Distribution. Potato stem borer is native to Europe, northern Asia, and Japan. It was first found in North America in 1905 in Nova Scotia, but has since spread through eastern Canada west to Manitoba, and is occasionally damaging throughout this geographic range. In the United States potato stem borer is known from the northeastern and midwestern states. Hop vine borer, in contrast, is a native species, found across southern Canada and northern United States from coast to coast. Whereas potato stem borer tends to be more common in Canada, hop vine borer occurs frequently in the United States. Both species assumed greater importance as crop pests starting in the 1970s and 1980s, though the cause is unknown.
Host Plants. Potato stem borer is polyphagous, but it is known principally as a pest of potato, corn, and rhubarb. Crops attacked include barley, corn, hops, onion, potato, raspberry, rhubarb, strawberry, sugarbeet, tomato, and wheat. Several grasses can serve as hosts, including bromegrass, Bromus sp.; reed canary grass, Phalaris arundinacea; orchardgrass, Dactylis glomerata; and quackgrass, Agropyron repens. Hemp nettle, Galeopsis tetrahit; curly dock, Rumex cris-pus; and possibly other swamp or marsh dwelling plants are suitable broadleaf hosts (Giebink et al., 1992).
The major hosts of hop vine borer are hops and native perennial grasses, and this insect is a serious pest principally in areas where cultivated or wild hops grow. Increasingly, however, it has become a pest of corn. Among weed hosts preferred by ovipositing females are foxtail, Setaria spp.; quackgrass, A. repens; and to a lesser extent large crabgrass, Digitaria sangui-nalis; barnyardgrass, Echinochloa crusgalli; and fall panicum, Panicum dichotomiflorum. Larvae can survive on curly dock, but growth is poor.
Natural Enemies. Several native parasitoids of potato stem borer are known. Egg parasitoids include Telenomus sp. (Hymenoptera: Scelionidae), Tricho-gramma retorridum (Girault) (Hymenoptera: Ticho-grammatidae), and Centrodora sp. (Hymenoptera: Mymaridae). Parasitoids reared from larvae include Lydella radicus Townsend (Diptera: Tachinidae), Di-adegma sp., Campoletis sp., Ectopimorpha luperinae Cushman, and Glypta sp. (all Hymenoptera: Ichneu-
monidae). Reared from pupae are Therion sp. and Pter-ocormus sp. (both Hymenoptera: Ichneumonidae), but these species likely attack the larval stage. The most effective parasitoid in Ontario is Lydella radicus, and parasitism levels of 25-60% have been reported (West et al., 1983), but the other species seem to contribute little to the overall level of parasitism. Additional para-sitoids have been imported from Europe and released in Canada, including Macrocentrus blandus Eady and Clark (Hymenoptera: Braconidae) and Lydella stabu-lans Fallen (Diptera: Tachinidae).
Natural enemies of hop vine borer are less well known, but several predators and parasitoids were identified in New York. Among the ground beetle predators are Calosoma calidum Fabricius, Harpalus pen-sylvanicus De Geer, Pterostichus lucublandus Say, Pteros-tichus stygicus Say, and Amara impuncticollis Say (all Coleoptera: Carabidae). Hawley (1918) suggested that the ground beetles consumed the egg, larval, and pupal stages of hop vine borer. Parasitoids identified from New York included Microplitis gortynae Riley, Aenoplex sp., and Synaldis sp. (all Hymenoptera: Braco-nidae), and Lespesia frenchii Williston (Diptera: Tachi-nidae).
Life Cycle and Description. Potato stem borer and hop vine borer are similar in biology and appearance. They display one generation per year, with the egg serving as the overwintering stage. Eggs hatch in April-May, pupation typically occurs in July, and adults are found from late July-September.
periods are reported to be longer, about 6-9 weeks in Wisconsin (Giebink et al., 1984) and 9-12 weeks in New York (Hawley, 1918). The developmental threshold of hop vine borer is about 5°C, slightly lower than that of potato stem borer, which is about 7°C (Giebink et al., 1985). The larva is whitish, but the early instars bear rose to purplish bands on the thoracic and abdominal segments. Thereafter the bands fade in hop vine borer, whereas potato stem borer larvae bands tend the remain evident. The head capsule is yellow in potato stem borer but brown in hop vine borer. The larvae measure about 2-3 mm at hatch, but eventually attain a length of 30-50 mm.
Hop vine borer larva.
Hop vine borer larva.
Moths emerge in August-September, mate and begin egg production within a few days of emergence. Fecundity is estimated at 400-1200 eggs.
Life history of potato stem borer was given by Deedat et al. (1983). Life history of hop vine borer was given by Hawley (1918) and Giebink et al.
Larvae feed initially on grasses growing as weeds among or near crop plants, then usually switch to larger grasses such as corn, or broadleaf plants such as hops, potato, or curly dock. On perennial grasses the feeding occurs above-ground, but after the feeding switch at about the fourth instar larvae tunnel below-ground into the base of the stem and roots. Some of the perennial grasses and other plants may have sizable underground rhizomes, roots, or stems that allow complete larval development, but this aspect of larval biology is poorly known.
Damage to corn by potato stem borer was described by Deedat and Ellis (1983), who reported that over 90% of seedling corn plants were infested in some fields. Small plants, such as two-leaf stage seedlings, may be completely severed by the entry of potato stem borer, and this damage resembles cutworm injury. With larger seedlings, however, larvae may burrow within the stem, feeding just above the roots, until larvae attain the fifth or sixth instar. Such mature larvae tend to remain below-ground, outside the stem, entering only to feed. Early signs of larval feeding are leaf or plant wilting; later signs are death and disintegration of the plant. Young plants perish within a few days of larval attack. Plants that have attained the eight-leaf stage are slow to wilt and die, but eventually perish. Larvae often destroy 3-4 plants during the course of their development.
Cultural Practices. Cropping practices can help alleviate injury by Hydraecia spp. Of foremost importance is weed management. Larvae often invade crop fields from weedy fence rows, resulting in considerable damage along field margins. Thus, insecticidal treatment of the crop periphery, or destruction of grasses and weeds by burning or application of herbicides, can reduce injury. The critical period for weed management is early in the season, typically April or May before susceptible annual crops are available. The presence of wild hops is often related to the occurrence of hop vine borer, whereas potato stem borer is positively affected by the presence of marshy areas where several alternate host plants may occur. Potato stem borer is most likely to be widespread in fields that are heavily infested with grasses (Deedat et al., 1982).
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