Distribution. First found in North America near Boston, Massachusetts in 1917, European corn borer quickly spread to the Great Lakes region. By 1948 it was established throughout the midwestern corn-growing region and eastern Canada. It now has spread as far west as the Rocky Mountains in both Canada and the United States, and south to the Gulf Coast states. European corn borer is thought to have originated in Europe, where it is widespread. It also occurs in northern Africa. The North American European corn borer population is thought to have resulted from multiple introductions from more than one area of Europe. Thus, there are at least two, and possibly more, strains present. The presence of an eastern or New York strain, and a midwestern or Iowa strain, is evident because different pheromone blends are required to capture moths from each population. Both strains sometimes occur in the same area (Eckenrode et al., 1983).
Host Plants. European corn borer has a very wide host range, attacking practically all robust herbaceous plants with a stem large enough for the larvae to enter. However, the eastern strain accounts for most of the wide host range, the western strain feeding primarily on corn. Among vegetable crops injured are beet, broccoli, celery, corn, cowpea, eggplant, lima bean, pepper, potato, rhubarb, snap bean, spinach, Swiss chard, and tomato. Vegetables other than corn tend to be infested if they are abundant before corn is available, or late in the season when senescent corn becomes unattractive for oviposition; snap and lima beans, pepper, and potato are especially damaged. In North Carolina, for example, potato is more attractive than corn at peak emergence of the first moth flight, and more heavily damaged (Anderson et al., 1984). Other crops sometimes attacked include buckwheat, grain corn, hop, oat, millet, and soybean, and such flowers as aster, cosmos, dahlia, gladiolus, hollyhock, and zinnia. Corn is the most preferred host, but many thick-stemmed weeds and grasses also support European corn borer, especially if they are growing amongst, or adjacent to, corn. Some of the common weeds infested include barnyardgrass, Echinochoa crusgalli; beggarticks, Bidens spp.; cocklebur, Xanthium spp.; dock, Rumex spp.; jimsonweed, Datura spp.; panic grass, Panicum spp.; pigweed, Amaranthus spp.; smart-weed, Polygonum spp.; and others. A good list of host plants was given by Caffrey and Worthley (1927).
Natural Enemies. Native predators and parasites exert some effect on European corn borer populations, but imported parasitoids seem to be more important. Among the native predators that affect the eggs and young larvae are the insidious flower bug, Orius insidious (Say) (Hemiptera: Anthocoridae); green lace-wings, Chrysoperla spp. (Neuroptera: Chrysopidae); and several lady beetles (Coleoptera: Coccinellidae) (Jarvis and Guthrie, 1987; Andow, 1990). Insect preda tors often eliminate 10-20% of corn borer eggs. Avian predators such as downy woodpecker, Dendrocopos pubescent (Linnaeus); hairy woodpecker, D. villosus (Linnaeus); and yellow shafted flicker, Colaptes auratus (Linnaeus) have been known to eliminate 20-30% of overwintering larvae.
Native parasitoids include Bracon caulicola (Gahan), B. gelechiae Ashmead, B. mellitor Say, Chelonus annulipes Wesmael, Macrocentrus delicatus Cresson, and Meteorus campestris Viereck (all Hymenoptera: Braconidae); Gambrus ultimus (Cresson), G. bituminosus (Cushman), Itoplectis conquisitor (Say), Campoletis flavicincta (Ashmead), Nepiera oblonga (Viereck), Rubicundiella pertur-batrix Heindrich, Vulgichneumon brevicinctor (Say) (all Hymenoptera: Ichneumonidae); Dibrachys carus (Walker) and Eupteromalus tachinae Gahan (both Hyme-noptera: Pteromalidae); Syntomosphyrum clisiocampe (Ashmead) (Hymenoptera: Eulophidae); Scambus pter-ophori (Ashmead) (Hymenoptera: Hybrizontidae); Tri-chogramma nubilale Ertle and Davis and T. minutum Riley (both Hymenoptera: Trichogrammatidae); and Archytas marmoratus (Townsend) and Lixophaga sp. (both Diptera: Tachinidae). Although many species of native parasitoids are known, native parasitoids rarely cause high levels of corn borer mortality.
Exotic parasitoids numbering about 24 species have been imported and released to augment native parasi-toids. About six species have successfully established. Among the potentially important species is Lydella thompsoni Herting (Diptera: Tachinidae), which may kill up to 30% of second generation borers in some areas, but has disappeared or gone into periods of low abundance in other areas. Other exotic parasi-toids that sometimes account for more than trivial levels of parasitism are Eriborus terebrans Gravenhorst (Hymenoptera: Ichneumonidae), Simpiesis viridula (Hymenoptera: Eulophidae), and Macrocentris grandii Goidanich (Hymenoptera: Braconidae) (Burbutis et al., 1981; Andreadis, 1982a; Losey et al, 1992). A comprehensive review of biological control agents imported in the first half of the 1900s was published by Baker et al. (1949).
Several microbial disease agents are known from corn borer populations. The common fungi Beauveria bassiana and Metarhizium anisopliae are sometimes observed, especially in overwintering larvae. The most important pathogen seems to be the microsporidian, Nosema pyrausta, which often attains 30% infection of larvae and sometimes 80-95%. It creates chronic, debilitating infections that decrease longevity and fecundity of adults, and decreases survival of larvae that are under environmental stress (Hill and Gary, 1979; Andreadis, 1984). Unfortunately, N. pyrausta also infects the parasitoid M. grandii (Andreadis, 1982b).
Life table studies conducted on corn borer populations in Quebec with a single annual generation perhaps provide insight into the relative importance of mortality factors (Hudon and LeRoux, 1986c). These workers demonstrated that egg mortality (about 15%) was low, stable and due mostly to predators and parasites. Similarly, mortality of young larvae, due principally to dispersal, dislodgement, and plant resistance to feeding was fairly low (about 15%) but more variable. Mortality of large larvae during the autumn (about 22%) and following spring (about 42%) was due to a number of factors including frost, disease and parasitoids, but parasitism levels were low. Pupal mortality (about 10%) was low and stable among generations. The factor that best accounted for population trends was considered survival of adults. Dispersal and disruption of moth emergence by heavy rainfall are thought to account for high and variable mortality (68-98%, with a mean of 95%), which largely determines population size of the subsequent generation. Overall generation mortality levels were high, averaging 98.7%.
Weather. There are many reports that weather influences European corn borer survival. Heavy precipitation during egg hatch, for example, is sometimes given as an important mortality factor (Jarvis and Guthrie, 1987). Low humidity, low nighttime temperature, heavy rain, and wind are detrimental to moth survival and oviposition. However, during a 10-year, three-state study, Sparks et al. (1967) reported no consistent relationship between weather and survival.
Life Cycle and Description. The number of generations varies from one to four, with only one generation occurring in northern New England and Minnesota and in northern areas of Canada, three to four generations in Virginia and other southern locations, and usually two generations in the northern United States and southern Canada. In many areas generation number varies depending on weather, and there is considerable adaptation for local climate conditions even within strains. For example, though the developmental rates of single-generation strains are lower than multiple-generation strains, at northern locations such as Prince Edward Island the single-generation strain develops quickly (Dornan and Stewart, 1995). European corn borer overwinters in the larval stage, with pupation and emergence of adults in early spring. Diapause apparently is induced by exposure of last instar larvae to long days, but there also is a genetic component. Moth flights and oviposition usually occur during June-July and August-September in areas with one to two generations annually. In southern locations with three generations, moth flights and oviposition typically occur in May, late June, and
European corn borer larva.
European corn borer larva.
European corn borer pupa.
European corn borer pupa.
about 12 days. The developmental threshold for pupae is about 13°C.
Adult. The moths are fairly small, with males measuring 20-26 mm in wingspan, and females 25-34 mm. Female moths are pale yellow to light brown, with both the forewing and hind wing crossed by dark zigzag lines and bearing pale, often yellowish, patches. The male is darker, usually pale brown or grayish brown, but also with dark zigzag lines and yellowish patches. Secondary host plants and adjacent grassy areas play a significant role in the mating behavior of adults, as adults rest and mating takes place in such areas of dense vegetation, called "action sites." Retention of droplets from rainfall and dew in this dense vegetation stimulates the sexual activity of females.
Moths are most active during the first 3-5 h of darkness. The sex pheromone has been identified as 11-tet-radecenyl acetate, but eastern and western strains differ in production of Z and E isomers. The western strain produces a blend that approximates 97:3 Z:E, whereas the eastern strain uses a blend of 3:97 Z:E. The pre-oviposition period averages about 3.5 days. Duration of oviposition is about 14 days, with oviposi-tion averaging 20-50 eggs per day. The female often deposits 400-600 eggs during her life span, though there are also estimates of mean fecundity of about 150 eggs in some locations. Total adult longevity is normally 18-24 days. (See color figures 208 and 209.)
Brindley and Dicke (1963), Brindley et al. (1975), Hudon and LeRoux (1986a,b) and Hudon et al. (1989) published reviews of the biology and management of European corn borer. Detailed biology was presented in Vinal and Caffrey (1919), and Caffrey and Worthley (1927). Sex pheromone blends were identified (Klun and Robinson, 1971; Kochansky et al., 1975; Showers et al., 1974). Beck (1987) reviewed corn borer seasonal biology, and offered interesting insight into the pher-omonal races or strains. Rearing procedures were given by Reed et al. (1972). European corn borer was included in the larval key by Capinera (1986) and the moth key by Capinera and Schaefer (1983). A key to stalk borers associated with corn in southern states was presented by Dekle (1976); this publication also included pictures of the adults. A key to pyralid borers
was also included by Stehr (1987). A key to common stalk boring caterpillars is included in Appendix A.
This is a very serious pest of both sweet and grain corn, and before the availability of modern insecticides this insect caused very marked reductions in corn production. Young larvae feed on tassels, whorl, and leaf sheath tissue; they also mine midribs and eat pollen that collects behind the leaf sheath. Sometimes they feed on silk, kernels, and cobs, or enter the stalk. Older larvae tend to burrow into the stalk and sometimes the base of the corn ear, or into the ear cob or kernels. Feeding by older larvae is usually considered to be most damaging, but tunneling by even young larvae can result in broken tassels. The presence of 1-2 larvae within a corn stalk is tolerable, but the presence of any larvae within the ear of sweet corn is considered intolerable by commercial growers, and is their major concern. European corn borer is considered to be the most important sweet corn pest in northern production areas, and second-generation borers are the principal source of ear damage. Heavily tunneled stalks of grain corn suffer from lodging, reducing the capacity for machine harvesting. Lodging is not a serious threat to sweet corn. Boring by corn borers also allows several fungi to affect corn plants.
In crops other than corn, the pattern of damage is variable. European corn borer larvae damage both the stem and fruit of beans, pepper, and cowpea. The temporal occurrence of fruit affects susceptibility to injury, of course; in Wisconsin, snap beans 14-30 days from harvest were susceptible to damage by larvae, but young plants and fruit near harvest suffered little damage (Sanborn et al., 1982b). In celery, potato, rhubarb, Swiss chard, and tomato, it is usually the stem tissue that is damaged. In beet, spinach, and rhubarb, leaf tissue may be injured. Entry of borers into plant tissue facilitates entry of plant pathogens. The incidence of potato blackleg caused by the bacterium Erwinia carotovora atroseptica, for example, is higher in potato fields with stems heavily infested by corn borers. Direct damage by corn borers to potato vines, however, results in negligible yield loss (Nault and Kennedy, 1996c).
Cultural Practices. Destruction of stalks, the overwintering site of larvae, has long been recognized as an important element of corn borer management. Disking is not adequate; plowing to a depth of 20 cm is necessary for destruction of larvae. Mowing of stalks close to the soil surface eliminates more than 75% of larvae, and is especially effective when combined with plowing (Schaafsma et al., 1996). Minimum tillage procedures, which leave considerable crop residue on the surface, enhance borer survival.
Diversified cropping is detrimental to corn borer population survival. Intercropping with red clover, for example, resulted in lower borer density (Lambert et al., 1987).
Early planted corn is taller and attractive to ovipositing female moths, so late planting has been recommended, but this is useful mostly in areas of only a single generation per year. If a second generation occurs, such late-planted corn is heavily damaged. Planting border rows of a highly attractive variety of corn to surround a less attractive variety has been investigated in France (Derridj et al., 1988). The attractive variety, especially if it is an early flowering cultivar, receives most of the eggs of moths dispersing into the field. If treated with insecticide or destroyed, this border row trap could provide protection for the main corn crop.
Soil conditions can affect corn borer oviposition patterns. Research conducted in Ohio demonstrated that corn grown in rich organic soils were not as attractive to moths as low-protein plants grown in conventionally fertilized soil (Phelan et al., 1996).
Host-Plant Resistance. Extensive breeding research has been conducted, and resistance has been incorporated into grain corn, especially against first generation borers. A principal factor in seedling resistance to young larvae is a chemical known as DIMBOA, which functions as a repellent and feeding deterrent (Klun et al., 1967). It has proven difficult to incorporate the known resistance factors into sweet corn without degradation of quality. However, some progress has been made in producing commercially acceptable resistant cultivars, especially when hostplant resistance is complemented by use of other sup-pressive tactics such as application of Bacillus thurin-giensis (Bolin et al., 1996).
Pepper cultivars differ in their susceptibility to corn borer. Hot pepper cultivars are most resistant, and most green bell peppers are susceptible.
Biological Control. Biological control has been attempted repeatedly in sweet corn and other vegeta bles susceptible to European corn borer attack. Bacillus thuringiensis products can be as effective as many chemical insecticides, but often prove to be less effective than some (Bartels and Hutchison, 1995). Most single-factor approaches, with the exception of newer formulations of Bacillus thuringiensis, have proven to be erratic. Release of nativeTrichogramma spp. (Hymenop-tera: Trichogrammatidae), for example, provides variable and moderate levels of suppression (Andow et al., 1995). In Massachusetts, an egg parasitoid normally associated with a related Ostrinia species in China was released. This new parasitoid, Trichogramma ostriniae (Hymenoptera: Trichogrammatidae), may prove useful for augmentative biological control programs, but seems susceptible to disruption by adverse weather (Wang et al., 1997). The effect of egg parasitoids is enhanced by application of Bacillus thuringiensis (Losey et al., 1995; Mertz et al., 1995). Application of pathogens such as Nosema pyrausta and Vairimorpha necatrix (Microsporida: Nosematidae) has been proven to have benefit under experimental conditions (Lewis et al., 1982), but a commercial product has not been developed.
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