Distribution. Striped cucumber beetle, Acalymma vittatum (Fabricius), is found throughout the United States and Canada east of the Rocky Mountains, and most of Central America. It is not usually considered a severe pest in the Rocky Mountain and Great Plains states, nor in the southeast. However, in the midwestern and northeastern states, and in eastern Canada, it is the most important pest of cucurbits.
Acalymma trivittatum (Mannerheim), called the western striped cucumber beetle, replaces A. vittatum west of the Rocky Mountains. Both Acalymma species are very similar in appearance, biology, and damage potential.
Host Plants. The host range consists principally of plants from four families: Cucurbitaceae, Rosaceae,
Leguminosae, and Compositae. Although apple, pear, green beans, soybeans, okra, eggplant, potato, and many other crop plants have been fed upon occasionally by striped cucumber beetle, crop injury is usually limited to cucurbit crops. Rosaceous hosts are important in the early spring, before cucurbit crops are available. At this time, beetles feed greedily on the blossoms of trees and shrubs. Alternate hosts become important again in the autumn when cucurbit crops have declined, but late in the season Compositae are favored by adults. Larvae feed on roots of cucurbits and, though damaging, are of less concern than adults.
Natural Enemies. Numerous natural enemies of striped cucumber beetle have been determined, but only a few are significant. Probably the most important parasitoid is Celatoria setosa (Coquillett) (Diptera: Tachinidae), which parasitizes adult cucumber beetles, and was found consistently to parasitize 10-40% of the population in Ohio (Houser and Balduf, 1925). The wasp, Syrrhizus diabroticae Gahan (Hymenoptera: Braconidae) also attacks adult beetles, though the incidence of parasitism was found to be less. The nematode Howardula begnina Cobb (Nematoda: Allan-tonematidae) penetrates newly hatched beetle larvae in the soil, matures in the adult beetle, and is dispersed by the female during the act of oviposition. The adult beetle may contain large numbers of nematodes within her body cavity, which undoubtedly affects her longevity and reproductive capacity, but the adults are not very pathogenic. Rates of nematode infection of 10-25% are common throughout the summer in Ohio. A number of general predators, including soldier beetles (Coleoptera: Cantharidae), ground beetles (Coleoptera: Carabidae), shield bugs (Hemi-ptera: Pentatomidae), damsel bugs (Hemiptera: Nabi-dae), and birds, as well as fungi attack striped cucumber beetle.
Life Cycle and Description. Striped cucumber beetles require about 40-60 days to complete a generation, but development time is largely a function of weather, and values given by various authors differ greatly. The number of generations varies from one in New Hampshire, to two in Ohio and Connecticut, and three in Texas. Adults overwinter and become active early in the spring, taking flight when the air temperature reaches about 18°C.
Egg. The eggs are deposited in the soil about the base of plants or along vines, usually within 1.5 cm of the soil surface. Females sometimes crawl into cracks in the soil and deposit their eggs at greater depth. Chittenden (1923) reported maximum egg production of over 1500 per female, but indicated that the
Striped cucumber beetle larva.
average was 400-500. The oval eggs are yellow or orange, and measure 0.5-0.7 mm long and 0.3-0.5 mm wide. The length of the egg stage is about 5-10 days.
Good accounts of cucumber beetle life history were provided by Garman (1901), Chittenden (1923), Houser and Balduf (1925), and Isely (1927).
Striped cucumber beetle causes damage directly by its feeding and indirectly through transmission of plant diseases. In the spring, adults feed on young seedlings, often causing complete defoliation. Brewer et al. (1987) studied the effects of adult feeding on zucchini seedling survival in Louisiana, and noted significant levels of mortality when they attained densities of 10 beetles per plant or greater. If infestation was delayed until the second or third true leaves were formed, yields were not reduced. Beetles also selectively feed on cucurbit blossoms, if they are available, and may prevent good fruit set. They even attack the fruit, making unsightly pits in the rind. Smooth-skinned melons are especially prone to injury. Because adults seek to avoid heat, beetles may accumulate on the underside of fruit and cause damage there. Larvae feed below-ground on the roots and above-ground on the stems, usually maintaining contact with the soil. Occasionally, larvae will tunnel from the soil directly into mature fruit.
Striped cucumber beetle is an important vector of plant diseases. Latin and Reed (1985) demonstrated the ability of larvae to increase the incidence and severity of Fusarium wilt through their root feeding behavior. Adults harbor bacterial wilt, Erwinia tra-cheiphila, within their bodies during winter months and then apparently vector the disease in the spring. Disease transmission is accomplished when contaminated-fecal material comes in contact with wounds caused by beetle feeding. Although the importance of cucumber beetles in disease transmission is well-established, the role of beetles in aiding the overwintering of the bacterium is less certain (Harrison et al., 1980b). Alternate, asymptomatic host plants such as weeds are likely involved. Cucumber beetles also transmit squash mosaic virus (Gergerich et al., 1986).
Sampling. Plant kairomones such as mixtures of indole with trimethoxybenzene and cinnamaldehyde have been used to make highly attractive sticky traps for monitoring beetle densities (Lewis et al., 1990). These chemical lures are most effective when combined with visual stimuli, such as yellow-sticky traps, and positioned at the height of the plant canopy (Hoffman et al., 1996a). Visual examination is often used to estimate adult density, with suppression initiated at densities of 0.5-1.0 beetles per plant. Brust and Foster (1999) suggested that a threshold of one beetle per plant was adequate to protect cantaloupe in the midwest from economic infection with bacterial wilt.
Sampling plans, including sequential plans, were discussed by Burkness and Hutchison (1997,1998).
Insecticides. Insecticides are used routinely in commercial production of cucurbits to control striped cucumber beetle. Foliar protectants are common, but because the young, rapidly growing plants are very susceptible to injury, careful timing is important. Granular formulations are sometimes applied in a band over the plant row to protect the roots from larval tunneling. In general, striped cucumber beetles are not considered difficult to kill. Their ability to transmit disease exaggerates their impact, however, especially in the midwest. The need to control the beetles and their spread of cucurbit wilt disease diminishes as the plants reach maturity. If beehives are placed into a field to increase pollination, there should be at least a one-day interval between insecticide application and hive placement. Research in Indiana showed that insecticide applications to cantaloupe, based on thresholds of 0.5 beetles per plant before and one beetle per plant after fruit appeared, produced financial savings relative to scheduled weekly insecticide treatments (Brust et al, 1996).
As is the case with the closely related Diabrotica species, bitter cucurbitacins are potent arrestants and feeding stimulants for striped cucumber beetles, and can be used to enhance insecticidal control. Ferguson et al. (1983) reported a high correlation between cucur-bitacin content of squash and feeding damage by cucumber beetles. Spotted cucumber beetle is also affected by volatile chemicals produced by squash blossoms; indole is especially implicated. Combination of cucurbitacins with blossom volatiles and insecticide has been used experimentally to formulate an effective bait that suppresses beetle abundance and disease incidence as well as foliar insecticide, but is less disruptive to pollination (Brust and Foster, 1995). A cucurbitacin bait containing insecticide is available commercially.
Cultural Practices. Late planting is sometimes suggested to minimize damage by striped cucumber beetle. Cucurbits are cold sensitive and grow much faster under warm conditions, so late planting favors the ability of the plant to outgrow feeding injury by beetles. Screen or paper covers are sometimes used with young plants. The latter not only prevents defoliation by adults but also provides some protection from cold weather early in the season.
The use of resistant cultivars or species is widely practiced, when practical. The preference of cucumber beetles for summer squash has led to the recommendation that this be planted as an attractant and trap crop to prevent damage to cucumber. Michelbacher et al. (1955) stated that adults avoid heat and are attracted to moisture. Thus, melon fields were frequently invaded and damaged after irrigation.
Trap crops have some benefit for cucumber beetle control. Pair (1997) reported that small plantings of an attractive squash crop that were treated with systemic insecticide could suppress early season populations on nearby cantaloupe, squash, and watermelon.
Biological Control. Undoubtedly, some degree of control of rootworm larvae can be attained with stei-nernematid and heterorhabditid nematodes, but this has been well-studied only for western corn root-worm, Diabrotica virgifera LeConte. Under laboratory conditions, striped cucumber beetle larvae are very susceptible, and the nematodes can be delivered to the root zone of plants through trickle irrigation emitters (Reed et al, 1986).
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