Dalbulus maidis DeLong and Wolcott Homoptera Cicadellidae

Natural History

Distribution. The corn leafhopper occurs throughout the western hemisphere from the United States to Argentina. It is most destructive in tropical areas. In the United States it is damaging in the southern states from North Carolina to California. Apparently corn leafhopper overwinters only where host plants are available continuously, such as the Gulf Coast states and southern California, and disperses northward annually. It has been recovered as far north as Ohio. Its origin is likely Mexico.

Host Plants. The only crop affected by this insect is corn, and the life cycle is completed principally on this plant or other Zea spp. However, in the southeastern states gamagrass, Tripsacum dactyloides, can also support populations of corn leafhopper (Pitre, 1970) and in Mexico several Tripsacum spp. are hosts. Although corn leafhopper feeds on other crops such as barley, oat, rye, sorghum, sudangrass, sugarcane, and wheat, and even upon nongrass crops such as carrot and beet, these plants are not considered to be suitable for complete development of the life cycle.

Natural Enemies. The natural enemies of corn leafhopper are poorly known. Some ant species apparently prey upon them, because when ant populations are decreased leafhopper numbers increase (Perfecto, 1991). In Nicaragua, Anagrus sp. (Hymenoptera: Mymaridae) and Paracentrobia sp. (Hymenoptera: Tri-chogrammatidae) parasitize corn leafhopper eggs (Gladstone et al., 1994). In Mexico, Gonatropus bartletti Olmi (Hymenoptera: Dryinidae) and Eudorus sp. (Diptera: Pipunculidae) attack nymphs, and the fungi Beauveria bassiana and Metarhizium anisopliae infect nymphs and adults (Vega and Barbosa, 1990; Vega et al., 1991).

Life Cycle and Description. The life cycle can be completed in about 21 days, so several generations occur annually in warm climates. Two complete generations can be completed during the normal growth period of corn (Todd et al., 1991). Adults overwinter under cold conditions, and persist for up to seven months without breeding. They may feed on various grasses during this period, but Larsen et al. (1992) suggested that corn leafhopper could survive during the winter in Mexico without host plants if free water is available. These leafhoppers are gregarious and secrete honeydew, so they often are attended by ants.

  1. The eggs are deposited singly, but often in rows of up to eight, in the veins on the upper surface of young foliage in the whorl. Duration of the egg stage is five days at 24° C.
  2. Nymphs feed on the base of the leaf, or on the stem at the juncture of leaf and the stem. There usually are five instars, but occasionally a sixth is observed. Average nymphal body length is 0.8, 1.1, 1.7, 2.1, and 2.8 mm for instars 1-5, respectively. Mean nymphal development time is 2.5, 2.6, 2.5, 3.1, and 3.8 days for instars 1-5, respectively, when reared at 27°C. Nymphal develpment at variable temperatures usually requires about 15-18 days (range 11-19 days).
  3. The general body color of this insect is yellowish-white, with the pronotum yellowish. However, the top of the head is marked with two large black spots, and sometimes with additional dark pigmentation. The front wings are translucent and extend beyond the tip of the abdomen. Nielson (1968) reported the length of the adult male to measure 3.50-4.00 mm, the female 4.00-4.10 mm. Tsai (1988), however, gave values of 2.52-3.36 mm for males and 2.73-3.50 mm for females. Females oviposit at 21°C, but not at 18°C or lower. According to Davis (1966) females produce, on average, about 150 eggs and live for 25-50 days. Tsai (1988), however, reported fecundities of 400-600 eggs per female depending on temperature, and obtained mean adult longevity of 77 days when leaf-hoppers were reared at 27°C.

The biology was given by Davis (1966) and Tsai (1988). Triplehorn and Nault (1985) provided a description and a key to the members of the genus.

Damage

The adults and nymphs remove sap, causing wilting and yellowing. Honeydew production by leafhop-pers results in extensive growth of sooty mold (Bushing and Burton, 1974). Feeding by 4-5 adult leaf-hoppers causes severe wilt symptoms within five days, though the plants recover if the insects are removed (Hildebrand, 1954). However, this leafhop-per is important mostly because it is a disease vector. It is the most important leafhopper pest of corn, because it transmits corn stunt spiroplasma, maize bushy stunt mycoplasma, and maize rayado fino

Adult corn leafhopper.

virus. Symptoms of disease include discoloration, striping, and stunting. Severity of the disease depends on how early in the growth of the plant infection occurs, with greatest damage resulting from early infection (Hao and Pitre, 1970).

Management

  1. Nymphs and adults can be detected by visual examination of corn plants, concentrating attention on the whorl and base of the leaves. Adults can also be sampled with yellow water-pan traps (Vega et al., 1990). Although there is a positive relationship between insect abundance and disease incidence, yellow-sticky trap catches were shown not to be a good predictor of disease potential (Power et al., 1992).
  2. Systemic insecticides applied to the soil at planting or post-planting can provide protection against population establishment and increase during the early portion of the growing season. Planting time treatment, but not mid-season treatment, also reduces the incidence of corn stunt (Bhirud and Pitre, 1972). Infestation later may require foliar application of insecticides, with particular attention directed to application into the whorl.

Cultural Techniques. Cultural manipulations have frequently been reported to affect the incidence of corn leafhopper or disease, but the findings are not always consistent. Intercroppings of bean and corn were reported to have higher abundance of leafhop-pers than corn monocultures by Perfecto and Sediles

Dalbulus Maidis

Adult corn leafhopper.

(1992) and corn plantings with low plant density to have higher incidence of corn stunt (Power, 1989). In contrast, however, Castro et al. (1992) observed no effect of interplanting and density. Late plantings of corn are more subject to attack, and larger fields experience a lower incidence of attack. In California, crops of corn separated by a crop-free period experience little damage, but when cropped continuously leafhopper numbers attain high levels and considerable damage results (Bushing and Burton, 1974).

Disease Transmission. Diseases transmitted by corn leafhopper can cause severe economic losses. Significant damage to corn occurred in the late 1970s in Florida (Tsai, 1988), and in Nicaragua in the mid-1980s (Hruska et al., 1996). The basis for the disease outbreaks is not apparent, but in Nicaragua a possible explanation is the introduction of irrigated agriculture, which allows the disease to survive at high levels through the dry season. Corn stunt spiroplasma also survives in leafhoppers during the winter months, and its presence in the leafhopper increases survival rates at low temperature (Ebbert and Nault, 1994). Only a few hours of feeding are required for the insect to acquire corn stunt, but leafhoppers are not able to acquire the virus for 15-18 days after the disease is inoculated into a plant (Granados et al., 1968).

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