Jorō Spiders

Classification

Common name: Jorō spider

Scientific name: Trichonephila clavata L. Koch, 1878

Order: Araneae (spiders)

Family: Araneidae (orb-weaver spiders)

Description

Adult female jorō spiders are large and brightly colored, with body length up to 1 inch and a leg span up to 4 inches. The dorsum of the prosoma (cephalothorax) is covered with golden or silvery hairs. The opisthosoma (abdomen) is cylindrical, with bright yellow with grey-blue bands on the dorsum and an intricate yellow and grey-blue pattern with a large red mark on the venter. The legs are black with yellow bands (Figure 1).

Figure 1. Adult female jorō spider. Photograph by Benoît Segerer via iNaturalist, used under a CC BY-NC 4.0 license.

Immature female jorō spiders have a brown prosoma, a cylindrical opisthosoma with a black and yellow pattern, and black and yellow banded legs (Figure 2)

Figure 2. Immature female jorō spider. Note the difference in the pattern of the opisthosoma compared to the adult female in figure 1. Photograph by Paul Scott via iNaturalist, used under a CC BY-NC 4.0 license.

Adult male jorō spiders are much smaller and duller in color compared to females, with a body length of only 0.3 inches (Figure 3). The prosoma is light brown with lateral dark brown stripes and the opisthosoma has a dark brown central stripe.

Figure 3. Adult male jorō spider. Note the smaller size and brown coloration compared to females. Photograph by Denis via iNaturalist, used under a CC BY-NC 4.0 license.

Name

The Japanese name for jorō spiders is "jorō-gumo," which translates to "entangling or binding bride."

Distribution

Native range

Jorō spiders are native to Asia, including Japan (excluding Hokkaido), North and South Korea, China, Taiwan, Vietnam, and India (Figure 4).

Figure 4. Native range of jorō spiders, based on records from iNaturalist and Global Biodiversity Information Facility (GBIF).

Introduced range (current)

Jorō spiders were accidentally introduced into North America, likely through shipping containers or potted plant material, and were first discovered in three counties in Georgia in 2014. Jorō spiders have since spread throughout northern Georgia and adjacent areas of Alabama, North and South Carolina, and Tennessee (Figure 5). A single individual was also found in Oklahoma, although the person who reported it had traveled from northern Georgia and killed the specimen, so it likely did not represent an established population.

Figure 5. Current (2022) range of jorō spiders in North America, based on records from iNaturalist and Global Biodiversity Information Facility (GBIF).

Introduced range (potential)

Research has shown that Jorō spiders have a shorter lifecycle and can survive colder temperatures than related species (see Related species below). In their native range, Jorō spiders can be found in relatively cold areas – the average January temperature of northern Honshu, Japan is 25–32 °F, similar to most of Pennsylvania. So it is likely that they will be able to spread throughout eastern North America at least as far north as Pennsylvania and possibly further in warmer, coastal areas.  

Between 2014–22, Jorō spiders spread outward in all directions from the location they were initially found by 50–80 miles, or about 10 miles per year. At that rate, they may reach southeastern Pennsylvania in 35 years and northwestern Pennsylvania in 60 years. However, spiderlings are capable of moving tens to hundreds of miles via ballooning (see Life history below), so a storm blowing in the right direction at the right time of year may move them in large jumps. Additionally, as they increase in numbers, the risk of human-mediated transport to new areas also increases. This means that the immediate risk of Jorō spiders appearing in Pennsylvania is relatively small but will increase year over year as they spread naturally, via high wind events, and through accidental human transport.

Life history

Jorō spiders overwinter as eggs in a papery silk egg sac. In Japan, spiderlings emerge in early June and usually go relatively unnoticed. While small, they can be transported through the air through a process called ballooning, where the young spiders release silk and are picked up by the wind. Ballooning spiderlings can travel tens to hundreds of miles, especially if they are picked up by strong winds and storms.

In Japan, male spiders mature in late August and females in September and early October. In-depth phenologies have not been published for the United States, but mature females have been photographed as early as late August and as late as early December in Georgia.

Adult female jorō spiders create large webs that can be up to 10 feet wide. The webs are suspended between two or more objects, such as trees, bushes, light or fence posts, etc. Adult male spiders do not spin their own webs but are found in the webs of females (Figure 6). Female spiders lay a single egg sac which contains 400–500 eggs, which is usually attached to bark, leaves, or human structures.

 
Figure 6. Three male jorō spiders in the web of a female. Photograph by megcawood via iNaturalist, used under a CC BY-NC 4.0 license.

Medical importance

Jorō spiders do not have medically important bites. The fangs of jorō spiders are small and have difficulty piercing human skin. The spiders themselves are reluctant biters and it can be difficult to make them bite you even when the spiders are handled. The venom is weak, so when bites do occur they are less painful than a bee sting and only produce localized pain and redness that dissipates quickly without intervention. The author could not find any published reports of allergic reactions associated with jorō spider bites, although some people may be allergic to the venom.

Similar species

Golden silk orb-weavers (Trichonephila clavipes) are closely related to jorō spiders and, until jorō spiders were introduced into the United States, were the only Trochonephila species present in the New World (Figure 7). Golden silk orb-weavers are found in the Southeastern United States from Virginia to Florida, primarily along the Coastal Plain, west into Texas, and south throughout Central and South America (Figure 8). Prior to 2000, the species was not recorded north of southern North Carolina. It has since apparently moved northward into southern Virginia. Additionally, sporadic records include individuals from Pennsylvania, New Jersey, New York, and even southern Canada, although these spiders probably ballooned north as spiderlings and likely do no represent reproducing populations. The species was suggested as a model organism to track the impacts of climate change as it is expected to continue to move northward as average temperatures increase.

Golden silk orb-weavers are similar in size to jorō spiders. However, they can be distinguished based on the coloration. Female golden silk orb-weavers have a prosoma that is similarly covered in silvery hairs but the opisthosoma is dull orange to tan and covered with yellow spots. Additionally, the legs are brown with orange bands and have exaggerated hair brushes on legs I, II, and IV that look like fuzzy leg warmers.

Figure 7. Adult female golden silk orb-weaver. Note the differences in coloration and fuzzy leg warmers on legs I, II, and IV that distinguish it from jorō spiders. Photograph by rook70031 via iNaturalist, used under a CC BY-NC 4.0 license.

Figure 8. Range of golden silk orb-weavers in North and Central America. Dark green shows areas they are more commonly found, while light green shows areas with only scattered records. Note that they are rarely found as far north as Pennsylvania (but can occur here). Range based on records from iNaturalist and Global Biodiversity Information Facility (GBIF).

Garden spiders (Argiope spp.) are somewhat similar to jorō spiders and may be confused with them. Five species occur in North America: black and yellow garden spiders (A. aurantia) and banded garden spiders (A trifasciata) occur across the United States, including Pennsylvania; silver garden spiders (A. argentata) are found in southern California, Texas, and Florida; Florida garden spiders (A. florida) are found from North Carolina south to Florida); and Argiope blanda are restricted to southern Texas.

Argiope species are much smaller than Trichonephila species, with a body length of up to an inch and leg spans up to 2.5 inches. Both black and yellow and banded garden spiders have prosomas covered in dense, silvery hair, similar to jorō spiders. However, the opisthosomas are less cylindrical and differently patterned: black and yellow in black and yellow garden spiders (Figure 9) and striped with yellow, black, and silver in banded garden spiders (Figure 10). Additionally, Argiope species usually build a stablementum, which is an obvious pattern, often zigzags, which has an unknown function, into their webs while Trichonephila species do not (Figure 11).

Both black and yellow and banded garden spiders are common and abundant in Pennsylvania. They are especially noticeable when the adult females are active during the late summer and fall. Similar to jorō spiders and golden silk orb-weavers, neither species readily bite, even when handled, and if bites do occur the venom is weak and not medically important.

Figure 9. Black and yellow garden spider. Photograph by y82ws6 via iNaturalist, used under a CC BY-NC 4.0 license.

Figure 10. Banded garden spider. Photograph by Viajes_Biología via iNaturalist, used under a CC BY-NC 4.0 license.

Figure 11. Immagure black and yellow garden spider sitting on top of a stablementum. Immature jorō spiders rarely make a stablementum and adults never do, so the presence of a stablemenum in an orb web can help identify the maker of the web. Note the small stablemenum in Figure 9 as well. Photograph by Tom Kennedy via iNaturalist, used under a CC BY-NC 4.0 license.

Impact

Jorō spiders are a newly introduced species, so their impact is unclear. There are anecdotal reports of the spiders reaching high densities in Georgia and, due to the large size of their large webs, they may be nuisance pests when the webs are built around homes and other places people frequent.

The impact of jorō spiders on the environment and native species is not yet known. Like all spiders, they are predators that feed on insects and other arthropods, so will necessarily feed on native species. Whether they will capture and consume enough insect biomass to affect anything remains to be seen. It’s also not clear if they will compete with and displace native species, including golden silk orb-weavers, Argiope species, and other large, web-building spiders.

Control

Control measures have not yet been developed for jorō spiders. If control is desired, individual spiders can be removed manually, either by hand or by striking them with a stick or other object. Egg sacs that are located in the fall and winter can be destroyed before the spiderlings hatch. Biocontrol agents have not been investigated but may not be possible as there is likely to be a risk to related native species. Pesticide controls are not recommended at this point.

References

Bakkegard, K. A., and L. J. Davenport. 2012. Nephila clavipes (Araneae: Nephilidae): A model species for monitoring climate change in the Southeastern United States. Southeastern Naturalist, 11(4): 551–566. Available online.

Davis, A. K., and B. L. Frick. 2022. Physiological evaluation of newly invasive jorō spiders (Trichonephila clavata) in the southeastern USA compared to their naturalized cousin, Trichonephila clavipes. Physiological Entomology. doi.org/10.1111/phen.12385.

Global Biodiversity Information Facility (GBIF). 2021. Trichonephila clavata. Available online. Last accessed 8 March 2022.

Global Biodiversity Information Facility (GBIF). 2021. Trichonephila clavipes. Available online. Last accessed 8 March 2022.

Hoebeke, E. R., W. Huffmaster, and B. J. Freeman. 2014. Nephila clavata L Koch, the Joro Spider of East Asia, newly recorded from North America (Araneae: Nephilidae). PeerJ, 3: e763. Available online.

iNaturalist. 2021. Trichonephila clavata. Available online. Last accessed 8 March 2022.

iNaturalist. 2021. Trichonephila clavipes. Available online. Last accessed 8 March 2022.

Weems, H.V., Jr., and G.B. Edwards, Jr. 2020. Featured Creatures: golden silk spider. University of Florida circular EENY-229. Available online. Last accessed 8 March 2022.