The Rapid Spread of Joro Spiders: A Growing Nuisance in the US

The Joro spider (Trichonephila clavata), native to Asia, was recently discovered in the United States. They are found widely in Taiwan, Korea, China, and Japan. In the U.S., however, it is considered an invasive species. The Joro spider is believed to have been introduced to North America in shipping containers from East Asia, likely Japan or China, to Georgia in 2013. Since then, Joro spiders have been reported in several US states, primarily in the Southeast, as they continue to spread. 

Joro spiders have been confirmed in Georgia, South Carolina, North Carolina, Tennessee, Alabama, and Pennsylvania. Some reports also suggest they have been seen as far north as Massachusetts. Often mistaken for a banana spider, the palm-sized Joro spider is identifiable by its bright blue, red, and yellow colors, present mostly on the female. The Joro spider also spins a unique golden web.

Are Joro Spiders Venomous?

Arachnophobes might not like seeing a Joro dangling from their ceiling. The spider’s bright color schemes only highlight their size. Legs fully extended, these spiders are almost three inches across! That’s a noticeable difference from the average house spider, which measures somewhere between 3/16 to 5/16 of an inch.

The Joro spider uses venom to subdue its prey, usually small bugs. But this substance isn’t dangerous to larger creatures and is only medically significant to humans who have an allergic reaction. The bite itself poses no danger to humans or household pets. The Joro’s mouthparts aren’t large enough to cause more than mild symptoms, similar to a bee sting, and bites require little treatment.

Rapid Spread

While not harmful to humans, Joro spiders are negatively affecting native spider populations by outcompeting them for resources and prey. Since coming to the U.S., they’ve spread rapidly. Females are capable of depositing sacs of 400 to 1,500 eggs in bushes or trees. Hatching in the spring, hatchlings ride the wind on a strand of silk web. One benefit of the invasive Joro spider is that it eats a variety of other pests, including biting flies, stink bugs, and mosquitoes.

Joro spiders have now been sighted in at least six U.S. states, principally in the Southeast, with recent expansion into Pennsylvania. They lay their eggs in the fall (September to November) and overwinter in the egg sac before hatching in the spring, typically in April or May. Joro spiders generally reach maturity in about 6 months. Joro spiders generally reach maturity in about six months. Female Joro spiders mature in three to five months, while males mature earlier. They are considered mature in early September and die off by late November, but not before laying eggs. 

Ballooning is a form of aerial dispersal used by young Joro spiders, also known as spiderlings. The spiderling releases silk strands from its abdomen into the air. Wind or electrostatic charges carry the light silk and the attached spiderling, allowing it to become airborne. The spider can travel short distances or very long distances through this process. Only young Joro spiders balloon. Mature Joro spiders are too large and heavy to be carried by the wind. Ballooning is a key factor in the rapid spread of Joro spiders across regions.