2005. The Journal of Arachnology 33:193±195

SHORT COMMUNICATION

PREDATION BY ARGYRODES TRIGONUM ON TRIANGULARIS, AN INVASIVE SHEET-WEB WEAVER IN COASTAL MAINE

Jeremy D. Houser: Neuroscience & Behavior Program, Tobin Hall, University of Massachusetts, Amherst, Massachusetts 01003 USA Daniel T. Jennings: USDA, Forest Service. Northeastern Research Station, 686 Government Road, Bradley, Maine 04411 USA

Elizabeth M. Jakob: Psychology Department, Tobin Hall, University of Massachusetts, Amherst, Massachusetts 01003 USA

ABSTRACT. A female Argyrodes trigonum (Theridiidae) was observed feeding on a female Lin- yphia triangularis (), a recently established European immigrant in Maine. Multiple ob- servations of Argyrodes inhabiting L. triangularis webs suggest that this invasive sheet-web weaver is not immune to web invasions, kleptoparasitism or predation by A. trigonum. The potential impacts of A. trigonum on the invasion dynamics of L. triangularis are unknown, but likely to be minimal. Keywords: Kleptoparasitism, araneophagy, exotic

Members of the Argyrodes Simon 1864 we observed an adult female A. trigonum feeding (Theridiidae) are known for diverse and ¯exible on an adult female L. triangularis in the prey's web. foraging strategies. Although capable of spinning The predator-prey habitat was a roadside/coniferous small tangle webs of their own, many species for- forest-edge on the Schoodic Peninsula (Winter Har- age more often as kleptoparasites, web-stealers or bor, Maine). The L. triangularis was not extensively predators of other species (Cangialosi digested, suggesting that the capture by A. trigonum 1991, 1997). Argyrodes trigonum (Hentz 1850) is was recent. Several more A. trigonum were found common in the eastern U.S. (Exline & Levi nearby in the superstructure of L. triangularis webs. 1962), and is also found in Ontario (Levi & Ran- To the best of our knowledge, the earliest obser- dolph 1975) and QueÂbec (Paquin et al. 2001). vation of Argyrodes in the web of L. triangularis Cangialosi (1997) provides a thorough descrip- was on 28 August 1999, when D.T.J. collected a tion of the diverse and ¯exible foraging strategies juvenile Argyrodes sp. from a female-occupied web of this species. of L. triangularis in Pittston, Kennebec County, The European Hammock spider, Linyphia trian- Maine. gularis Clerck 1757 (Linyphiidae), has recently be- During August and September of 2003, surveys come established in parts of coastal Maine (Jen- of L. triangularis webs were conducted at four lo- nings et al. 2002) and apparently is spreading cations in Maine to determine whether the occu- inland. In some coastal habitats, such as those of pation of L. triangularis webs by A. trigonum is a Schoodic Peninsula in Acadia National Park, the common occurrence or better described as a nov- invasion has become severe, with population den- elty. Surveys were conducted in habitat favorable sities of L. triangularis reaching 12 individuals/m2. to L. triangularis, primarily seedlings, saplings, In these high-density areas, native linyphiids, such shrubs and forbs. At two of the locations, the pro- as Neriene radiata (Walckenaer 1841) and Pityoh- portion of webs containing at least one A. trigonum yphantes costatus (Hentz 1850), are now scarce was rather high: 11 out of 36, or 30.6% (Dixmont, (Houser, Jakob, & Jennings, pers. obs.). Penobscot Co.), and 19 out of 35, or 54.3% (Gar- During the summer of 2002, while studying the land, Penobscot Co.). At the other sites, however, invasion at Acadia N.P., we noticed A. trigonum in the frequency of A. trigonum was low; none were some webs of L. triangularis. On 24 August 2002, found among 33 webs surveyed at Guilford, Pis-

193 194 THE JOURNAL OF ARACHNOLOGY cataquis Co., and only 1 was found among 11 webs P. costatus appear to have declined more dramati- at Milbridge, Washington County. Multiple A. tri- cally than F. pyramitela. However, in New Hamp- gonum (always 2 or 3) were found in 11 of the 115 shire, Cangialosi (1997) found that A. trigonum (9.6%) webs surveyed, or 11 of the 31 (35.5%) uses N. radiata as hosts more often than either F. webs containing at least one A. trigonum. Although pyramitela or P. costatus. Comparable host-prefer- not found in these surveys, higher levels of occu- ence data including L. triangularis in addition to pancy are possible. D.T.J. observed 5 or 6 Argyro- native hosts are needed to better evaluate the im- des juveniles co-inhabiting a female L. triangularis pacts, if any, of A. trigonum on this invasion. Be- web in Garland, Penobscot County. cause of its host-generalist behavior, we suspect that At two of our survey sites, the proportion of L. A. trigonum has (and will have) minimal regulatory triangularis webs containing A. trigonum is com- impacts on populations of L. triangularis in Maine. parable to, if not higher than previously observed Instead, assemblages of natural enemies (e.g., par- rates of A. trigonum in other linyphiid webs. Over- asites, parasitoids, predators, and pathogens) may all, 27.0% of L. triangularis webs surveyed in 2003 be needed for control or containment of this inva- contained A. trigonum. In a large study of the for- sive spider. aging strategies of A. trigonum, Cangialosi (1997) The impetus for this note comes from the sharp recorded A. trigonum co-inhabiting 6.1% of N. ra- eye of Adam Porter, who made the initial discovery diata webs, 1.9% of P. costatus webs and 2.6% of and observation of the predation encounter on 24 Frontinella pyramitela (Walckenaer 1841) webs. August 2002. The 2003 surveys were conducted Our observed proportions may be high relative to with the much appreciated assistance of Nancy Jen- those reported in Cangialosi (1997) due to a variety nings and Frank Graham, Jr. We are grateful to Da- of factors, including differences in sampling meth- vid Manski, Chief Biologist, Acadia National Park, od, date or location, and therefore should be inter- for issuance of collecting permits, and to Park Bi- preted cautiously. ologist Bruce Connery for logistical support. Our observations support the already consider- Voucher specimens are deposited in the park col- able evidence that foraging behavior of A. trigonum lection of the Acadia National Park Research Cen- is very ¯exible. Species of Argyrodes have been ter, Bar Harbor, Maine. classi®ed as either host specialists, which use a va- riety of behavioral techniques to exploit their hosts, LITERATURE CITED or host generalists, which have a more limited be- Cangialosi, K.R. 1991. Attack strategies of a spi- havioral repertoire, but can take advantage of a der kleptoparasites: effects of prey availability large variety of hosts (Vollrath 1984; Whitehouse and host colony size. Behaviour 41: 1988). Cangialosi (1997) has argued that a host 639±647. generalist would also bene®t by having a variety of Cangialosi, K.R. 1997. Foraging versatility and the techniques at its disposal and presents A. trigonum in¯uence of host availability in Argyrodes tri- as an example. The behavioral repertoire of A. tri- gonum (Araneae, Theridiidae). Journal of Arach- gonum is quite broad; in addition, hosts of A. tri- nology 25:182±193. gonum include (in various geographic locations) Exline, H. & H.W. Levi. 1962. American spiders of linyphiids, agelenids, theridiids, and araneids the genus Argyrodes (Araneae, Theridiidae). Bul- (Larcher & Wise 1985; Cangialosi 1997). The abil- letin of the Museum of Comparative Zoology ity of A. trigonum to exploit an introduced exotic 127:75±204. species is further evidence that it is a host gener- Jennings, D.T., K.M. Catley & F. Graham, Jr. 2002. alist, and that its foraging success is not, at least at , a Palearctic spider (Ara- present, likely to be driven by co-evolution with neae: Linyphiidae) new to North America. Jour- any particular host. nal of Arachnology 30:455±460. The possible effects of A. trigonum on the inva- Larcher, S.F. & D.H. Wise. 1985. Experimental sion of L. triangularis are unclear. Because A. tri- studies of the interactions between a web-invad- gonum makes use of both L. triangularis and native ing spider and two host species. Journal of Ar- host webs, it could mitigate or exacerbate the ef- achnology 13:43±59. fects of the invader on native populations. It would Levi, H.W. & D.E. Randolph. 1975. A key and be useful to know the relative preference, if any, checklist of American spiders of the family Ther- that A. trigonum has for native linyphiids vs. the idiidae north of (Araneae). Journal of invader. Host-web structure, particularly the amount Arachnology 3:31±51. of barrier silk, affects selection and occupancy po- Paquin, P., N. DupeÂrre ,& R. Hutchinson. 2001. tentials of A. trigonum (Cangialosi 1997). Super®- Liste reÂviseÂe des AraigneÂes (Araneae) du QueÂ- cially, the semi-dome shaped webs of L. triangu- bec. Pp. 5±87. In Contributions aÁ la connaissance laris are more similar to those of N. radiata and P. des AraigneÂes (Araneae) d' AmeÂrique du Nord. costatus than the bowl and doily webs of F. pyr- (P. Paquin & D.J. Buckle, eds.). Fabreries, Sup- amitela. At the Schoodic study site, N. radiata and pleÂment 10. HOUSER ET AL.ÐPREDATION BY ARGYRODES 195

Vollrath, F. 1984. Kleptobiotic interactions in in- speci®city and choice of host in Argyrodes an- vertebrates. Pp 61±94. In Producers and scroung- tipodiana (Araneae, Theridiidae). Journal of Ar- ers: Strategies of exploitation and parasitism. achnology 16:349±355. (C.J. Barnard, ed.). Grom Helm, London & Syd- ney. Manuscript received 28 February 2003, revised 8 Whitehouse, M.E.A. 1988. Factors in¯uencing December 2003.