DISCUSSION
By studying 50 colonies from five species of stingless bees, we showed
that the number of brood cells and external activity are the biological
parameters that best relate to colony size, regardless of the striking
differences in their size and life history traits. To measure bee colony
size and relate it to biological parameters closes important knowledge
gaps in the life histories of several widely distributed Amazonian
stingless bees, but also identifies useful proxy measures of colony size
for colony management and multiplication.
The number of brood cells was the best predictor of colony size;
however, its measurement involves an intensive handling of colonies, and
it is impossible to obtain in natural nests. External activity was also
positively correlated with population size, and it is feasible to obtain
in managed or natural colonies; we hypothesize this relationship occurs
because it is linked with the number of foragers (the last task in the
life of workers; Michener 1974; Sakagami 1982; Wille 1983), which are
responsible for keeping the food stocks in optimal levels, and thus in
capacity to nourish the immatures and adult bees.
External activity is commonly used for measuring the “strength” of the
colony; together with the number of adult bees, the number of brood
cells and the number of food pots (Hilário et al. 2000; Gostinski et al.
2017). External activity is a metric that can be easily assessed even in
natural colonies, where the measurement of brood combs and food stocks
is impossible to obtain without destroying the colony (Hilário 2007).
The relationship of colony size with the other measured biological
parameters varied among the different species. The food stocks only
presented a positive relationship with the number of adult bees inM. fasciculata . For the other species, there was no relationship,
as shown in the general model, including all species. Regarding the
egg-laying rate, in S. aff. postica we observed high
levels of variation in the relationship with the adult population,
probably due to the high mortality of immature bees observed in this
species (Figure S4).
Among the five species studied, S. aff. postica had the
largest population, corroborating previous estimates made by Lindauer
and Kerr (1960; around 15,000 adult workers), while P. minimapresented the smallest population (mean colony size around 400), much
higher than the estimate of 175 bees made by Wille and Michener (1973).
For M. fasciculata , our measures presented a smaller population
than the estimate of 776 adult bees, which may be due to methodological
differences between studies (Kerr et al. 2001). Another possibility is
that the since the study of Kerr et al. (2001) was made with other
populations of M. fasciculata , in a region approximately 600 km
from our study site and with different vegetation physiognomies. Thus,
differences in the estimates made for M. fasciculata (and forP. minima ; Wille and Michener 1973) may be due to populational
variation and the resources available to colonies in the different
areas. For F. longipes and M. flavolineata , there were no
previous estimates of their population size.
The size of a colony in social insects is linked with several aspects of
their life-history, such as foraging strategies and reproduction rates
(Oster and Wilson 1978; Planque et al. 2010). Species with large
populations, like we found in S. aff. postica and F.
longipes potentially have large numbers of workers involved in defense
and resource gathering, and therefore the colony’s consumption of
resources may increase proportionally. In contrast, small populations,
such as P. minima , although potentially not able to collect large
amounts of resources, may have smaller resource demands, thus
compensating for the small number of available foragers. However,
relationships between consumption/gathering of resources are yet to be
studied in stingless bees, with one of the reasons being the perceived
difficulty in estimating colony size. As such, our results will help
future studies investigate this relationship.
From a practical perspective, knowledge on stingless bee colony sizes is
highly relevant for their use as crop pollinators (Giannini et al.,
2020). For example, in the Amazon region, Scaptotrigona aff.postica visit crops of economic importance, such as Rambutan
(Nephelium lappaceum L.) and pollinate others, such as Açaí palm
(Euterpe oleracea Mart) (Ricon-Rabanales 2015; Campbell et al.
2018). However, for crop pollination, clear recommendations on colony
stocking densities are necessary for the development of effective
managed pollinator programs, as already defined for Apis
mellifera (Vaudo et al. 2012). This number depends on the foraging
range of the colonies, the number of foragers, and the number of flowers
that need to be pollinated within croplands (Kuhn-Neto et al. 2009;
Rands and Whitney 2011). Thus, our data can be combined with existing
information on species’ foraging ranges to develop novel managed
pollinator protocols using native bee species for several important
regional crops (Campbell et al. 2019; Araújo et al. 2004).
Finding an efficient method for estimating population size is important
for commercial rearing of stingless bee colonies, for monitoring colony
health, and for future scientific studies. Our study is the first to
count the total number of bees (adult and immature) and correlate it
with intrinsic factors in meliponine colonies. Previous studies provided
formulae to estimate colony size but did not validate these formulae
with biological data (Ihering 1930; Aidar 1996), and a recent study
showed that colony size of stingless bees has been overestimated
(Valadares et al. 2021). Since the direct measure of the number of adult
bees involves invasive sampling methods (Valadares et al. 2021), our
study provided a reliable approach to estimate the number of adult bees
by using the external activity. On the other hand, our study shows that
estimating population size will not be possible with a general formula
that works for all Meliponini.
For building a reliable and feasible estimation method for population
size, future studies should focus on single species or genus. It is also
important to collect larger and broader samples to deal with
disturbances caused by natural variation and thus understand the error
range of the method. External activity is a good parameter to start
developing a formula, because it is a variable that is easy to obtain in
natural or managed colonies, and it is highly correlated with population
size in most species. Future studies also must focus on other bee genera
and verify possible variation in interspecific population size related
with different life-history traits, as well as with temporal
fluctuations.