Group Size and Composition

The size of a cockroach aggregation is ultimately controlled by its resource base. If food and water are adequate, the surface area of undisturbed dark harborage limits population size (Rierson, 1995). Favorable habitats can result in enormous populations. Roth and Willis (1957), for example, cite a case of 100,000 B. germanica in one four-room apartment. As with many other characteristics of urban and laboratory cockroaches, however, high population size and the tendency to form large aggregations are not typical of cockroaches in general. Although species that inhabit caves often live in large groups, individuals of most species are not at all crowded in nature. In Hawaii, aggregations of Diploptera punctata in dead dry leaves consisted of 2-8 adults, together with 5-8 nymphs (WJB and L. Kipp, unpubl. data). Researchers who study agonistic or mating behaviors of cockroaches in the laboratory are invariably amazed when they are unable to observe these activities in the field. Small groups of cockroaches are sometimes observed feeding and pairs may be seen copulating, but never in high numbers (Bell, 1990). In one 3-yr field study of cockroach behavior, only four instances of agonistic behavior were recorded, while in laboratory cages agonistic behavior occurred nearly continuously among males (WJB, unpubl. obs.).

Age- and sex-related variation in grouping tendencies are commonly reported in cockroaches (Gautier et al., 1988) and are no doubt related to the mating system and age-dependent fitness biases unique to a species or habitat. In most tested cockroaches the early instars have the strongest grouping tendencies, and in some they are the only stages that display gregarious behavior (e.g., Hafez and Afifi, 1956).All developmental stages are found in aggregations of B. germanica and P. americana, but young nymphs have the greatest tendency to remain in tight groups (Ledoux, 1945; Wharton et al., 1967; Bret et al., 1983; Ross and Tignor, 1986b). At hatch, neonates maintain a distance from each other, but aggregate as soon as the exoskeleton has hardened (Dambach et al., 1995). The gregarious behavior typical of young cockroaches is retained into later developmental stages in some species. Exceptions lie among the cave cockroaches, where older insects may show the strongest grouping tendencies; these differences appear related to habitat stratification. Adults and older nymphs are typically found aggregated on the walls of caves or hollow trees, utilizing crevices if present, and young nymphs burrow in guano or litter on the substrate (e.g., Blaberus colloseus, Blab. craniifer, Blab.

giganteus, Eublaberus posticus) (Brossut, 1975; Farine et al., 1981; Gautier et al., 1988). Nonetheless, Darlington (1970) found that young nymphs of Eub. posticus aggregate strongly, but they do so independently of older stages, and aggregation pheromone is produced by all developmental stages of both Eub. distanti and Blab. craniifer (Brossut et al., 1974). Laboratory assays seldom take into account the habitat preferences of different stages, and we know nothing of the social tendencies of young cave cockroaches while under organic debris. Age-related distributional differences are known within the large affiliative aggregations typical of pest cockroaches. Young B. germanica typically cluster in the middle of the aggregation (Rivault, 1989).Fuchs and Sann (1981,in Metzger, 1995) found that first- and second-instar B. germanica create small independent aggregations and do not mingle with older conspecifics until the third instar.

There is a complex relationship between sex ratio, sexual status, and grouping behavior in affiliative aggregations. Ledoux (1945) noted that male nymphs of B. germanica showed significantly stronger aggregation tendencies than groups of females. Adult females of this species have the most influence on group composition, but these effects are moderated depending on the demographics of the group in question (Bret et al., 1983). The reproductive status of females was a factor, with gravid females promoting the strongest grouping behavior. The maturity of the egg cases carried by females was also influential. Adult males typically show little gregarious-ness and spend the least amount of time in shelters. The loss of gregarious behavior in males typically coincides with sexual maturity and the onset of competition for mates (Rocha, 1990).

An examination of group composition in the cockroaches listed by Roth and Willis (1960) indicates that aggregations of lesser known species in several cases do not contain adult males. The basic unit of some affiliative aggregations appears to be the uniparental family: groups of mothers together with their offspring. Species mentioned include females and young of Ectobius albicinctus found beneath stones (Blair, 1922), of Polyphaga aegyptica and Polyp. saussurei found in rodent burrows (Vlasov, 1933; Vlasov and Miram, 1937), and of Arenivaga grata collected from guano in bat caves (Ball et al., 1942). There are also occasional reports of cockroach aggregations consisting entirely of females, for example, Arenivaga erratica in burrows of kangaroo rats (Vorhies and Taylor, 1922), and aggregated females and dispersed or territorial males in Apotrogia sp. (= Gyna maculipennis) (Gautier, 1980).

Nothing is known about the immigration of unaffili-ated cockroaches into established conspecific groups. Discrete aggregations collected in the field often mix to gether freely in the laboratory (e.g., Panesthia cribrata— O'Neill et al., 1987), but this is quite different from a solitary insect attempting to join an established group under natural conditions. When two isolated young nymphs of P. americana are placed in contact with each other, they undergo a "ritual of accommodation"which may become aggressive (Wharton et al., 1968). Behaviors include "sampling" each other's deposited saliva with palpi or antennae, stilting, tilting their bodies, bending their abdomens, antennal fencing, leg strikes, and biting. The decision to accept new members into the aggregation can be important when changing ecological conditions (e.g., food availability) alter the relationship between group size and fitness (Giraldeau and Caraco, 1993).

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