Cockroache; Ecology, behavior & history - W.J. Bell

unclear, however, whether the insects are faithful to the
group, to the physical location, or both.
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 gregariousness
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 unaffiliated
cockroaches into established conspecific groups.
Discrete aggregations collected in the field often mix to-
134 COCKROACHES