Rev. Biol. Trop. 46(4): 1141-1143, 1998 www.ucr.ac.cr WWW.ots.ac.cr www.ots.duke.edu
Nest architecture of group-living wasps Microstigmus (Hymenoptera: Sphecidae): role of the pedicel as a def�nse against ants
Gretthehun G. Grajalesl and Willian T. Wcislol.2 • "',
SrnithsoÍlÍan Tropical Research Institute, Apartado 2072 Balboa, República de Panamá. 2 Address correspondence to: W.T. Wcislo, Srnithsonian Tropical R,esearch Institute, Unit 0948, APO AA 34002-0948 USA; Fax: 507-232-5978 (Panamá); e-mail: [email protected]
Received 26-11-1998. Corrected 21-VIII-1998. A�cepted 1O-IX-1998
Abstraet: The nest pedicel of a social sphecid wasp Microstigmus nr. thripoctenuslplays an important role in protecting the brood againstant predators,as inferred froq¡data on artificial nests.. � pair oC artificial nests with or without pedicels was suspended Cromihe undersides oC leaves (Carludovica pa'mata) for 30 rnin to assess rates of discovery and removal of food (tuna bait) by ants (N = 36 pairs). Three-hundred lmd forty-thlee individuals oC five ant species removed food at a higher frequency from nests wihout pedicels (83.3% oC' 36 I nests) than fromnests with pedicels (11.1 % of 36 nests).
Key words: Brood predation, ants, nest architecture, maternal care, deCense.
Ant predation is thought to bea majot factor MATERIALS ANOIMETIiODS that evolQtionarily shaped nest architecture of tropical paper'wasps '(Hymenoptera: Vespidae) Observatio s of natural and artificialnésts (e.g., Jeanne 1975, Wenzel 1991, Matthews were made be een 22 September and 15 1991). One nest feature common to many October 1997 on Barro Colorado Island, species is a pedicel, 'a slender support stru�ture Republic of P ama (for details on the site" see for the nest envelope or cells. West-Eberhard Leigh et al. 1 O). We' constructed replicas of (1969) and Jeanne(1972, 1975) hypothesized nests of Micrbstigmus nr. thripoctenus by tbat its slender forin reduces,access to thebrood gIuing saw-dqst to threads (pedicels), and ' by ant predators, Most species in' the genus attached them ,o ceIls constructed of cIay; cells Microstigmus (Hymenoptera: Sphecidae) also were baited "fith smalI pieces of tuna. Tbe constructa pedicel that suspends theirnests from dimensions ofl artificial nests were similar to the undersides ofleaves (Matthews 1968, West natural ones I(e.g., Matthews 1968, West Eberhard 1977, Melo and Matthews 1997). Eberhard 1977): "pedicels" were -15 mm, , and Tbus, thesewasps de an independanttest of "nests" were .¡)ear-shaped and -17 mm long.. pro,vi , West-Eberhard's 'and Jeanne's hypothesis. To AdditionalIy, we constru�t� replicas without assess ,the function of the pediceI as a defense, pedicels. 00 eacl:t leaf ofdiffereot individuals we constructedartificial Microstigmus-like nests of Carludoyica palmata R. & P. 1798 with and without pediceIs,, baited them in the (CycIanthacea), we attached two artificial field,and determined the pereentage discovered oestS- ooe with and ooe without a pedicel. byants. TbethPead was ioserted ioto a smallhoIe made 1142 REVISTA DE BIOLOGIA TROPICAL witl!_a needle, and the artificial nest was substrate, or from a decreased probability of suspended from the underside of a leaf. Non encountering a nest because the slender pendant nests were pinned directIy to the pedicel reduces the point of contact between undersides of leaves. Each of 36 nest pairs the nest and substrate. Alternatively, the were observed for· 30 min, and 3 replicates pedicel might be too difficult to descend. were observed during one day. During several hundred hours of Differences in fre.quencies of attacks.by observations of nests of M. comes in Costa ants on artificial nests with and without Rica, Matthews and Starr (unpubl. data, cited pedicels were comparedwith a X2 test. in Matthews 1991) rarely observed foraging Voucher specimeils of thé model species, ants crawl onto a nest pedicel, and ant Microstigmus Dr. thripoctenus, are deposited in predation does not seem to be a major source the G. B. Fairchild Museum of Invertebrates, of . brood mortality in M. comes (Mátthews University of Panama, and the Snow 1991). Matthews and Starr (op. cit.) showed Entomological Division,. Natural History that Solenopsis ants routinely walked down Museum, University of Kansas. pedicels during experimental manipulations, and thus are capable of reaching a Microstigmus nest. Matthews (1991) RESULTS suggested that low rates of predation are related to the small biomass in Microstigmus Nests without pedicels were attacked by nests, and the small payoff to predators. A 343 individuals of 5 ant species (Edatomma slender pedicel further reduces the payoff. sp., Pheidole sp., Monomorium sp., Formica sp. and Camponotus sp.), while only 5 individuals of Ectatomma sp. discovered nests ACKNOWLEDGMENTS suspended from pedicels. Ants removed bait from a significantly greater proportion o'f This studybegan as GGG'sproject in a artificial nests without pedicels (83.3% of 36 field course sponsored by the Smithsonian trials) thanfor ,nests with pedicels (11.1% of 36 Tropical Research Institute (STRI), and was triaIs) ( x2 == 37.66, d.f. = 1, P <: 0.001). Nine continued with support from STRI general ants began descending the artificial pedicels, research funds to WTW. Gabriel Melo kindly stopped halfway, and returned· to the leaf, helped with identifying the Microstigmus, and indicatirÍgthey Were capable of descending the Mary Jane West-Eberhard provided helpful structure. ilnd incisive cornments on an earlier draft.
DISCUSSION RESUMEN
In our study, ants discovered and La suspensión pedicelar en la arquitectura del nido removed food more frequently from artificial de la avispa social Microstigmus nr. thripoctenlls juega nests without pedicels than those with un importante papel en la protección de las crías contra los depredadores, 'especialmente hormigas, basado de pedicels. These findings suggest that tbis estudios con nidos artificiales. Debajo de las hojas de architectural feature' can impede ant Carludovica palmata un nidoártificial con pedicelo y predation, witbout a chemically-defended otro sin pedicelo fueron suspendidos por 30 min para pedicel like that kl10wn for sorne paper wasps determinar la frecuencia de descubrimiento y (N = (Vespidae) (e.g., Jeanne 1972, 1979; Kojima eliminación de un cebo (atún) 36 pares). El porcentaje de depredación fue menor (11.1%) en los 1993). Instead, this deterrence may result nidos con pedicelo que en nidos sin pedicelo (83.3%). Se from a decreásed liklihóod that' ants detect observaron 343 individuos de cinco especies de potential food that is farther from tbe hormigasatacando los nidos artificiales. GRAJALES & WCISLO: Nest architecture oC group-living wasps Microstigmus 1143
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