The Joro spider, a large and brightly colored orb-weaver, has become widespread across the southeastern United States. These arachnids, known for their yellow and black patterns, construct large, golden-hued webs often seen stretched between trees and utility lines. Understanding the Joro spider’s position in the local food web is important for comprehending its rapid establishment and what animals are helping to control its population growth.
Identifying the Joro Spider
The adult female Joro spider, Trichonephila clavata, is easily distinguished by her size and coloration. The female’s body can measure up to one inch long, with her striped legs giving her a total span of up to four inches. Her abdomen is yellow with broad, bluish-green bands on the top surface, complemented by red markings on the underside.
The legs are black with bands of bright yellow. In contrast, the adult male Joro spider is significantly smaller and less colorful, typically measuring only about a quarter of an inch in body length with a drab brown appearance. The large, three-layered orb webs these spiders build also have a subtle golden tint when viewed in sunlight.
Documented Natural Controls
In its new habitat, the Joro spider is consumed by a variety of generalist predators. Birds are frequently documented consumers, including species like the Blue Jay, Northern Mockingbird, and various other songbirds. These avian predators often target spiders lower to the ground or in exposed locations, as they generally avoid flying directly into the large, sticky orb webs.
Other opportunistic predators include small mammals like squirrels, reptiles, and amphibians such as green tree frogs and lizards. These animals snatch the spiders from the edges of their webs or consume them when they are away from the main structure.
Certain native insect predators also play a role, most notably specialized spider wasps, such as the mud dauber. Mud dauber wasps paralyze the Joro spider with a sting before sealing the arachnid inside a mud nest to serve as food for the developing larva. This predation targets mature spiders, effectively removing adult females from the reproductive cycle.
The Joro’s reproductive stage is also vulnerable, as a single female can produce an egg sac containing up to 500 eggs. These dense white silk sacs are often laid on exposed surfaces, making them susceptible to generalist predators like ants, or to attack by native parasitoid insects.
Why Predators are Scarce in New Habitats
Despite the list of generalist predators, the Joro spider population has rapidly increased across the US, largely due to a concept known as “enemy release.” The Joro spider is native to East Asia, primarily regions of Japan, China, and Korea. In their native range, the spiders have co-evolved with a complex web of specialist parasites, pathogens, and predators that specifically target the species at different life stages.
When the Joro spider was inadvertently introduced to North America, it left behind the specialized natural controls that kept its population in check. Native generalist predators are not numerous enough or specialized enough to significantly suppress the spider’s rapid population growth. The absence of these specialized enemies provides the Joro spider with an advantage, allowing more individuals to survive to maturity and reproduce in high numbers without the constant ecological pressure they face in their home continent.