Hydraecia immanis Guenee Lepidoptera Noctuidae

Natural History

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.

  1. The eggs are laid in 2-3 parallel rows between the stem and leaf sheath of grasses with a split leaf sheath. The number of eggs ranges from about 30300 per clutch (Levine, 1986a). In shape, the eggs are a flattened sphere. They measure 0.64-0.82 mm in diameter and 0.31-0.51 mm in height. The edges of the eggs are marked with about 100 narrow, branching ridges, but ridges are absent from the center of the egg. They are white when first deposited, but turn reddish-brown, and then black just before hatching. The egg cluster is covered with a transparent film. They are often deposited in August-September and hatch in April-May—a duration of about eight months.
  2. There are six instars. Mean (range) of head capsule widths is about 0.33 (0.30-0.38), 0.60 (0.410.76), 0.97 (0.89-1.11), 1.43 (1.25-1.75), 2.24 (2.002.50), and 3.50 (2.75-4.06) mm, respectively, for instars 1-6. Larval development time is about 30-70 days when reared at 24-27°C. Deedat et al. (1983), for example, gave mean instar-specific development times of potato stem borer as 4.9, 3.8, 3.8, 3.2, 4.4, and 11.5 for instars 1-6, respectively. In the field, however, larval
Hop vine borer eggs.

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.

  1. Pupation occurs in the soil at a depth of 24 cm. Pupation normally occurs in July, with a duration of 4-6 weeks in the field in New York, but only 20-30 days under laboratory conditions. The pupa is dark brown, and measures 15-28 mm long. The tip of the abdomen bears two short spines.
  2. The adult is light brown, with pinkish or greenish tints. In hop vine borer the front wings bear broad light-colored transverse lines bordered with brown, and some darker gray to olive brown shaded areas. In potato stem borer the overall color and pattern are similar, but the transverse lines on the front wings are narrow and dark. The hind wings are grayish with dark veins. The wingspan is 40-50 mm. The moths were pictured by Rings and Metzler (1982).

Hop vine borer larva.

Hop vine borer larva.

Adult potato stem borer.
Adult hop vine borer.

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.

  • 1984). Developmental biology of both species was described by Giebink et al. (1985). Culture of Hydraecia spp. on artificial diet was described by West et al.
  • 1985) and Giebink et al. (1985). A sex pheromone has been described for potato stem borer (Teal et al., 1983; Burns and Teal, 1989).

Damage

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.

Management

  1. Adults can be sampled with blacklight traps, and the sex pheromone of these species may eventually prove useful. Levine (1989) used temperature summation, about 1700 degree-days above a threshold of 5.3°C, to estimate peak moth flight by hop vine borer. Larvae can be sampled by dissecting seedlings, but the below-ground portion should also be included in the sample. Wilting plants are a good indication of infestation by larvae.
  2. Persistent insecticides applied to crop plants and soil can provide some suppression of larvae, including those that disperse into crop fields from nearby weedy vegetation. However, better crop protection can be attained by applying insecticide directly to the source of many larvae, weedy fence rows (Deedat et al., 1982). Insecticidal control alone is often inadequate if the borers are abundant in the proximity of a susceptible crop.

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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