Distribution. This native species can be found throughout the United States, and often disperses to southern Canada during the summer. Formerly limited to central and western North America east of the Appalachian Mountains, the eastern dispersal of alfalfa caterpillar in the early 1900s was helped by the clearing of forests and widespread culture of alfalfa. It remains most common, and damaging, in the southwest.
Host Plants. Alfalfa caterpillar feeds on several forage legumes, particularly alfalfa, sweet clover, white clover, and hairy vetch. Red clover is not suitable for development, and adults rarely oviposit on this plant. Wild hosts include many species of milkvetch, Astragulus spp.; trefoil, Lotus spp.; clover, Trifolium; and vetch, Vicia spp. Among vegetable crops sometimes consumed are bean and pea, but not cowpea.
The adults feed on a succession of nectar-producing flowers, though the sequence varies between localities. In northern Virginia, the preferred flowers are dandelion, Taraxacum spp., and winter cress, Barbarea spp., during the first generation; followed by dogbane, Apocynum sp., and clover, Trifolium spp., during the second generation; then by milkweeds, Asclepias spp., in the third generation; and goldenrods, Solidago spp., aster, Aster spp., and tickseed sunflower, Bidens coronata, during the fourth to fifth generations (Opler and Krizek, 1984).
Natural Enemies. Natural enemies often are effective to keep this insect from becoming very abundant, and consequently, it generally is not a serious pest. An egg parasitoid, Trichogramma sp., occasionally destroys 50% of the eggs. As it develops rapidly, completes two generations in the very time it takes the host to complete a single generation, this egg parasitoid builds to high levels by the end of the season when caterpillar populations are high (Wildermuth, 1914). Young caterpillar larvae are attacked frequently by Apanteles flaviconchae Riley (Hymenoptera: Braconi-dae), with over 50% of them destroyed during some years. Michelbacher and Smith (1943) reported that when this parasitoid was abundant during the first generation of its host, the alfalfa caterpillar did not reach damaging levels. Other braconid parasitoids of lesser importance include Apanteles cassianus Riley, A. medicaginis Muesebeck, Meteorus autographae Mue-sebeck, M. laphygmae Viereck, and M. leviventris (Wes-mael). Other wasps known from alfalfa caterpillar are Itoplectis vidulata (Gravenhorst), Thyrateles instabilis (Cresson), Nepiera benevola Gahan, Hyposoter exiguae (Vierick), and Pristomerus spinator (Fabricius) (all Hymenoptera: Ichneumonidae). A pupal parasite, Pteromalus puparum Linnaeus) (Hymenoptera: Ptero-malidae), can be very important, sometimes parasitizing up to 60% of the pupae (Wildermuth, 1920). Among other parasitoids known from alfalfa caterpillar are Euphorocera claripennis (Macquart), E. omissa (Reinhard), and Lespesia archippivora (Riley) (all Diptera: Tachinidae).
The importance of predators has not been determined, but several are known. Ants (Hymenoptera: Formicidae) and lady beetles (Coleoptera: Coccinelli-dae) kill the caterpillar larvae, and robber flies (Diptera:
Asilidae) capture adults. Pupae of alfalfa caterpillar are sedentary, and therefore easy prey, and are consumed by the softwinged flower beetle, Collops vittatus Say (Coleoptera: Melyridae), larvae of corn earworm, Heli-coverpa zea Boddie (Lepidoptera: Noctuidae), and likely many others.
A disease caused by a cytoplasmic polyhedrosis virus sometimes causes spectacular alfalfa caterpillar population collapse late in the season. However, as happens with many insect diseases, these become apparent only during seasons when other natural enemies are ineffective and caterpillar densities are high.
Life Cycle and Description. The adults are observed from March-May until late November over most of the ranges of this species. The number of generations varies with latitude and elevation. In northern regions and in the Rocky Mountains, only two generations are reported annually. However, three to five generations occur over most of the United States, five to six are reported from southern California and Arizona, and seven generations occur in Louisiana. During mid-summer a complete life cycle can be completed in 30 days. In northern regions this species survives periods of cold weather in the pupal stage, though survival in northern latitudes is poor. In southern regions, alfalfa caterpillar tends to pass the cold periods in the larval stage.
Alfalfa caterpillar egg.
Alfalfa caterpillar egg.
Adult. The adults usually are orange-yellow butterflies with a broad black band distally on the upper surface of each wing. A large black spot is also found near the mid-point of each forewing. In females, but not males, the black marginal band contains light spots. The undersurface of each wing, in both sexes, is uniformly pale yellow. The wingspan measures about 50-58 mm in males, and about 62-64 mm in females. The upper surface of the male's wings reflects ultraviolet light. This light reflection is visible to butterflies and serves to separate alfalfa butterfly from a co-occurring species, Colias philodice Godart, and to minimize hybridization (Silberglied and Taylor, 1978), though pheromones are also involved in mating (Sappington and Taylor, 1990). The adults of alfalfa butterfly are quite variable in appearance, and white variants are sometimes observed. Also, larvae exposed to cold temperature produce, after a lag of one generation, adults with wings that are mostly yellow, but containing some orange. Larvae exposed to warm temperature, in contrast, produce adults with orange wings (Tuskes and Atkins, 1973). Common sulfur butterfly, C. philodice, is most easily confused with alfalfa butterfly, but it is yellow rather than yellow-orange. Hybrids between these two species are not uncommon.
The biology of alfalfa caterpillar was described by Wildermuth (1914, 1920), though Floyd (1940) and Michelbacher and Smith (1943) added important observations. A useful synopsis was included in Opler and Krizek (1984). Methods of rearing were given by Taylor et al. (1981). (See color figure 199.)
Alfalfa caterpillar larvae are defoliators. Young larvae initially feed on the leaf surface or make small holes in leaves, but soon consume large quantities of leaf material. Although they are recorded from several types of bean and pea, damage to vegetable crops is usually minor. The pupal stage is an imortant contaminant of peas processed for freezing and canning.
Vegetables generally are not at risk unless the caterpillars are very abundant on other crops or weeds. Thus, it is useful to monitor nearby alfalfa crops for the presence of caterpillars. The larvae and pupae are difficult to detect visually on foliage, and a sweep net is often used for sampling. If larvae are abundant in alfalfa, especially early in the season, it is often beneficial to harvest the alfalfa early because such harvesting can result in caterpillar mortality, and subsequent crops may avoid infestation. Infested crops can also be treated with insecticide or Bacillus thuringiensis applied to the foliage.
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