Hybrid zone movement
For eight of the transect-years, the best fit of was the model with free
minimum and maximum frequencies and no tails, for six transect-years it
was the model with fixed frequencies and no tails, and for one
transect-year it was the model with free frequencies with tails (Table
3; Supplementary Figures S1 and S2). For the Massachusetts transect, the
analyses for 2009, 2011, and 2012 resulted in cline shapes with minimum
or maximum frequencies that did not approach 0 and 1 (Supplemental
Figure S2). This result was likely due to the fact that for those years,
we did not sample far enough into the Bruce spanworm (2009) or winter
moth (2011 and 2012) dominated areas. Consequently, these years were
left out of Figure 7 as the centers could not be accurately estimated
(Table 3). Cline analyses of the hybrid zones along the Massachusetts
and Connecticut transects indicated that the center of the hybrid zones
moved westward during our study period (Figures 7 and 8). Along the
Massachusetts transect, in 2007 the center of the hybrid zone was
located approximately 131 km from trap T00, and by 2018 the center of
the hybrid zone was located approximately 103 km from trap T00 (Table 3
and Figure 7). During our survey period, the location of the center of
the hybrid zone moved 3.1 km from 2007 to 2008, then moved more rapidly
from 2008 to 2010 (6.2 km/yr), then moved steadily westward from
2014-2017 (2.4 km/yr), before retreating eastward 7.7 km in 2018. Along
the Connecticut transect, in 2016 the center of the hybrid zone was
located approximately 109 km from trap CT01, in 2017 it moved
approximately 8.9 km west, and in 2018 the center of the hybrid zone
retreated approximately 3.1 km east (Table 3). It is therefore possible
that while we did not sample this transect prior to 2016, that a similar
westward shift occurred along the Connecticut transect as we found along
Massachusetts. For both transects, the mean population density of moths
increases dramatically just east of the hybrid zone center in the region
where winter moth is dominant (Figures 7 and 8). For the Massachusetts
transect, the cline width varied from 9.62 to 69.42 km across years with
a mean of 39.03 ± 7.01 km, and for the Connecticut transect, the cline
width varied from 26.82 to 50.3 km across years with a mean of 40.18 ±
6.97 km (Table 3).