Drivers of adult insect emergence, termination, and duration
In comparison to other aspects of insect phenology, the drivers of adult
emergence are relatively well understood. The emergence of many insect
species is controlled by temperature because the growth rate of immature
stages increases at warmer temperatures
(Gilbert and Raworth,
1996; Hodgson et al., 2011). Life history traits, particularly diapause
stage, are also known to be an important predictor of when insects
emerge (Scott and
Epstein, 1987; Altermatt, 2010b). Our results confirm earlier emergence
in warmer areas and in species that diapause as adults.
Less is known about drivers of termination and duration of adult
insects. For many insects, photoperiod is likely predictive of activity
termination since it is a primary cue in many insects to initiate
diapause (Tauber and
Tauber, 1976; Denlinger, 2002). Although photoperiod is often the main
driver, diapause induction has also been related to temperature, food
availability, moisture, and chemical cues (Danks, 2007). Our results
also suggest that many factors contribute to the termination of adult
activity and indicate that these processes may be linked to temperature
seasonality, timing of the first fall freezes, or resource depletion
caused for example by drought. In multivoltine insects, we expected
longer activity durations in warmer regions, due to the production of
additional adult generations in areas with longer growing seasons
(Altermatt, 2010a;
Zeuss et al., 2017). It is less clear how temperature influences the
duration of univoltine insects. Because obligate univoltine species
cannot produce additional generations in warmer regions, we expected
more consistent activity durations across temperature gradients,
regardless of emergence dates. However, we did not find an interaction
between temperature and voltinism, suggesting similar temperature-driven
increases in duration regardless of voltinism. This surprising result
could be explained by multiple mechanisms for extending adult activity,
including lengthening timing of activity of cohorts of adults, more
generations per season, and reduced adult synchrony of univoltine
populations in warm areas. However, this result may be due to our
simplistic trait coding system labeling semi-, parti-, and merovoltine
species as “not univoltine”. While this does not impact our finding of
longer activity periods for univoltine species in warmer regions, more
finely scored voltinism states may help elucidate trait-mediated
phenology responses to temperature. We also note that our understanding
of life history traits, particularly voltinism and migratory behavior,
is incomplete across broad temperature gradients. Species that are
documented in the literature as non-migratory and univoltine may in fact
have undescribed migratory patterns
(Robinson et al.,
2009) or potentially multiple generations in warmer regions that remain
unreported.
One of the challenges with understanding phenological sensitivities is
that regional contexts may impact not only the strength of sensitivity
but also its direction. Li et al. (2019) showed, for example, that
urbanization shifts from advancing flowering in cold regions to delaying
in warmer ones. Here we also find strong evidence of these
region-specific contextual effects. One of the clearest examples is that
the influence of precipitation and life history traits on insect
duration varies with regional temperature. Warm, wet areas demonstrated
longer adult insect duration than cooler, wetter regions. This
interaction is expected, considering that snow contributes to annual
precipitation in cool regions and snowmelt date delays the beginning of
activity more than the end
(Stemkovski et al.,
2020). These regional effects also included life history interactions
with temperature. We found that detritivores terminated activity later
and had longer durations than carnivores, potentially due to detritus
being more available later into the fall compared to green leaf
materials or prey items. This effect is especially strong in warm
environments, where detritus can accumulate year round.
We also expected a complex interaction between temperature and
urbanization, similar to the results of Diamond et al. (2014) for
butterflies in Ohio, which documented earlier emergence in cold areas
with high human population density and delayed emergence in warmer, more
urbanized areas. While we found such an interaction, it was in the
opposite direction than expected: insects generally emerged later in
cool, urbanized areas and earlier in warm, urbanized areas, in
comparison to corresponding rural areas. Due to our species selection
protocol, our species list likely overrepresented exploitative species
since these are commonly observed. Exploitative butterflies showed
smaller delays in first appearance in warm and urbanized areas (Diamond
et al., 2014), potentially, in part, explaining our differing results.
We encourage future work exploring how urbanization impacts insect
phenology and at what spatial scales urbanization influences are most
apparent.