Eusocial wasp biology in Monteverde is affected
by elevation. Many polistine species occur only at
particular elevations (Appendix 4.3). However, a num-
ber of species exhibit a "cliff edge effect"; that is, they
build nests at atypically high or low elevations along
steep drop-offs, a pronounced pattern along the east-
ern end of the San Luis valley. In the course of obser-
vations made in different seasons between 1988 and
1996, I have found Polybia aequatorialis nests, usu-
ally restricted to higher elevations in Monteverde,
below the San Luis waterfall down to 1150 m (Fig.
4.16). Conversely, Mischocyttarus atrocyaneus and
Synoeca septentrionalis, which are common in San
Luis, have been found nesting along the cliff edge
above 1350 m in Monteverde.
Insect elevational migration, which is best studied
in butterflies (see Sec. 4.5.2), is exhibited in Monte-
verde by Polistes instabilis. This species does not nest
in Monteverde but is common in the seasonally dry
lowlands to the west, below 600 m. Nonnesting ag-
gregations of migrant females begin forming on build-
ings and large forest trees in Monteverde before the
onset of the dry season, beginning around late Octo-
ber. Males join the aggregations later. The wasps clus-
ter and are inactive during the evening and on cloudy
days but forage on sunny days. Aggregations of this
species also occur on Volcan Cacao at similar ele-
vations. There, dissected females did not have de-
veloped ovaries during the early aggregation season
(J. Hunt, pers. comm.). Changes in female reproduc-
tive physiology and the occurrence of copulation
(O'Donnell 1994) during the aggregation season war-
rant further investigation.
Seasonality, particularly in precipitation, can have
pronounced effects on tropical wasp life history. For
example, in the seasonally dry lowlands, many spe-
cies of eusocial wasps experience a period of dimin-
ishing prey abundance, which leads to reduced
colony growth during the dry season (R. Jeanne, pers.
comm., S. O'Donnell, pers. obs.). However, little evi-
dence exists for seasonal restriction of nest initiation
and colony development in Monteverde. I have found
brood of all stages in Polybia aequatorialis nests dur-
ing both wet and dry seasons and collected or reared
males (suggesting that colonies are in reproductive
mode) from nests in November-January and in May.
Some independent founding wasps may be more syn-
chronous in the timing of colony foundation than
Polybia. For example, the majority of Mischocyttarus
mastigophorous nests located in October 1995 surveys
were abandoned, and many of the colonies located in
December and January were recently initiated.
Independent founding wasps frequently nest on
human structures in Monteverde, particularly under
the eaves of buildings. Possible advantages of nesting
in these sites include reduced predation pressure from
ants (Jeanne 1979) and more favorable microclimate
(Jeanne and Morgan 1992). I have found Mischocyt-
tarus species nesting under leaves and on rootlets
in roadbanks and landslides. Nests of two swarm
founders, Polybia aequatorialis and P. raui, are also
often found on vegetation in road banks, built around
a slender vertical support (such as a rootlet) hanging
from a rooflike surface. The outer envelopes of Polybia
nests in Monteverde include many layers of paper and
numerous air pockets, presumably as insulation against
low temperatures and/or precipitation. Whether enve-
lope thickness varies with elevation within species is
unknown. Wasps in the genus Agelaia usually nest
in cavities in live trees (e.g., A. yepocapa) or in fallen
logs (e.g., A. panamensis and A. xanthopus). Agelaia
areata is unusual in building envelope-covered nests
among tree branches.
Nearly all species of Monteverde eusocial wasps
have dark brown or black bodies, particularly species
occurring at higher altitudes. Pale-colored exceptions
(Agelaia areata and A. yepocapa) include species that
nest in open sites. Some Monteverde species that span
a wide range of elevations have reduced pale or yel-
low markings at higher sites (Polybia diguetana,
P. raui, and Mischocyttarus mastigophorus). Wasp
body coloration may be adapted to life in Monte-
verde's climate (cool temperatures, limited insola-
tion). Darker coloration may allow wasps' bodies to
warm to flying temperatures more quickly.
Parasites and parasitoids (including several fami-
lies of wasps, flies, and mites) make their homes in
nests of eusocial wasps. Parasitoids in the family
Trigonalyidae in Agelaia and Polybia colonies occur
in the Monteverde area. Several of these trigonalyids
are close Wasmannian mimics (parasites that re-
semble their hosts) of the vespid wasps with which
they were collected.
Surface raiding army ants are important predators
of neotropical eusocial wasps (Chadab 1979). Wasps
have few defenses against army ants if their nests are
discovered (West-Eberhard 1989), although species
with costly nests or nest sites (e.g., cavities in trees)
may defend against ant raids more vigorously (O'Don-
nell and Jeanne 1990). Epipona niger and Agelaia
areata nest high up in trees in Monteverde, possibly
to avoid army ant predation. Polybia rejecta nest almost
exclusively in association with Azteca ants, often in
Cecropia trees, thereby gaining protection from army
ants (F. Joyce, pers. comm.).
Little is known about vertebrate predators of
Monteverde's eusocial wasps. Birds and other visu-
ally oriented predators have probably been an im-
portant selective factor in the evolution of wasp col-
oration patterns and mimicry, but avian responses to130 Insects and Spiders