Tussock moths in the genus Orgyia are small moths that are best known because of their attractive larvae.
In some years the larvae are very numerous and become a problem when they leave their host plants to search for suitable sites to spin their cocoons. Only the three species that are found in Florida will be discussed here. Orgyia detrita (the fir tussock moth) is the most common of the species in Florida followed by Orgyia leucostigma (the whitemarked tussock moth) and finally Orgyia definita (the definite tussock moth), which is rare in Florida (Foltz 2004).
Much of the older literature places the tussock moths in the family Liparidae and more recently in the Lymantriidae. They are now classified in the subfamily Lymantriinae in the family Erebidae (Beadle & Leckie 2012). Orgyia leucostigma was formerly placed in the genus Hemerocampa. For a detailed taxonomic history and synonyms, see Ferguson (1978).
Orgyia detrita: Coastal Plain from Long Island to Florida and Gulf States west to Texas (Ferguson 1978, Wagner 2005, Orgyia detrita entry at North American Moth Photographers Group web site). It is uncommon in the northern parts of its range.
Orgyia leucostigma: Entire eastern United States and west to Minnesota and Texas (Ferguson 1978, Orgyia leucostigma entry at North American Moth Photographers Group web site). The form that occurs from South Carolina to Texas is subspecies Orgyia leucostigma leucostigma (Godfrey 1987).
Orgyia definita: Entire eastern United States. Most common in Northeast and mid-Atlantic states (Ferguson 1978, Orgyia definita entry at North American Moth Photographers Group website).
Larvae are 1–1.5 inches in length. They are characterized by hair pencils of black setae that extend forward from the prespiracular verrucae of the prothorax, a dorsal hair pencil of black setae on the eighth abdominal segment, dorsal tussocks on the first four abdominal segments, and mid-dorsal glandular structures on abdominal segments six and seven.
Orgyia detrita has two common color forms in Florida, a dark form and a light form. The sub-dorsal areas (sides) can be a dark gray as in Figures 1 and 2, or they can be light gray to light yellow as in Figure 3. The sides of Orgyia leucostigma are light in color, similar to the light form of detrita. There is a white or yellow line on each side of the dark mid-dorsal line of leucostigma (Ferguson 1978, Godfrey 1987). Orgyia detrita has bright orange spots along the back and sides, while the spots on leucostigma are yellow (Foltz 2004).
Detrita and US populations of leucostigma have bright red heads, while definita are unique because of their tan or yellow heads. Definita is also lighter in body color than the other two species (Foltz 2004).
The dorsal glandular structures on segments six and seven of leucostigma are bright red, those of detrita are orange, and those of definita are pale yellow.
Cocoons and Pupae
Cocoons are constructed of silk and setae from the caterpillars. They are usually found in protected places—in furrows in bark, undersides of limbs, in tree cavities, under loose bark, and often under the soffits of buildings. Also, they are commonly spun in dense masses among the foliage of epiphytic bromeliads (Tillandsia spp.).
Pupae appear hairy and have patches of dorsal spatulate setae ("vesicles" of Mosher 1916) on abdominal segments 1–3. The antennae of male pupae are longer and broader than those of females and the wings of male pupae are longer than those of female pupae. The wings of female pupae reach only slightly beyond the anterior margin of the fourth abdominal segment while those of male pupae extend nearly to the posterior margin of the segment (Mosher 1916).
Adults are dimorphic. Males are small, relatively dull-colored moths with prominent bipectinate antennae. At rest, they hold their first pair of legs in an outstretched position. The genus name Orgyia (Greek for "the length of the outstretched arms" [Borror 1960]), is based on this pose. Wingspreads of Orgyia species are 2.0–3.5 cm (0.78–1.4 in).
For photographs of pinned and spread specimens of males of the Lymantriinae, see Ferguson (1978). Males are difficult to distinguish, but fresh specimens of Orgyia leucostigma and Orgyia definita have a purplish tint that is lacking in Orgyia detrita. Detrita also lacks the whitish tornal spot of leucostigma and definita (Ferguson 1978).
The literature frequently describes the females as being wingless. Actually, they are brachypterous (short-winged) but cannot fly. At present, females can be identified to species only by association with their respective larvae (or in the case of Florida Orgyia detrita, by association with their egg masses).
Life Cycle and Biology
Orgyia detrita is univoltine (one generation per year), while the other two species are bivoltine in Florida (Foltz 2006). In Florida, the overwintering eggs begin to hatch in late February. After hatching, the young larvae feed on the remaining egg mass and then spin a silk thread that they use to "balloon" for dispersal (Thurston 2002). Because adult females are flightless, ballooning by young larvae is the major mode of dispersal. Ballooning is also important given their propensity for spinning cocoons off their host plants (i.e., on buildings, fences, and other artificial objects).
By the second instar, the larvae are already recognizable because of their short hair pencils. Young larvae eat holes in leaves. Older larvae are leaf-edge feeders.
Caterpillars reach maturity and wander in search of sites to spin their cocoons in early April in Florida.
Adults emerge from mid-April to early May. The flightless females remain on their cocoons and release a sex pheromone to attract males. The sex pheromones of Orgyia detrita and Orgyia leucostigma have been characterized (Grant et al. 2003, Gries et al. 2003).
After mating, the females lay a mass of eggs directly on the cocoon and cover them with a protective covering. Detrita and definita females cover their eggs with a secretion and then rub setae from their bodies onto the secretion to form a protective layer over the eggs. Leucostigma females cover their eggs with a frothy secretion but do not cover the secretion with setae (Ferguson 1978). The egg stage is the overwintering stage for all three species.
Orgyia detrita: Although the common name is "fir tussock moth", the only documented hosts are oaks and bald cypress (Taxodium distichum) (Ferguson 1978).
Orgyia leucostigma: Polyphagous. Heppner (2003) listed plants belonging to 116 genera that have been reported as hosts. A few common hosts include oak, cherry, hackberry, and willow.
Orgyia definita: Only willow (Salix sp.) has been confirmed as a host in Florida, but other host plants are also likely (Heppner 2003). Common hosts in other parts of its range include oak, maple, hackberry, birch, and willow (Wagner 2005).
The medical importance of Orgyia species caterpillars is well-documented in the scientific (Diaz 2005, Gilmer 1925, Goldman et al. 1960, Knight 1922) and clinical dermatology (Hossler 2009 & 2010 ) literature. Pruritic (itching) dermatitis due to tussock moth caterpillars has been reported to be a problem at child day-care centers and elementary schools in Florida (Atrubin et al. 2012, Atrubin & Granger 2006, Cruse et al. 2007). Contact with the cocoons produces the same symptoms.
The caterpillars may be contacted when they drop from the host trees or when they wander from the trees in search of a place to spin their cocoons. Homeowners develop dermatitis from contact with the cocoons while removing them from the soffits of houses. Hairs in the cocoons retain their urticating capability for up to a year or longer.
Most of the urticating hairs are in the dorsal tussocks of the caterpillars (Knight 1922), but a few are also found on the lateral verrucae and intermingled with the black plume hairs of the hair pencils (Gilmer 1925). Gilmer (1925) conducted histological studies of the urticating setae of Orgyia leucostigma and found that each seta has a venom gland at its base. The venom has not been adequately characterized.
Welts resulting from contact with Orgyia hairs usually appear within minutes and subside by the next day, but itching and erythema commonly continue for another day or two. People apparently vary somewhat in their sensitivity to Orgyia species hairs. Goldman et al. (1960) studied the histopathology of a sensitive person and reported perivascular concentrations of eosinophils and leucocytes beneath the irritated areas.
Tussock moth larvae have various natural enemies. Medina and Barbosa (2002) looked at predation of small and large Orgyia leucostigma larvae in a temperate forest and suggested that birds were the major predators of large larvae but most mortality of smaller larvae was probably due to failure to find a suitable host during ballooning dispersal and also possibly to predation by invertebrate predators in the leaf litter. Large ground beetles (Henn et al. 2009) and Polistes paper wasps (Castellanos et al. 2011) have also been reported to attack the larvae up in the trees.
Orgyia caterpillars are infected by nuclear polyhedrosis viruses (Baculovirus) (Cunningham 1972) and cytoplasmic polyhedrosis viruses (Cypovirus) (Hayashi and Bird 1968). Those infected with nuclear polyhedrosis virus typically die in a characteristic pose—hanging limp by their prolegs.
Larvae and pupae are killed by various parasitoids. Foltz (personal communication) counted egg masses on cocoons and found that there were far less than the 50 percent that would be expected based on a 50:50 ratio of females to males obtained by laboratory rearings. In the year 2000, only 70 of 730 (9.6%) cocoons he examined had egg masses, suggesting a high rate of mortality. He has suggested that levels of parasitism of pupae often approach 50 percent (Foltz 2004, 2006).
Parasitoids of Orgyia detrita and Orgyia definita have not been well-studied, but those of Orgyia leucostigma are well documented. There is little doubt that Orgyia detrita and Orgyia definita also have many parasitoids.
Arnaud (1978, pp. 632–633) listed the following tachinid parasitoids of Orgyia leucostigma: Bessa selecta (Meigen), Carcelia amplexa (Coquillet), Carcelia perplexa Sellers, Carcelia yalensis Sellers, Compsilura concinnata (Meigen), Drino inconspicua (Meigen)*, Euphorocera claripennis (Macquart), Euphorocera edwardsii (Williston), Exorista lobelia Coquillet (currently Nilea lobelia [Coquillet]), Exorista mella Walker, Leshenaultia spp., Lespesia aletiae (Riley), Lespesia frenchii (Williston), Patella leucaniae (Coquillet), Phorocera spp., Sisyropa spp., Winthemia datanae (Townsend), and Winthemia quadripustulata (Fabricius).
*This species is not currently recorded from N.A. north of Mexico (O'Hara 2012).
Names from Arnaud (1978) have been updated by O'Hara and Wood (2004) and O'Hara (2012).
Control of the caterpillars is difficult because by the time they are migrating from the trees, it is too late. In Florida, feeding damage to large trees by Orgyia species does not usually harm the trees. However, they may occasionally be sufficiently numerous to completely defoliate large trees. Also, large numbers of larvae blown onto small landscape trees may result in severe defoliation.
If control measures are required, chemical insecticide or Bacillus thuringiensis applications recommended for control of other caterpillars should be effective. For current control recommendations, contact your local UF/IFAS Extension agent.
Insects are very popular in human culture. Images of butterflies and moths are common in movies, art, jewelry, and fabrics. Although the fir tussock moth is not a highly familiar moth even to most entomologists, an image of an adult male does appear in a popular design used on ornamental paper, wall art, journal covers, purses, and fabric (Tim Holtz, personal communication).
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