Communal nesting, rare in the crabronid wasps, has been recorded for various species in the Spilomenina clade of the Pemphredoninae. A new communally nesting species, Spilomena socialis, is described from peninsular Malaysia where it nested on buildings at Bukit Fraser. The nest consists of a group of closely spaced clusters of vertically oriented cells attached to walls, and is constructed of tiny pieces of vegetal and mineral origin, parts of insects, and fungal hyphae bound together by silk secreted from each female wasp’s abdominal gland. Nests contained up to 39 cells (average 10.4 cells, N = 35). Nest entrances were at the upper end of the cells and were protected on one side by a “roof”. Cells constructed side-by-side have their roofs connected to form a tube that allowed access to all the cells. Nests were inhabited by 1–13 females (average 4.3 females per nest, N = 21) and 0–4 males, the overall sex ratio being 0.22. Ovarian development among the females in a nest varied. In 8 of 20 nests with 3 or more females only one female had developed ovaries, but female size (measured as head width) did not correlate with ovarian development. Cells are apparently progressively provisioned with thrips, and are often re-used. Adult females cooperatively defend the nests against intruders.

Social wasp, ovarian development, sociality, thrips

In the superfamily Apoidea, which includes the apoid wasps of the family Crabronidae, nesting habits vary from strictly solitary to eusocial. The majority of the more than 8000 described crabronid species are characterized by solitary nesting habits but some members of one clade in the subfamily Pemphredoninae, the Spilomenina, consisting of the genera Arpactophilus, Microstigmus, Spilomena, and Xysma, have evolved relatively complex social behavior (Matthews 1991).

Reproductive division of labor has so far only been ascertained in the Neotropical Microstigmus comes Krombein, where nests are usually founded by solitary females and relatedness between female colony members varies from 0.6 to 0.7 indicating the presence of mother-daughter associations with single-mated foundresses (Matthews 1968, Ross and Matthews 1989a and b). Females of M. nigrophthalmus Melo perform trophallactic exchanges, practice oophagy and differ in their activities on the nest (Melo and Campos 1993, Lucas et al. 2011). However, ovarian development was comparable in all colony females. In the Australian Arpactophilus mimi Matthews and Naumann, an average of three females occupied and progressively provisioned the nest, but displayed no differences in ovarian development (Matthews and Naumann 1989).

Most of the 80+ species of the cosmopolitan genus Spilomena nest in preexisting cavities in twigs, decayed wood and structural timber and prey on thrips, psyllids, aphids or coccids (Bohart and Menke 1976). However, communal nests have been reported in a few species. An Australian ground-nesting species, S. subterranea, McCorquodale and Naumann, had an average of 2.5 females with similarly developed ovaries per nest (McCorquodale and Naumann 1988). In an undetermined species from Costa Rica, West Eberhard (1977) reported a nest with ten females differing widely in size but all having comparable ovarian development and practicing progressive provisioning of the larvae. Some nests of another undetermined Spilomena species from southeastern Brazil contained as many as four females (Carvalho and Zucchi 1989).

We here describe a new species of communally nesting Spilomena found in the Central Mountain Range of Peninsular Malaysia and present information on its nest structure and composition, and other features of the biology.

Nests of Spilomena socialis were found at Bukit Fraser (1600 m; 03°42.77'N, 101°46.32'E) a mountain resort at the higher elevations of the Central Range of Peninsular Malaysia, in the state of Pahang. Entire nests were collected in February and March in both 2004 and 2007. For each colony all adults present were captured and the nest cells were removed from the substrate. Eggs, larvae and pupae were recorded for each nest. Adults and immature brood were preserved in 70% ethanol. Some nests were mounted on pieces of hard cardboard and others preserved in 70% ethanol.

Adult females were dissected to determine ovarian development, classified from 0 to 3 (maximum development = mature egg ready to be laid) according to their relative size. We photographed the head of each female present in each colony and used the open source software ImageJ to measure the maximum width of the head (reported in pixels).

Brief ethological observations were performed on some colonies, and simple experiments were conducted in an attempt to ascertain the defensive reactions of the adults against ants.

To better describe the relationship among small clusters of nests, and the use of different nests by wasps, we built an association network in which two nests were considered “associated” when a wasp passed from one nest to the other, or visited two nests consecutively. The resulting graph was a weighted and directed network in which each nest represents a node and their associations the edges. Degree (i.e., the sum of the strength of all edges connected to a node) was the only centrality measure calculated for each node. The analysis was based on two days of video observation of five adjacent nests, from 0900 to 1900 h. The graph (network) was obtained using NET-DRAW 2.097.

Digital images were captured using Microptics Digital Lab equipment. All statistical analyses were performed using the statistical program SPSS® 13.0 for Windows®.

Spilomena socialis colonies are comparable to those of several other species of the subtribe Spilomenina. For example, the maximum number of females (11) is similar to that reported for other species (13 in Microstigmus comes (Matthews 1991), 6 for M. nigrophtalmus (Melo 2000, Lucas et al. 2011), 26 for M. sp. (West-Eberhard 1977), 10 in Arpactophilus mimi (Matthews and Naumann 1989), Spilomena subterranea (McCorquodale and Naumann 1988), and a Costa Rican Spilomena sp. (West-Eberhard 1977)). Their colonies also display a well-defined, strongly female-biased sex ratio; males while regularly present in the nests, appear to contribute very little to the colony life.

Some females in a colony display fully developed ovaries, as occurs in M. nigriphtalmus, but in more than a third of the colonies examined with at least three females only one possessed mature eggs. However, it was not possible to demonstrate a reproductive division of labor between the various females in a nest, as occurs in M. comes, because other females in the colonies display at least some ovarian development. Whether wasp age is the key factor in ovarian development, and whether age correlates with epicuticular chemical profiles as occurs in some species of Stenogastrinae wasps (Turillazzi et al. 2004, Bridge and Field 2007, Baracchi et al. 2010), must await future studies. Unlike the situation in M. comes (where egg laying females are significantly larger than the population average) no size difference between females with fully developed and undeveloped ovaries was found in S. socialis.

Despite the relatively limited behavioral observations it appears that females practice progressive provisioning and cooperate in defense of the nest, patrolling the cells and attacking approaching ants and conspecific intruders. In contrast, males seem not to take part in the nest defense as was reported in M. nigrophtalmus (Lucas et al. 2011). Progressive provisioning is also reported for M. nigrophtalmus (Melo 1992, 2000), while M. comes mass provisions its cells (Matthews 1968). Patrolling and provisioning behaviors peak during morning and late afternoon similar to activity profiles reported for other tropical wasps like hover wasps (Turillazzi 1988, Baracchi et al. 2009, 2013).

In S. socialis, nests are composed of a series of cells clustered on flat substrata. The ‘invention’ of the tube connecting nest cells permits access to the immature brood. However, this architecture not only gives adults the possibility to monitor and guard groups of cells, but also provides a place where they can rest and interact with other adult individuals. The connecting tube may also constitute a barrier to further social evolution by physically limiting the space available for adult interactions. Other than patrolling, no interactions among these wasps have been observed outside of the nest or in the area surrounding the nest clusters.

In no case were cells found superimposed on others, a design that is more energetically expensive to create, due to the greater amounts of material needed (Jeanne 1975). Examples of nests composed of cells attached to flat substrata can be found also in Stenogastrinae wasps. In the genus Liostenogaster, for example, L. topographica Turillazzi presents a quite peculiar nest architecture with cells arranged along ribs of material (Baracchi et al. 2009), while in L. vechti Turillazzi, a series of contiguous cells form rings or brackets with their openings facing a central area where the adults rest (Turillazzi 1988). In both cases, however, adult individuals are not separated by architectural barriers and all the cells of the nest can be directly accessed by any member of the colony.

Construction of a nest has always been regarded as an important (and probably obligatory) characteristic of social insects (Hansell 1996). However, the vast majority of crabronids also construct nests, but are not communal or social. Thus there must be other factors that predispose evolution toward communal living in species like S. socialis. Unless preexisting cavities such as hollow twigs are ‘rented’ for nests, there is the need to acquire, transport, and assemble suitable nest building materials. This in turn creates opportunities both for cooperation and for making more complex nests. The latter is greatly facilitated through the use of silk to bind and shape nest materials. Thus the possession of silk-producing glands on the 6^th metasomal tergite of females, unique to the Spilomenina (Melo 1997), may be a key preadaptation facilitating the evolution of communal behavior in this clade (Matthews 1991, Melo 2000). That nest architecture can strongly influence social wasp evolution, is generally accepted (Jeanne 1975, Wenzel 1991, Matthews 1991, Hansell 1996), and this aspect of S. socialis biology merits further investigation.

We thank Henry Barlow and Simon Hok for their support in Malaysia. Tommy McElrath, University of Georgia, assisted with the photographs of the new species, and David G. Notton, of The Natural History Museum, London, UK kindly compared the new species to Turner’s type of S. obliterata.