Cardinal predation of the invasive Jorō spider Trichophila clavata (Araneae: Nephilidae) in Georgia

The invasive Jorō spider (Trichophila clavata L. Koch: Araneae: Nephilidae) (Figure 1) was found to be established in Georgia in the southeastern United States in 2014 (Hoebeke et al. 2015). This native of eastern Asia has become abundant in northern Georgia and it has spread to three adjacent states (Tennessee, South Carolina, North Carolina), and northeast to West Virginia and Maryland, which is currently its most northern occurrence (iNaturalist, last accessed 22 August 2024). Recent modeling of the Jorō’s potential distribution in North America, using its known occurrences in its native Asia, predicted that it could spread to the Great Lakes region (Nelsen et al. 2023). An earlier study (Davis and Frick 2022) found that the Jorō spider has a higher metabolism and is more cold tolerant than its congener Trichophila clavipes (L.), whose distribution in the United States is limited to the southeastern states. This suggested that the Jorō spider could spread beyond the southeastern United States to colder areas, which it already has done. Giulian et al. (2024), used Max Ent modeling to predict the areas of North America that are climatically suitable for the Jorō spider. The model found a land area of 2,566,547 km² that was above 50 % suitability for the spider, which would be a 20-fold increase from the 120,000 km² recently estimated to be the area of occupation (Chuang et al. 2023).

Recent surveys of the Jorō and native orb weavers in northern Georgia found this invasive spider to be the most abundant at just over half of the 103 surveyed sites (Nelsen et al. 2023). Surveys to census the Jorō and native orb weaving spiders in 25 Atlanta area forest sites during 2022 and 2023 found the Jorō to be the most abundant spider in all the forests (R. Pemberton, unpublished data). The spider’s large size, abundance, frequent occurrence in human habitats (Davis et al. 2024a), and rapid geographic spread has alarmed the public, a perspective which is due, in part, to the news media’s sensationalism and portrayal of the spider as a threat to people (Chuang et al. 2023). Davis and Anerao (2023) described the Jorō as timid, after determining that when the spider was startled it froze in position much longer than other studied spiders. When I inadvertently touched Jorō webs during my censusing of the spiders, the occupants either froze or ran away. Their large 1 m wide webs and the high densities of these webs in invaded forests suggest significant capture and predation of flying insects. DNA metabarcoding of prey remains in Jorō webs, of their dissected gut contents, and their fecal matter indicated that the spider has a broad diet, but specific prey types (insect families, genera and species) have yet to be determined (Grabarczyk et al. 2023). A survey of the contents of 62 Jorō webs in Georgia found that species of Hymenoptera were the most abundant prey (52 %) followed by Hemiptera (24 %) and Diptera (6 %) (Davis et al. 2024b). The same study found that Jorō spiders readily consumed two native butterflies placed in their webs but not the poisonous Monarch butterfly.

Figure 1:

Invasive Jorō spider (Trichophila clavata). A. ventral surface showing distinctive red markings and B. dorsal surface, scale bar = 3 cm.

The abundance and potential ecological impacts of the Jorō spider raises questions about what might be predating or failing to predate the spiders in its invasive range. Some invasive species become extremely abundant because they have escaped the natural enemies which regulate their populations in their native regions (Liu and Stiling 2006). The only reported predation of the Jorō in its invasive range is by mud daubing wasps (Hymenoptera: Sphecidae), which provisioned their brood cells with juvenile Jorō spiders (E.R. Hoebeke and L.A. Taylor, personal communication; as cited in Chuang et al. 2023). Schronce and Davis (2022) reported observing a bird, a Northern Cardinal (Cardinalis cardinalis L. [Passeriformes: Cardinalidae]) perched on a Jorō spider web, lunge at the female spider in the web, which escaped. The bird (hereafter referred to as Cardinal) then picked out and ate the insect prey trapped in the web.

During surveys of Jorō spiders, severely torn or destroyed Jorō webs were periodically encountered, often with the central part of the orb missing (R. Pemberton, unpublished data). The resident female Jorō spiders also were missing, and at times one or several male Jorōs were still present in the remains of the webs. The web remnants were recognizable as Jorō webs because they were composed of golden silk. The only other orb weaver in the eastern United States to make webs with gold colored silk is T. clavipes, which is uncommon in the Atlanta region. Damaged orb webs without a spider present are strong evidence of a predator attack (Horton 1982).

To obtain egg masses to bait for possible egg parasitoids and egg predators in Atlanta forests, female Jorō spiders were captured and kept in white screen cages (approximately 90 × 30 × 30 cm) placed on the back porch of an Atlanta residence. On 26 November 2023, a male Cardinal was observed to be perched on the top of one of these cages and vigorously pecking at a spot on the cage, just above the large female Jorō. The bird continued pecking at the spot for about a minute, in an attempt to catch the spider, before flying off.

To confirm that Cardinals can predate Jorōs, a test was conducted soon after the bird’s attempted attack on the caged Jorō. A large female Jorō was tethered with a thread by its pedicel and then hung from the trellis-like frame above the porch, approximately 2 m above the porch floor and approximately 3 m from where the Cardinal predation attempt of the caged Jorō was observed. The tethered spider was alive and occasionally moved its legs but did not attempt to climb the thread or make other movements. The tethered spider was observed for an hour and toward the end of that period, a female Cardinal flew in and attacked the spider three times. In each of these attacks the bird grabbed the spider with its beak and attempted to fly away with it, but the tethering thread held the spider in place. At the end of the attacks the Jorō was dead but not dismembered. The Cardinal attacking the tethered Jorō was a female, while the one attempting to predate the caged Jorō was a male. This indicates that Cardinals of both sexes can predate the spiders, and two individual birds attempted to do so within about an hour when these observations were made.

The frequency of attack on the tethered Jorō, three attacks in 1 h, raises the question of how Jorō spiders can be abundant if this degree of attack occurs in nature. There were approximately 20 Jorō spiders in the garden below the porch where the attacks occurred. The most likely reason is that the tethered spider was not in a web. Jorō webs are usually composed of the orb, the region of prey capture, and barrier webs which are a tangle of webs opposite either or both faces of the orb. Barrier webs have been thought to block the approach of flying predators such as birds and dragonflies (Robinson and Robinson 1973). Barrier webs were found to deter bird predation of T. clavipes (Higgins 1992).

Cardinals are common birds in residential areas in the eastern United States and are frequent visitors at backyard birdfeeders. Due to their consumption of seed at birdfeeders, Cardinals are popularly thought of as seed feeders, but are known to be omnivores (De Graaf et al. 1985). In a study of common farm birds in Florida, the Cardinal was found to be the most abundant functional insectivore, making longer foraging bouts and taking more prey than other species (Jones et al. 2005). Cardinals exhibited peak survival at intermediate and upper portions of the rural-to-urban gradient, due in part to their ability to take advantage of diverse foods (Evans et al. 2015). Cardinals are one of the most common birds in the Atlanta region (Atkins and Wilson 2011) and may be responsible for the destroyed webs and missing female Jorō spiders observed during surveys (R Pemberton, unpublished data). The Cardinal predation attempts of Jorō spiders reported here support the observation of a Cardinal lunging at a Jorō spider (Schronce and Davis 2022) as predatory behavior. The surveys and Cardinal attack on the caged and tethered Jorō spiders occurred during the autumn after many other insectivorous bird species had already migrated. The Cardinal’s food adaptability may enable it to use the new food resource that the abundant Jorō spiders represent, perhaps lessening the abundance of these new invaders. Other birds may also predate Jorō spiders. Although this note provides evidence of bird predation of the Jorō spider, more research is needed to understand what factors influence the great abundance of this invasive spider. Direct observations of Jorō spider predation by Cardinals, and possibly other birds, in wild situations are needed. Such observations are, however, difficult in the dense forest vegetation in which most of the spiders occur. The use of tethered Jorō spiders (living or humanly killed), could increase our knowledge about bird predation and other natural enemies of these invasive spiders