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The Purple Pitcher Plant as a Spider Oviposition Site
Marc A. Milne

Southeastern Naturalist, Volume 11, Issue 4 (2012): 567–574

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2012 SOUTHEASTERN NATURALIST 11(4):567–574 The Purple Pitcher Plant as a Spider Oviposition Site Marc A. Milne* Abstract - Spiders’ nesting sites may vary depending on species-specific requirements and environmental conditions. I report on the use of leaves of Sarracenia purpurea (Purple Pitcher Plant) as oviposition sites by 5 species of spiders in Virginia and North Carolina. The presence of egg sacs, webbing, and a protective female spider inside the pitchers confirmed the spiders’ use of the carnivorous plant trap as a safe and secure nest rather than a deadly mechanism by which many spiders have commonly become victims. I also collected spiders in the surrounding environment to dete rmine if this microhabitat is used by uncommon species or ones that are often found nearby. Almost all spiders found within pitchers of S. purpurea were also found in the surrounding environment, but not at high densities near S. purpurea. The decaying pitchers of S. purpurea may create an ideal home for many spider species in environments where suitable oviposition sites are hard to come by. Introduction Spiders employ a range of behaviors to protect their egg sacs. Some place them in webs or silk-lined tunnels, under stones or bark, or in rolled-up leaves; others grasp them with their chelicerae or attach them to their own abdomens (Foelix 2010). Silk sacs surround the eggs to help prevent water loss, desiccation, and changes in temperature and humidity (Opell 1984). Although the egg sac’s silk also aids in defense against predatory or parasitic organisms, adults of certain species often take up residency near the egg sac in order to further aid in the survival of the juveniles or eggs (Foelix 2010). Maternal care for the egg sac among spider species ranges from abandonment to guarding and aiding in freeing the eclosing spiderlings from the egg sac (Foelix 2010). The carnivorous Sarracenia purpurea L. (Purple Pitcher Plant) is a low-lying herbaceous plant that uses water-filled pitcher-shaped leaves to trap, kill, and digest arthropod prey (Schnell 2002). Sarracenia purpurea thrives in sunny bogs and marshes along the Gulf Coast, North on the Atlantic edge of North America and West across Southern Canada, the largest range of any North American carnivorous plant (Schnell 2002). Pitchers catch the most prey during the first two weeks of opening (Fish and Hall 1978), yet pitchers persist during the full growing season, often in a slowly senescing state whereby the pitcher leaf decays from the top down over the course of the year (Schnell 2002). Many spiders have been shown to associate with carnivorous plants, usually as either kleptoparasites (Anderson and Midgley 2002; Cresswell 1991, 1993; Rymal and Folkerts 1982; Schnell 2002) or victims (Heard 1998, Judd 1959, Wray and Brimley 1943). Flying insects regularly oviposit in the liquid of S. purpurea pitchers, helping to create a phytotelmatous community. However, the only known organism that commonly utilizes an entire dry pitcher as its home is Exyra rolandiana *University of North Carolina, Greensboro, NC 27412; mamilne@unc 568 Southeastern Naturalist Vol. 11, No. 4 Grote (Noctuidae) (Pitcher Plant Moth), which spins egg sacs inside first-year pitchers and feeds on the pitcher tissue (Schnell 2002), thus demonstrating a parasitic-like relationship (Schnell 2002), while the plant’s relationship with the macroinvertebrate phytotelmatous community has been shown to be at least partially mutualistic (Bradshaw and Creelman 1984, Heard 1994, Mouquet et al. 2008). For spiders, the use of S. purpurea pitchers as oviposition sites has previously been noted for only one family of spiders (Lycosidae, as a refuge for a spider with an egg sac) (Hubbard 1896, Jones 1935). Similarly, Rymal and Folkerts (1982) note a relationship between spiders and an unspecified Sarracenia species. The goal of my study was to determine which spider taxa commonly build webs inside S. purpurea pitchers in the mid-eastern United States, if these spiders select for specific states of senescence in the pitchers when choosing their web location, and if these spiders were also commonly abundant in the surrounding environment. Methods The Highlands Botanical Station (HBS; 35.05°N, 83.19°W) in Highlands, NC (Macon County) and the Joseph Pines Preserve (JPP; 37.05°N, 77.24°W) near Waverly, VA (Sussex County) were searched for spiders residing in S. purpurea pitchers. HBS was sampled once a week through 8 weeks in June and July 2007, and JPP was sampled once a month for 6 months from April–September 2008. The sampled area at HBS is a marshy area on the outskirts of an 11-acre garden, containing approximately 700 S. purpurea rosettes (consisting of approximately 16,554 pitchers when counted in late June) along a lake edge. The Joseph Pines Preserve is an artificially managed, periodically burned, 100-acre site that contains approximately 32 S. purpurea rosettes (consisting of approximately 1100 pitchers when counted in mid-September). During each sampling period, all pitcher plants at the preserve were checked for spider residents with egg sacs. Checking for spider residency consisted of examining the insides of each pitcher on a plant for living spiders with egg sac(s). When spiders were found, they were collected and preserved for later identification. The number of egg sacs present with each spider was counted. The level of senescence for each resided pitcher was determined through observation as either “no senescence”, “partial senescence”, or “complete senescence”. Senesced pitchers are from the previous growing season, begin their decay from the distal portion of the leaf, and often lack the presence of water due to holes ripped in the decayed tissue. To estimate the abundance of spiders in the surrounding environment, ten 25-m2 square plots were created: 2 plots were placed within the pitcher plant population (henceforth, “pitcher plant plots”) and 2 were adjacent to the population (henceforth, “non-pitcher plant plots”) at HBS, while there were 3 plots of each kind created at JPP. Each plot was sampled for spiders using 3 methods: sweep netting, shrub beating, and pitfall trapping. Each technique was conducted 4 times over 2 months at HBS, and pitfall trapping was conducted once a month over 6 months while sweep netting was done once every other month at JPP (shrub beating was not conducted at JPP due to the lack of lar ge bushes). 2012 M.A. Milne 569 Sweep netting consisted of waving a sweep net (0.5 m diameter) over grassy vegetation. The entire plot was sweep netted unless vegetation was too large. Large vegetation (height > 0.25 m; stem width < 3 cm diameter) was sampled via a beating sheet (shrub beating; 71 cm2). Plants that had a stem width larger than 3 cm in diameter were considered “trees”, and their foliage was not sampled. Pitfall trapping consisted of using 147.9-ml (5-oz) cups filled half-full with soapy water. Each pitfall trap was placed in the ground, flush with the forest floor. Five pitfall traps were placed in each plot. Four of the 5 pitfall traps were placed at approximately 1 m from each corner of the plot while the fifth pitfall trap was placed at the center of each plot. After 1 week, the pitfall traps were collected and the spiders preserved. All spiders were identified to species using Ubick et al. (2005) and associated taxonomic keys. Data were tested for normality and homogeneity of variance. If data did not conform to these assumptions, then they were transformed. Comparisons of diversity between pitcher plant and non-pitcher plant plots at each location were done using independent t-tests. Figure 1. Adult female Enoplognatha caricis living inside a senesced pitcher of S. purpurea with 4 egg sacs (c). 570 Southeastern Naturalist Vol. 11, No. 4 Results Twelve Enoplognatha caricis Fickert (Fig. 1), 8 Pirata insularis Emerton, 2 Theridion frondeum Hentz (Cobweb Weaver), 1 Eperigone maculata Banks, and 1 Clubiona rhododendri Barrows adult females were observed with egg sacs inside S. purpurea pitchers at HBS, while 1 adult female Rabidosa rabida Walckenaer (Rabid Wolf Spider; Fig. 2) was observed with an egg sac residing in Figure 2. Adult female Rabidosa rabida living inside a partially senesced pitcher of S. purpurea. 2012 M.A. Milne 571 a pitcher at JPP. The number of egg sacs held by each spider varied by taxa; the number of egg sacs held by E. caricis ranged from 1–5 (mean = 1.5, SD = 1.17, mode = 1), and the E. maculata was seen with 3 egg sacs, while all other spider types were seen with only 1 egg sac each (Table 1). All spiders were observed to be residents of the pitchers as opposed to victims, as webbing spanned the inner aperture of the pitchers, seeming to prevent the spiders’ capture and allowing them to move freely around and out of the leaf. All spiders were found inside partially or fully senesced pitchers. Seven-hundred and sixty-nine spiders in 18 families were found in the surrounding environment: 345 at HBS and 424 at JPP (Table 2). The most common species found among both locations were P. insularis (110), T. frondeum (24), Bathyphantes pallidus Banks (14), Pelegrina galathea Walckenaer (10), and C. rhododendri (9). The species found with an egg sac in a pitcher Table 1. Total numbers of spiders by species found with egg sacs in pitch ers. Species # in pitchers with egg sacs Mean number of egg sacs HBS Clubiona rhododendri 1 1.0 Enoplognatha caricis 12 1.5 Eperigone maculata 1 3.0 Pirata insularis 8 1.0 Theridion frondeum 2 1.0 JPP Rabidosa rabida 1 1.0 Table 2. Total numbers of spiders by family found in pitcher plant (PP) vs. non-pitcher plant (NPP) plots. HBS JPP Family NPP PP NPP PP Agelenidae 0 0 1 0 Araneidae 6 14 21 15 Atypidae 0 0 0 1 Clubionidae 17 1 1 0 Corinnidae 0 0 0 2 Dictynidae 0 1 1 0 Gnaphosidae 0 0 3 7 Hahniidae 0 0 4 4 Linyphiidae 50 36 21 31 Lycosidae 95 13 61 52 Miturgidae 0 0 1 0 Oxyopidae 2 1 13 5 Philodromidae 0 0 3 0 Pisauridae 4 2 12 10 Salticidae 5 23 49 68 Theridiidae 41 14 2 2 Thomisidae 5 6 11 18 Unknown 2 7 2 3 Total 227 118 206 218 572 Southeastern Naturalist Vol. 11, No. 4 at JPP (R. rabida) was also found in the non-pitcher plant plots (3), but not in the pitcher plant plots. At HBS, C. rhododendri (9) and E. maculata (1) were found in non-pitcher plant plots but not in pitcher plant plots, E. caricis was not found within either plot type, and P. insularis and T. frondeum were found in both plot types. However, P. insularis and T. frondeum were found at much higher densities in the non-pitcher plant plots than in the pitcher plant plots (n = 78 for P. insularis in non-pitcher plant plots, and n = 9 in pitcher plant plots; n = 22 for T. frondeum in non-pitcher plant plots, and n = 2 in pitcher plant plots). There was no significant difference in the diversity of spiders between pitcher plant and non-pitcher plant plots at HBS (t = 0.95, df = 34, P = 0.35) or JPP (t = 0.91, df = 34, P = 0.11). Discussion Spiders are prone to capture by S. purpurea (Heard 1998, Judd 1959, Wray and Brimley 1943). Therefore, if spiders were to build webs in actively trapping pitchers, they must first prevent capture and make the pitcher s afe for residency. This precaution may be why the spiders observed in this study resided in senesced pitchers—pitchers that no longer posed a significant threat to arthropods (Fish and Hall 1978). Because these older, senesced leaves no longer function for prey capture, the plant is unlikely to be at a disadvantage by housing such spider residents, making the relationship most likely a commensalism, though more quantification should be done to confirm this hypothesis. The abundance of each species found in both locations varied from very abundant (P. insularis) to a single representative (E. maculata). However, these numbers should be considered underestimates because the many immatures that were found could not be identified to species (fully developed reproductive parts are usually required to identify spiders to species). The diversity and abundance of spiders with egg sacs found in pitchers was much lower than that which was found in the environment at both locations (Tables 1, 2). Almost all spiders that had egg sacs in pitchers were also found in the environment, but were either absent or at low densities within plots that held pitcher plants. These differences in density could have been due to differences in other vegetation types and amounts between pitcher plant and non-pitcher plant plots, which would affect the density of spiders. However, these differences may also indicate a preference by some spider species for habitat without pitcher plants or, perhaps, that these species are frequently captured by pitcher plants and are therefore at low densities nearby. Since all of the species that were found with egg sacs in pitchers were either absent or at low densities near pitcher plants, the leaves of S. purpurea may be a specific microhabitat that is preferentially utilized by these species for oviposition. More quantification should be done to determine the cause of this variability in spider density. Only a few scattered reports exist of spiders using carnivorous plants as oviposition sites (Hubbard 1896, Jones 1935, Rymal and Folkerts 1982). Spiders residing in S. purpurea may gain an advantage over spiders using other 2012 M.A. Milne 573 microenvironments to raise their young. Pitchers are protected on multiple sides by the inner leaf surface. These leaves may present a microenvironment similar to the funnel webs of Agelenopsis or Sosippus, which are covered laterally by webbing and have a rear retreat (Brady 1962, Foelix 2010). Interestingly, spiders of the genus Agenelopsis build funnel-webs that funnel into S. purpurea pitchers and often retreat to the bottom of the pitcher when disturbed (M.A. Milne, pers. observ.). Moreover, S. purpurea pitchers may present an architectural “jackpot” for spiders seeking shelter from predators or protection for their young. This advantage may be particularly useful in bog habitats where protective vegetation is scarce. Acknowledgments I would like to thank Deborah Waller for her advice, James Costa for allowing me to sample at the Highlands Biological Station, Phil Sheridan for allowing me to sample on the Joseph Pines Preserve, the American Arachnological Society for a student grant, and the Highlands Biological Station for the Martina Wadewitz Haggard grant. Literature Cited Anderson, B., and J.J. Midgley. 2002. It takes two to tango but three is a tangle: Mutualists and cheaters on the carnivorous plant Roridula. Oecologia 132:369–373. Bradshaw, W.E., and R.A. Creelman. 1984. Mutualism between the carnivorous Purple Pitcher Plant and its inhabitants. American Midland Naturalist 112:294–304. Brady, A.R. 1962. The spider genus Sosippus in North America, Mexico, and Central America (Araneae, Lycosidae). Psyche 69:129–164. Cresswell, J.E. 1991. Capture rates and composition of insect prey of the pitcher plant Sarracenia purpurea. American Midland Naturalist 125:1–9. Cresswell, J.E. 1993. The morphological correlates of prey capture and resource parasitism in pitchers of the carnivorous plant, Sarracenia purpurea. American Midland Naturalist 129:35–41. Fish, D. and D.W. Hall. 1978. Succession and stratification of aquatic insects inhabiting the leaves of the insectivorous pitcher plant, Sarracenia purpurea. American Midland Naturalist 99:172–183. Foelix, R.F. 2010. Biology of Spiders. Oxford University Press, New York, NY. 432 pp. Heard, S.B. 1994. Pitcher-plant midges and mosquitoes: A processing chain commensalism. Ecology 75:1647–1660. Heard, S.B. 1998. Capture rates of invertebrate prey by the pitcher plant, Sarracenia purpurea L. American Midland Naturalist 139:79–89. Hubbard, H.G. 1896. Some insects which brave the dangers of the pitcher-plant. Proceedings of the Entomological Society of Washington 3:314–318. Jones, F.M. 1935. Pitcher plants and their insect associates. Pp. 25–30, In M.V. Walcott (Ed.). Illustrations of North American Pitcher Plants. Smithsonian Institution, Washington, DC. 34 pp. Judd, W.W. 1959. Studies of the Byron Bog in Southwestern Ontario X. Inquilines and victims of the pitcher-plant Sarracenia purpurea L. Canadian Entomologist 91:171–180. Mouquet, N., T. Daufresne, S.M. Gray, and T.E. Miller. 2008. Modeling the relationship between a pitcher plant (Sarracenia purpurea) and its phytotelma community: Mutualism or parasitism? Functional Ecology 22:728–737. 574 Southeastern Naturalist Vol. 11, No. 4 Opell, B.D. 1984. A simple method for measuring desiccation resistance of spider egg sacs. Journal of Arachnology 12:245. Rymal, D.E., and G.W. Folkerts. 1982. Insects associated with pitcher plants (Sarracenia; Sarraceniaceae) and their relationship to pitcher plant conservation: A review. Journal of the Alabama Academy of Science 53:131–151. Schnell, D.E. 2002. Carnivorous Plants of the United States and Canada. Timber Press, Portland, OR. 468 pp. Wray, D.L., and C.S. Brimley. 1943. The insect inquilines and victims of pitcher plants in North Carolina. Annals of the Entomological Society of America 36:128–137.