: Halictophagidae). Of these, the strepsipterans are least important, because they do not induce much mortality and infect a small percentage of the leafhopper population. The dryinids (Gonatopus spp.) are not often reared from leafhoppers, suggesting low levels of parasitism, but as the adults are predatory, the impact of these wasps is difficult to gauge. In California, the egg parasitoid Polynema eutettixi Girault (Mymaridae), and the nymphal and adult parasitoid Pipunculus industrius Knab (Pipunculidae), were reported to be most important by Stahl (1920). However, Meyerdirk and Hessein (1985), in a study of the egg parasitoids in California, indicated that Anagrus giraulti Crawford (Hymenoptera: Mymaridae) and Aphelinoidea sp. (Hymenoptera: Trichogrammatidae) were the most important of the five egg-attacking species collected. Although egg parasitoids sometimes cause up to 90% parasitism of eggs during the summer, and overwintering populations of beet leafhopper may experience up to 25% parasitism, parasitoids are often not reliable for suppressing beet leafhopper. Pipunculus and other parasitoids are often left behind when leaf-hoppers disperse, so dispersing leafhoppers initially escape from many of their important natural enemies. Several parasitoids have been introduced to the western United States from the Mediterranean area, particularly northern Africa; however, they have failed to establish (Clausen, 1978).

Birds, spiders, and other general predators also affect leafhopper populations. The predator most commonly observed to feed on these leafhoppers is western big eyed bug, Geocoris pallens Stal (Hemiptera: Lygaeidae), but green lacewings (Neuroptera: Chryso-pidae) also prey on these leaf hoppers. Unspecified fungal diseases affect overwintering populations, and horsehair worms (Nematomorpha) have been observed occasionally (Severin, 1933).

Weather. As leafhoppers inhabit harsh environments during the winter, they are susceptible to high levels of mortality during this period. In northern areas, the overwintering adults cannot feed during cold periods, so many die due to starvation and dehydration. In southern areas, there is less cold stress, but adults also suffer during foggy weather. Also, desiccation of host plants is a major cause of winter mortality in southern desert areas.

Life Cycle and Description. A complete generation requires 1-2 months. Several generations develop annually. In northern areas such as Washington, Oregon, Idaho, and Utah, three generations is the norm. In warmer areas such as California and Arizona, five generations is usual. Normally, 1-2 generations occur in the spring months on the early season host plants, with an additional 2-3 generations on summer hosts. Fertilized females overwinter, become inactive when it is cold, and active again when weather is favorable. Males perish during the winter months.

  1. Egg deposition commences about the time the winter host plants begin their spring growth. The eggs are deposited singly within a slit in the tissue of the leaves and stems. The petiole and leaf midrib are preferred sites, but leaf margins are sometimes selected. The eggs are elongate and slightly curved, with the posterior end tapering almost to a point. They measure 0.06-0.07 mm long and 0.018 mm wide, and are white to yellow. Each female may deposit 300400 eggs, though Meyerdirk and Moratorio (1987) reported only about 100 eggs per female when reared at 20-25°C and up to about 200 eggs at 32°C. Eggs hatch in 5 to 40 days, depending on temperature. Harries and Douglass (1948) reported mean egg durations of 26.3,11.9, 7.4, and 5.6 days at 18°, 24°, 30°, and 35°C.
  2. At hatching, the nymphs are transparent white. They begin to feed almost immediately, and within a few hours acquire a greenish color. There are 5 -6 instars, and the larger instars usually are spotted with black, red, and brown on the thorax and abdomen. Mean head widths are 0.33, 0.43, 0.54, 0.37, and 0.84 mm for instars 1-5, respectively. Maximum body lengths average 1.13, 1.52, 1.88, 2.38, and 3.20 mm, respectively, for instars 1-5. Development time normally ranges from 2-6 weeks, but at 32°C the mean duration of instars 1-5 is 3.4, 2.3, 2.4, 3.1, and 3.7 days, respectively.
  3. The adult measures 3.4-3.7 mm long and about 1 mm long. Adults are variable in color. The color is lighter at high temperature. During the summer they are usually uniform whitish or greenish, in the autumn they acquire some dark spots dorsally, particularly on the forewings. During the winter they become mostly dark. During the summer months, mating occurs within a few days of adult emergence, whereas during the autumn this period is prolonged. Oviposition may commence 5-10 days after mating, or may be protracted during cool weather. Mean oviposi-tion rates exceed 10 eggs per day at temperature above
Circulifer Tenellus
Beet leafhopper nymphs.

32°C, but the rate decreases at lower temperature. Duration of the adult stage is reported to be up to 4-5 months during the winter, though Harries and Douglass (1948) reported maximum mean longevity of about 82 days when overwintering females were held at about 15.5°C. Similarly, Meyerdirk and Mora-torio (1987) reported mean longevity of about 40-90 days at various temperatures, but some individuals lived more than four months. Overwintering adults benefit from availability of water, but atmospheric humidity is not a significant factor.

The biology and physical description were given by Severin (1930, 1933) and Cook (1967). Phenology and host relations were presented by Douglass and Cook (1954) and Cook (1967). Physical ecology was treated by Harries and Douglass (1948).

Picture Beet Leaf Hopper

Adult beet leafhopper, light form.

Dalbulus Maidis Virus

Adult beet leafhopper, dark form.

Adult beet leafhopper, light form.

Adult beet leafhopper, dark form.

Damage

Beet leafhopper is known as a serious pest in the western states principally because it transmits curly top virus. It is the only known vector of this plant disease. Signs of infection vary greatly among crops, but vein clearing, rolling or curling of leaves, stunting of the plant, and discoloration are common. Young plants are most injured, and often fail to produce fruit, or produce deformed fruit. Commercial vegetable production is infrequent in some southwestern areas owing to a high incidence of curly top.

Beet leafhopper also transmits Spiroplasma citri, causing ailments known as stubborn disease in citrus and brittle root in horseradish (Liu et al., 1983; O'Hayer et al., 1984). A mycoplasma-like (mollicute) organism is also transmitted to vegetable crops (Golino et al., 1987,1989; Shaw et al., 1993).

Management

  1. Visual examination and sweeping of plants are usually used to detect beet leafhopper. The mere presence of leafhoppers in susceptible crops usually stimulates insecticide applications. In some locales, sampling also is directed to breeding areas, so advance warning of leafhopper densities and damage potential can be obtained. The presence of high densities in rangeland breeding areas sometimes results in insecticide treatment to prevent dispersal of leafhoppers to crop production regions.
  2. Breeding is often confined to relatively small areas, making it feasible to apply insecticides and to eliminate a high proportion of the leafhoppers before they disperse to crops. Applications of insecticide to such areas can be made in the spring, before overwintered females have deposited eggs, or after the eggs have hatched but before the leaf-hoppers have matured into adults. More commonly, however, insecticides are applied directly to susceptible crops in areas that are prone to attack by leaf-hoppers. A common practice is to apply systemic insecticides at planting to protect seedlings.

Cultural Practices. Broadleaf weeds are common hosts of beet leafhopper, and it is desirable to replace them on rangeland with nonhost perennial grasses. Similarly, it is important to prevent rangeland from being damaged by overgrazing and fire, as this fosters the development of annual broadleaf weeds. As annual broadleaf weeds are replaced by late succes-sional plants such as sagebrush, Artemisia tridentata, the abundance of leafhoppers drops markedly (Fox, 1938). Similarly, weed suppression in areas with crops deprive leafhoppers of good alternate hosts. Russian thistle is particularly hazardous because it is a good host.

Host-Plant Resistance. Varieties that are resistant to curly top are available for most crops. This approach to manage the pest is widely used, but sometimes the resistant varieties bear horticultural characteristics that make them less suitable for commercial production, or resistance level varies due to horticultural practices such as date of planting (Martin and Thomas, 1986). Resistance, at least in tomatoes, appears to be due to lower frequency of feeding on the resistant varieties (Thomas and Martin, 1971; Thomas and Boll, 1977). Resistance in beans is temperature sensitive, with high temperature reducing resistance (Silbernagel and Jafri, 1974).

Disease Transmission. Curly top survives the winter both within the leafhopper and within some host plants. The leafhopper acquires the disease through feeding. About three hours of feeding are usually necessary for acquisition, and an incubation period of 4-7 h is necessary before successful transmission. Both nymphs and adults are vectors. Once infected, the leafhopper remains infected for the duration of its life. It cannot transmit the disease to its progeny through the egg, however. The proportion of the leafhopper population that is capable of transmitting the virus varies from 4-80%, thus greatly affecting damage potential. Different strains of the virus also display varying pathogenicity.

Leafhoppers feed on, and infect, numerous hosts. Apparently they reject, or select plants for prolonged feeding, only after an extended period of feeding during which curly top virus can be transmitted (Thomas, 1972). Insecticides often fail to kill leafhoppers before disease transmission occurs, but within-field transmission can be greatly reduced due to mortality of the vectors.

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