Strymon melinus Hbner Lepidoptera Lycaenidae

Natural History

Distribütion. This insect is found throughout the United States and southern Canada. Its range also extends south through Central America to South America. Apparently it is absent from the Caribbean.

Host Plants. The caterpillar stage of gray hair-streak feeds on the widest array of plants of any butterfly, but few are crop plants. Serious damage is limited to bean and cotton. Because of its injury to the latter plant, this species is also known as "cotton square borer." Vegetable crops eaten include cowpea, lima bean, okra, pea, and snap bean. This insect is sometimes associated with such field crops as alfalfa, hops, lespedeza, and sweet clover. Numerous other plants are consumed by this insect, including such Leguminocae as Astragalus, Casia, Desmodium, Lupinus, and Trifolium spp.; such Malvaceae as Hibiscus, Malva, and Sphaeralcea spp.; such Polygonaceae as Eriogonum, Polygonum and Rumex spp; and many others. Although the preferred habitat is open areas inhabited by early successional plants and shrubs, most agricultural weeds are exempt from attack by this insect. Scott (1986) provided a list of larval hosts. Adults collect nectar from goldenrod, Solidago spp.; dogbane, Apocy-num spp.; milkweed, Asclepias spp.; sweet clover, Meli-lotus spp.; and other flowering plants (Opler and Krizek, 1984). (See color figure 32.)

Natural Enemies. Natural enemies generally are effective at suppression of gray hairstreak numbers, so little crop damage occurs. In Texas, for example, the first generation is abundant, but in subsequent generations caterpillars tend to be less and less abundant as parasitoids take a toll. The most important parasitoid is a gregarious species, Apanteles thecloe Riley (Hymenoptera: Braconidae). This species attacks the caterpillar, with larvae emerging from the host to pupate in white silken cocoons on the back of the caterpillar. Other, less important parasitoids are Octosmicra sp. (Hymenoptera: Chalcididae), Aplomya theclarum (Scudder), and Lespesia sp. (both Diptera: Tachinidae). Predators are poorly known, but adults are taken by robber flies (Diptera: Asilidae).

Life Cycle and Description. There are two generations annually in the northern portion of gray hair-streak's range, and 3-4 generations in the central and southern states. About 40 days are required for a complete generation. In Texas, butterflies are observed in all months except December and January. The winter is passed in the pupal stage. Females begin oviposition in February or March in the south, but not until May in the North.

  1. The eggs are spherical in shape, but slightly flattened. They are pale-green to yellowish-white. They measure 0.6-0.7mm in diameter, and 0.2-0.3 mm long. They are deposited singly, usually on the underside of leaves. Caged adults have been observed to produce 40 -70 eggs each, but this is believed to be an underestimate of true fecundity because this species fares poorly in captivity. Oviposition occurs most frequently in the morning and early evening. Mean duration of the egg stage is 5.5 days (range 4-6 days).
  2. There are five instars. The mean (range) duration of each instar is 3.6 (3-5), 3.4 (3-5), 3.5 (36), 4.8 (3-6), and 7.7 (5-11) days, respectively. Thus, total larval development requires about 23 days. At hatch, larvae measure only about 1 mm long, but even-
Gray hairstreak larva.

tually grow to measure 12-16 mm long and 4.56.5 mm wide. Initially the larvae are slender, but with each succeeding molt the larva becomes relatively broader, so at maturity it is rather stout. The caterpillar in all instars is covered with stout hairs or bristles. The body color is generally green, but sometimes brownish. Larvae normally feed on the leaf surface during the first three instars, and thereafter display a tendency to bore into pods or other reproductive tissues. (See color figure 97.)

  1. Near the completion of larval development the larva attaches itself with silk to the plant, usually adjacent to the feeding site. After a quiescent period of 2-3 days the larva molts into the pupa. The pupa is yellow to brown, and bear blackish spots that vary in size, frequency, and intensity. It measures 7.510.5 mm long. The sides of the pupa bear spines arranged regularly in rows. Duration of the pupal stage is about 9.6 days (range 8-11 days).
  2. The adults usually emerge in the morning, and 2-3 days are required before oviposition commences. The butterflies are small, measuring only 25-32 mm in wingspan. In general appearance they are blue-gray. The dorsal surface of the wing is steel-gray, with the lower surface pale-gray. The hind wing is marked dorsally with a reddish-orange spot. The wings are fringed with long scales, and each hind wing bears two thin extensions or tails—one long and the other short. Adults live at least 10 days in confinement, but likely live considerably longer in nature. In southern latitudes, butterflies apparently are dormant during the brief cold periods of winter, and become active during warm weather. Adult males perch on bushes and trees and patrol their territory to watch for intruders. They are aggressive about defending their territory not only from males of the same species, but other butterflies, wasps, and even hummingbirds (Alcock and O'Neill, 1986). (See color figure 203.)

A comprehensive treatment of gray hairstreak biology was given by Reinhard (1929).


Foliage consumption by early instar larvae is insignificant, but damage may occur as larvae burrow into

Adult gray hairstreak.

bean or okra pods. On beans, larvae often burrow only partially into the pod, then move to another location, which may be on the same or another pod. Only in home gardens is this damage likely to be significant.


These insect cause only incidental damage and infrequently require suppression. Foliar insecticides are generally very effective.

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