10.5061/DRYAD.FJ6Q573QX
Westby, Katie
0000-0003-1403-5362
Washington University in St. Louis
Medley, Kim
Washington University in St. Louis
Data from: Cold nights, city lights: artificial light at night reduces
photoperiodically induced diapause in urban and rural populations of Aedes
albopictus
Dryad
dataset
2020
Tyson Research Center
2021-07-09T00:00:00Z
2021-07-09T00:00:00Z
en
https://doi.org/10.1093/jme/tjaa139
10863 bytes
4
CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
As the planet becomes increasingly urbanized, it is imperative that we
understand the ecological and evolutionary consequences of urbanization on
species. One common attribute of urbanization that differs from rural
areas is the prevalence of artificial light at night (ALAN). For many
species, light is one of the most important and reliable environmental
cues, largely governing the timing of daily and seasonal activity
patterns. Recently, it has been shown that ALAN can alter behavioral,
phenological, and physiological traits in diverse taxa. For temperate
insects, diapause is an essential trait for winter survival and commences
in response to declining daylight hours in the fall. Diapause is under
strong selection pressure in the mosquito, Aedes albopictus; local
adaptation and rapid evolution has been observed along a latitudinal
cline. It is unknown how ALAN affects this photosensitive trait or if
local adaptation has occurred along an urbanization gradient. Using a
common garden experiment, we experimentally demonstrated that simulated
ALAN reduces diapause incidence in this species by as much as 40%. There
was no difference, however, between urban and rural demes. We also
calculated diapause incidence from wild demes in urban areas to determine
if wild populations exhibited lower than predicted incidence compared to
estimates from total nocturnal darkness. In early fall, lower than
predicted diapause incidence was recorded but all demes reached nearly
100% diapause before terminating egg laying. It is possible that nocturnal
resting behavior in vegetation limits the amount of ALAN exposure this
species experiences potentially limiting local adaptation.
We collected live Ae. albopictus at six sites (demes) during July and
August, 2019; three locations were in back yards within the city of Saint
Louis, MO and three locations were in oak-hickory forest habitat at Tyson
Research Center (TRC), located approximately 38 kilometers outside of
Saint Louis city (Fig. 1). TRC is a 2,000 acre, primarily forested,
environmental field station bordered by state and county parks (38°31′N
90°33′W). To facilitate independence, collection sites were at least one
kilometer apart which is the estimated maximum distance that adult Ae.
albopictus will disperse naturally. At each site, four seven-litre black
buckets were secured with ground stakes and provided initial inoculums of
rain water and leaf detritus collected from the forest floor at TRC to
attract oviposition; mosquitoes were allowed to colonize buckets
naturally. We collected eggs weekly on seed germination paper that had
been secured above the water line in each bucket. Egg papers were dried
and stored in an environmental chamber set at 24OC and a 16:8 light:dark
cycle. To establish colonies, eggs were stimulated to hatch in batches by
submersing them in a solution of 0.35g/L of DifcoTM Nutrient Broth
(Becton, Dickenson, and Company) in deionized (DI) water. After 48 hours,
larvae were sieved, identified as Ae. albopictus and reared in group pans
containing 10mL DI water and 0.0015g bovine liver powder per larva. Each
of the six lab colonies were composed of mosquitoes collected from the six
field sites during at minimum three collection events, providing at least
460 larvae per colony to produced F1 adults. F1 adults, in turn, produced
the eggs used in the experimental manipulation to measure diapause
incidence. We continued to collect eggs from the three city sites through
the week of October 9th at which time no more eggs were laid.
For the experimental manipulation, we stimulated F1 eggs to hatch using
the afore described protocol. Four hundred first instar larvae were
counted out for each deme and reared in group pans with four litres of
water and 0.6g of bovine liver powder. The pans were then covered with
mosquito netting to prevent oviposition and were placed in the field under
the forest canopy at TRC. We had two sites at TRC, 1.5 kilometers apart,
one we experimental illuminated and one we did not. Our experimentally
illuminated site consisted of two high pressure sodium lights, one 75 Watt
(Lithonia Lighting, OFL 70s 120 LP BZ M4) and the other 35 Watt (Eaton
Lighting, W-35-H/PC) strapped to trees approximately 2.5 meters from the
ground and positioned in a way that our larvae and adults received between
9 and 11 lux depending on their position on the table to which they were
strapped. This amount of illuminance was chosen as it is commonly recorded
in urban areas and was considered an intermediate level. No detectable
light at night was measured at the non-ALAN treatment site (Dr. Meter
LX1330B Digital Illuminance Meter). Caged larvae and adults were also
strapped to a table at the non-illuminated site; the position of all cages
was rotated every two days. Larvae were placed in the field on September
20th which corresponds with a photoperiod of 12:13 hours of daylight at
the latitude (38.3ON) of this study and is predicted to be short enough to
produce diapause incidence in Ae. albopictus nearing 100% (Urbanski et
al., 2012 see supplemental material). Temperatures were logged (Onset HOBO
MX2301) at both sites to ensure that there were no significant differences
between sites, as temperature is also known to influence diapause rates in
this species. By the week of October 6th the temperatures in the field
were dipping below 10OC at night and adult mosquitoes were not taking
blood meals, nor was there any observed mating, despite very frequent
mating occurring in colony cages for this species. Thus, we moved the
cages into the laboratory in environmental chambers set at 21OC with a
12:12 light:dark cycle. One chamber was set at complete darkness during
the night cycle and the other chamber was manipulated but inserting two
high pressure sodium lights, one at the top and the other at the bottom of
the chamber. The lights were covered with aluminum foil to dim the lights
to achieve illumination comparable to the field manipulation, and ranging
between 8 and 12 lux. All cages were rotated inside chambers every two
days. All of the eggs used to calculate diapause were laid after the
mosquitoes had been moved into the chambers. Temperatures in the two
environmental chambers varied by approximately 0.3OC. Adults were fed by
placing definbrinated bovine blood (Hemostat Laboratories, Dixon, CA) into
hog intestinal casing, warmed in a water bath, and placed on top of each
cage. Eggs were collected in black cups lined with seed germination paper
and filled with leaf infusion to stimulate oviposition. Egg papers were
replaced every three to five days and stored in humidified bags in the
chamber from which they were collected. After eggs were allowed to
embryonate for two weeks, they were hatched with the aforementioned
hatching protocol except that they were dried and hatched a second time.
After all hatched larvae were counted (from both hatching events), the egg
papers were bleached to determine embryonation [53]. Viable embryos that
had not hatched were counted under a dissecting microscope and classified
as being in diapause. We calculated the proportion of diapause eggs as the
number of embryonated but unhatched eggs divided by the total number of
embryos (hatched and unhatched). The effects of deme origin (urban or
rural) on diapause incidence (proportion of eggs in diapause) was analysed
using a GLMM using a quasi-binomial distribution with deme origin (urban
or rural), light treatment (exposed to ALAN or not), and their interaction
as predictors. Statistical analyses were performed in SAS 9.4.
We also calculated the proportion of eggs in diapause laid by wild demes
at the three urban sites from which we collected experimental mosquitoes.
We were unable to do this at the three rural locations because two other
species, Aedes triseriatus and Aedes japonicus, which are not readily
distinguishable as either eggs or pharate larvae, co-occur with Ae.
albopictus in rural habitat. We are confident, however, that our urban
collections were Ae. albopictus because all of the eggs collected at these
sites in the previous weeks and identified for the colonies were Ae.
albopictus. Moreover, we have two years of data from urban Saint Louis
that contained no other Aedes species (Westby et al. unpublished). We
determined proportion diapause for two egg papers collected weekly from
the three sites for the weeks of September 4th through October 2nd after
which no eggs were collected. Proportion diapause was determined using the
same methods as for the experimental demes and we report the mean and
standard error proportion for the two egg papers.