Around 70 species of non-native, human-introduced spiders have been observed in North America. A handful of these species have established themselves across much of the continent, including Tegenaria domestica (the domestic house spider or barn funnel weaver) and Cheiracanthium mildei (the yellow sac spider), but most are found more locally where they were first introduced. A common theme among introduced spider species is a preference for living in and around houses and other buildings. Most non-native spider species are inconspicuous in appearance or behavior and avoid the public eye.
Joro spiders (Trichonephila clavata) are anything but inconspicuous. Impressive size, flashy colors, and a talent for spinning large webs make female Joro spiders hard to miss.
An adult female Joro spider at Mill Creek Nature Center in Gwinnett County, Georgia
Trichonephila clavata (formerly placed in the genus Nephila) is a large orb-weaver native to much of Eastern Asia, including parts of China, Japan, and Korea. As with other species in this genus, including Georgia's resident Trichonephila clavipes, males (4 – 8 mm) are significantly smaller than females (17 – 30 mm) and do not spin orb-webs of their own, preferring instead to join the female on hers. Due to their smaller size and unobtrusive behavior, males are less commonly observed.
The first confirmed sightings of Joro spiders in the United States came from northeastern Georgia (Barrow, Jackson, and Madison counties) in 2014. Residents reported that they had been seeing Joro spiders for a couple of years. While these earlier reports are not confirmed, these statements and the rapid range expansion afterwards suggest that the Joro spider was indeed introduced a few years prior to 2014.
Joro spiders are univoltine (females produce 1 egg-sac per year). Female Joro spiders deposit between 400 and 1500 eggs in their one egg sac. The species passes the winter in the egg sac and spiderlings hatch the following spring. Individuals become sexually mature as summer gives way to autumn. In Georgia, adult females can be seen from August through November. A few persist into December, but these individuals are usually on their last legs (literally and figuratively).
Figure 1: Joro spider observations on iNaturalist from introduction through 2018
The initial entry point is not known, but was likely somewhere along the I-85 corridor. This interstate is flanked by a large number of warehouses and distribution facilities. Many accidentally-introduced insect and spider species are transported on shipping containers, plant materials, or wooden pallets. There is no reason to suspect that the introduction story of Joro spiders was any different.
While the Joro spider attained a B-list celebrity status on local news after its discovery, it never ascended to the level of national news media stardom that the so-called Murder Hornet captured last year. Since the initial flurry of attention, the Joro spider has descended into relative anonymity. But spiders don’t care for such frivolity anyways, they just do their own thing.
Though Joro spiders are large and imposing, they pose no threat to people
I turned to iNaturalist data to see what T. clavata has been up to in the decade since its introduction. The answer I found? “Enjoying the balmy Georgia weather.”
Figure 2: The present range (as of 2020) of Joro spiders in North America
T. clavata has now been sighted in at least 24 Georgian counties – Banks, Barrow, Cherokee, Clark, Cobb, Dawson, DeKalb, Douglas, Forsyth, Franklin, Fulton, Gwinnett, Habersham, Hall, Jackson, Lumpkin, Madison, Monroe, Oconee, Oglethorpe, Rabun, Stephens, Walton, White – and 1 county in South Carolina (Anderson). If we take the I-85 corridor near the cities of Braselton and Hoschton to be the original entry-point, Joro spiders have expanded their range roughly 50 miles northward, 60 miles to the south, 40 miles westward, and 50 miles to the east.
Figure 3: Joro spiders have greatly expanded their range in less than a decade
How are Joro spiders so successfully expanding their range within Georgia?
Two modes of transport might explain their rapid range expansion. First, adult spiders (or egg sacs) may be carried to new destinations by cars, trucks, and trains. This could explain the easternmost point in South Carolina, which is situated along the I-85 corridor. Second, ballooning of spiderlings immediately after hatching, a dispersal strategy employed by many spiders that utilizes air currents to travel long distances, could explain range expansion into areas away from major transportation corridors.
Joro spiders currently occupy a band of Georgia reaching from the 33rd to the 35th parallel. A glance at iNaturalist observations in Asia reveal that Joro spiders range between the 12th parallel (in Vietnam) and the 41st parallel (in Japan) in its native range.
The Joro spider is most likely only just beginning to explore North America. My anecdotal experience is that they are equally at home in backyards and neighborhoods as they are in riparian corridors and deciduous woodlands.
Figure 4: Hypothetical latitudinal range limits of the Joro spider in North America based on iNaturalist data from its native range
How is T. clavata impacting native species? We don't know yet. One possible competitor in the native “large orb-weaver” guild is the yellow garden spider, or writing spider (Argiope aurantia). There is reason to hope that the newcomer may not directly compete with A. aurantia – Joro webs tend to be higher above the ground compared to those of writing spiders and after looking at iNaturalist data, I believe there may be a slight seasonal mismatch as well.
Figure 5: Seasonal trends in iNaturalist observations of the Joro and writing spider in Georgia
Observation frequency of writing spiders in Georgia appears to peak in mid-September while the peak for Joro spiders is in early October. A brief foray into iNaturalist data is not a substitute for standardized field data but I think this potential difference is worth investigating further.
Argiope aurantia, a native orb-weaver that may compete with the Joro spider
Kleptoparasitic dewdrop spiders have been observed in Joro webs. These small spiders intrude onto the webs of their larger neighbors and pilfer small prey items. At least someone is happy to host Georgia's newest arachnid.
There is still much to learn about the spider-sized earthquake that may or may not be sending ecological shockwaves ricocheting across northern Georgia. But if iNaturalist data tell us anything, it is that Joro spiders are here to stay. The question is not whether they will spread, it is how far and how fast the wind will carry them.
References
Hoebeke ER, Huffmaster W, Freeman BJ. 2015. Nephila clavata L Koch, the Joro Spider of East
Asia, newly recorded from North America (Araneae: Nephilidae)
PeerJ 3:e763 https://doi.org/10.7717/peerj.763
Gavrilles, Beth. October 26, 2020. Like it or not, Joro spiders are here to stay. UGA Today.
Gavrilles, Beth. March 17, 2015. UGA scientists confirm first North American record of East
Asian Joro spider. UGA Today.
Podo, Kelsey. September 24, 2020. What are these spiders doing here? UNG team studying
invasive Joro spiders in Northeast Georgia. Gainesville Times. https://www.gainesvilletimes.com/life/animals/what-are-these-spiders-doing-here-ung-team-studying-invasive-joro-spiders-northeast-georgia/
Shearer, Lee. October 25, 2020. Meet your new Georgia neighbor, the Joro spider, an Asian
invader big as a hand. Augusta Chronicle.
All data points for T. clavata and A. aurantia observations accessed in April 2021 from https://www.inaturalist.org/home