Thelia bimaculata

Thelia bimaculata is widely distributed throughout the eastern United States (Funkhouser, 1915). Based on the distribution of its host species, Robinia pseudoacacia (Black Locust), T. bimaculata is likely to inhabit areas extending southwest through the Appalachian mountains to Northeastern Alabama (Huntley, 1990). (Funkhouser, 1915; Funkhouser, 1917; Huntley, 1990)

Thelia bimaculata is found in abundance on its preferred host tree, R. pseudoacacia (black locust). T. bimaculata prefers smaller trees in open, sunny areas such as the edges of wooded areas or near roads. Nymphs are generally found on tree trunks near the ground and adults are rarely found at heights over 5 m (Funkhouser, 1915). (Funkhouser, 1915)

As do nearly all species in the Membracidae, T. bimaculata has a distinct, exaggerated pronotum (Wood, 1993; Funkhouser, 1915). The pronotum is horned and extends posteriorly to a sharp point. Females are somewhat larger than males and have gray pronota with irregular brown markings. The males are dark brown with a distinctive, bright yellow band on either side of the thorax (Crew, 1927; Funkhouser, 1915). Males parasitized by parasitoid Aphelopus wasps will assume female coloration and size (see Ecosystem Roles) (Crew, 1927; Fenton, 1918). Nymphs are gray and have a spiny dorsal region. Adults are 11 mm in length (14 mm including horn) (Funkhouser, 1915). (Crew, 1927; Fenton, 1918; Funkhouser, 1915; Wood, 1993)

Females create a narrow slit in the base of the host tree and deposit from three to six eggs into the slit. This process is usually executed two to three times in succession. Thelia bimaculata overwinters in the egg stage and the first nymphs appear in late May to early June. Given the broad geographical range of T. bimaculata, timing and duration of the developmental processes may vary spatially.

The eggs are white, 2.6 mm long and 0.6 mm wide. They appear swollen two days before hatching and the nymphs later emerge through the top. Thelia bimaculata has five distinct instars. The first instar lasts around seven days and is distinguished by a whitish color, a long beak and abdominal spines. The second instar lasts an average of five days and is darkly colored, with diminished spines. The third instar lasts an average of six days and can be identified by a complete loss of thoracic spines and a swelling in the anterior region. The fourth instar also lasts six days on average. This instar exhibits increased swelling, visible wing pads, and dark markings on the abdomen. The fifth instar is the longest nymphal stage and lasts 8-15 days. This stage is marked by significant pronotal development, fully developed wing pads, and brown spotted legs. The total time of development from egg to adult is about one month. In the young adult, the pronotal horn is not hardned for a few hours and is subject to injury during this period. A notable distinction in ecdysis of T. bimaculata is that the molted integument does not remain attached to the host unlike other membracids, whose skins are often found on trees (Funkhouser, 1915; Funkhouser, 1917). (Funkhouser, 1915; Funkhouser, 1917)

Mating begins in early July and continues throughout the summer. During mating, the male and female will face opposite directions, with their caudal extremities touching. The process of copulation takes an average of 20 minutes (Funkhouser, 1915). (Funkhouser, 1915)

Membracids generally locate their mates through courtship calls (Dietrich, 2008). There is usually one brood per season, but more have been observed in closely related species under favorable conditions (Funkhouser, 1917). Funkhouser (1917), in a study of a population in New York state, found one brood per season, mating from early July through the end of the summer, individuals 30-40 days old at maturity, and females laying an average of 28 eggs per female. (Dietrich, 2008; Funkhouser, 1917)

Paternal care has never been reported in Membracidae. Although there is little or no information about maternal care in T. bimaculata, related genera have shown female egg guarding behavior. In the tribe Hoplophorionini, the mother aggressively protects her offspring and even alters the host plant to ease the feeding process for the nymphs. Egg guarding is more common among New World Membracidae than among Old World Membracidae (Wood, 1993). Females are known to remain abundant until after the first snow, whereas males are practically gone by the beginning of October (Funkhouser, 1915). This may be an indicator of some maternal care. (Funkhouser, 1915; Wood, 1993)

The average lifespan of membracid species observed in the Cayuga Lake Basin (including T. bimaculata) is approximately 14 months. Of this, eight and a half months are spent in the egg stage (Funkhouser, 1917). (Funkhouser, 1917)

An intriguing habit of adults is that they will rest on large branches or the trunk of their host in rows of twenty to forty individuals, all facing toward the trunk or, if on the trunk, toward the ground (see Harstaad, 2002). This may be a camouflage behavior to further their resemblance to the thorns found on black locust (Pseudacacia robinia) trees (Funkhouser, 1915).

Adult members of T. bimaculata can fly for short distances, making a buzzing sound, however they are generally not active. Most membracids prefer to walk and will only jump when preparing for flight. There have been no observations of hopping from twig to twig (Funkhouser, 1917). (Funkhouser, 1915; Funkhouser, 1917; Haarstad, 2002)

Migration of membracid species similar to T. bimaculata are notably slow. It is often the case that one tree is covered with a particular species, while a neighboring tree of the same species is completely uninhabited for successive seasons. Little movement is seen from the plant on which the eggs were laid during the lifetime of the membracid (Funkhouser, 1917). (Funkhouser, 1917)

Membracids generally locate their mates through courtship calls (Dietrich, 2008). They have limited vision, and are presumably very sensitive to the smell of their preferred host plant. (Dietrich, 2008)

Although specific observations of T. bimaculata feeding are scarce, all closely related species have similar feeding patterns. Both the nymphs and the adults use their beaks to pierce young stems and petioles of their host tree. In rare cases, feeding has been observed on the blades of the leaves (Funkhouser, 1917). Observations of T. bimaculata suggest that this species prefers older twigs with bark (Funkhouser, 1915). (Funkhouser, 1915)

Large robber flies have been seen carrying off individuals. One case of a toad attempting to eat the nymphs from the base of a tree has been reported, however the nymphs held on to the host tightly and made the process difficult for the toad. Some membracids have been seen caught in spider webs. The pronotal horn and sharp posterior of these leaf hoppers mimic the thorns of the host tree (see Moran, 2004) and deters many potential predators, including birds (Funkhouser, 1915). (Funkhouser, 1915; Moran, 2004)

Parasitism of the eggs has been noted, as some eggs are perforated and fail to mature. The exact parasite has not been identified. Nymphs and adults are occasionally parasitized by mites (Funkhouser, 1915).

Noteworthy parasites of T. bimaculata are wasps of the genus Aphelopus, which lay eggs in T. bimaculata nymphs. In cases where eggs were deposited in the first or second instar, fifty to sixty larvae will emerge and eat all of the contents of the host’s exoskeleton. If the eggs are deposited later and the parasites are not fully developed by the host’s fifth instar, the adult host becomes modified. Parasitized males show an increased body size and female coloration. Parasitized females will have decreased body size, but roughly the same form (Crew, 1927). The external genitalia of both are not functionally affected, but are reduced in size (Fenton, 1918).

Attendance by ants is an interesting mutualism observed in T. bimaculata. Nymphs and adults are attended in large numbers by ants, however a benefit to the ant is only seen in nymphs. The ant will collect a liquid secreted from the anal tubes of the nymphs. Stroking the nymph with its antennae will cause greater quantities of this secretion. When enemies (including collectors) approach, the ants will protect the nymphs by biting. Ant species with such behavior have been identified as Formica obscuriventris, Formica exsectoides, Camponotus pennsylvanicus, Crematogaster lineolata, and Prenolepis imparis (Funkhouser, 1915).

T. bimaculata are likely to be monophagous. The only host observed for this species is the black locust. Evidence indicates that the entire lifecycle of T. bimaculata is spent on this tree (Funkhouser, 1915). (Crew, 1927; Fenton, 1918; Funkhouser, 1915)

Positive impacts of T. bimaculata are unknown.

Although no specific economic impact has been described for T. bimaculata, closely related species have been identified as vectors of Xylella fastidiosa, a bacterial plant pathogen that causes bacterial leaf scorch in certain strains (Lashomb and Iskra, 2002). It is unlikely that T. bimaculata spreads the disease, however, because its host tree has not been shown to be affected. (Lashomb and Iskra, 2002)

This species has not been identified as needing special conservation. It's fate is dependent on the status of its host tree.