A peer-reviewed open-access journal

NeoBiota 18: 151-156 (2013)

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doi: 10.3897/neobiota. 18.5288 SHORT COMMUNICATION ty NeoBiota

www.pens oft. n et/j journa | s/ neob iota Advancing research on alien species and biological invasions

Could natural selection change the geographic
range limits of light brown apple moth
(Lepidoptera, Tortricidae) in North America?

Amy C. Morey', Robert C. Venette?, William D. Hutchison!

| Department of Entomology, University of Minnesota, St. Paul, MN, USA 2 Northern Research Station,
USDA Forest Service, St. Paul, MN 55108, USA

Corresponding author: Robert C. Venette (rvenette@fs.fed.us)

Academic editor: R. Magarey | Received 9 April 2013 | Accepted 27 May 2013 | Published 13 September 2013

Citation: Morey AC, Venette RC, Hutchison WD (2013) Could natural selection change the geographic range limits
of light brown apple moth (Lepidoptera, Tortricidae) in North America? In: Kriticos DJ, Venette RC (Eds) Advancing
risk assessment models to address climate change, economics and uncertainty. NeoBiota 18: 151-156. doi: 10.3897/

neobiota. 18.5288

We artificially selected for increased freeze tolerance in the invasive light brown apple moth.
Our results suggest that, by not accounting for adaptation to cold, current models of poten-
tial geographic distributions could underestimate the areas at risk of exposure to this species.

Forecasting future distributions of invasive insects is important for many manage-
ment and regulatory decisions. However, numerous challenges exist in creating accu-
rate, biologically relevant models and maps that are meaningful over time (Venette et
al. 2010). In particular, no models currently account for the potential of an invasive
species to adapt to a new environment. In fact, demographic models in invasion bi-
ology commonly treat species as “homogenous immutable entities” (Lee 2002). For
invasions by alien species in North America, adaptation to cold temperature may be
especially important at northern latitudes or high elevations; cold often prevents spe-
cies from surviving year round (Huey 2010, Venette 2013). For example, cold is likely
to constrain the future distribution of the light brown apple moth, Epiphyas postvittana
(Walker), a recent insect invader to North America. No model for E. postvittana cur-
rently accounts for the possibility of evolution of increased cold tolerance. The objec-
tive of this study was to determine if it was possible to artificially select for increased

Copyright Amy C. Morey et al. This is an open access article distributed under the terms of the Creative Commons Attribution License 3.0
(CC-BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

£52 Amy C. Morey et al. / NeoBiota 18: 151-156 (2013)

cold tolerance in this species, and if so, to begin exploring the subsequent geographical
repercussions. Epiphyas postvittana is considered to be predominately freeze intolerant
during its purported overwintering stage, the late instar larva (Burgi and Mills 2010),
but preliminary data suggests that a small proportion of the population may also be
freeze tolerant (Venette unpublished data). This phenomenon could be considered at
least “partial freeze tolerance” (Sinclair 1999), and enabled freezing to act as a strong
selection pressure for enhanced cold tolerance in our study.

Epiphyas postvittana eggs were obtained from USDA-APHIS (permit
P526P-11-03713). All subsequent rearing and experimentation was conducted in a
Biosafety Level 2 Containment Facility in St. Paul, MN. Eggs were held at 23°C,
60% RH, and resulting neonates were reared on artificial bean diet until late instars.
We cooled 4-6" instars (verified through head capsule measurement; Danthanarayana
1975) individually inside gelatin capsules at ~1°C/min to their supercooling point in-
side a -80°C freezer (modified from Carrillo et al. 2004). Once freezing occurred, lar-
vae were immediately returned to 23°C and given fresh diet. Mortality was measured as
failure to eclose. Surviving moths were randomly mated in 0.47L (16 fluid oz) contain-
ers with 1-3 individuals of each sex. Randomly selected offspring were subsequently
reared and supercooled as previously described. This procedure was repeated for nine
generations. A minimum of 102 larvae were supercooled in each generation. A control
population was maintained simultaneously and identically, save exposure to freezing.

Survival following freezing after nine generations was compared between the selected
and control populations using non-parametric cumulative incidence functions (CIF) in
SAS 9.3 (SAS Institute, Cary, N.C.) to address competing risk (Satagopan et al. 2004).
The competing risk was any individual that froze and survived. These estimates were then
used to calculate the temperature required to kill 50% (LT’,,) of each population.

We used NAPPFAST (Magarey et al. 2009) to map where temperatures might fall
below the bee of the selected or control populations. For each 10 x 10km grid cell,
NAPPFAST calculated the proportion of the last 10 years in which the lowest tem-
perature of the year was colder than the LT, for each population. We ran the model
with 3-D interpolated climate data.

After only nine generations of selection, the probability of survival following freez-
ing was significantly («=0.1) greater for the selected population than the unselected
control (Fig- 1seP=.0:078,-di= 1-47 =3.1 1): “The 1 for the unselected and selected
populations were estimated to be —16.5°C and —19.0°C, respectively.

Figure 2 illustrates the geographic significance of a putative increase in cold toler-
ance for E. postvittana. Dark blue areas indicate the most dramatic effect of cold, where
the LT,, was reached in 9-10 of the 10 years modeled. Cold was sufficient to exclude E.
postvittana in many northern areas (e.g., Minnesota, the Dakotas, Wyoming, and much
of Canada) using either the unselected (Fig. 2a) or selected (Fig. 2b) model. However,
for other mid-western, eastern, and southern states, there was an overall reduction in
the number of years where the LT’, was reached when using the selected population;
the light blue to dark green colors shifted north. For example, the unselected model
projected that nearly all of Michigan would reach the LT,, during 90-100% (dark

Could natural selection change the geographic range limits of light brown apple moth... 153

0.8 —e— Selected
—e— Unselected

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Cumulative Incidence of Mortality

0.0
-30 -25 -20 -15 -10 -5 0

Temperature (°C)

Figure |. Cumulative incidence of mortality of £. postvittana late instars with (n=179) and without

(n=109) nine generations of artificial selection for increased cold tolerance.

blue) of the years modeled. In contrast, the selected model projected that most of the
state would only reach the threshold between 50 to 80% of the modeled years (light
blue and white). Similarly, western states (e.g., Nevada and Idaho) showed an eastward
shift in the number of years that temperatures did not reach the LT...

Current risk maps that exist for E. postvittana (e.g., Fowler et al. 2009, Gutierrez et
al. 2010, Lozier and Mills 2011), acknowledge the importance of cold in shaping this
species’ potential U.S. range. However, the parameter(s) used to describe cold toler-
ance is/are assumed to be static. If natural selection follows a pattern similar to what
our research suggests, within a relatively short period, current models may underesti-
mate the risk of E. postvittana exposure in some areas in the future.

Uncertainty is inherent in pest risk models and contending with it is an ongoing
area of research (Venette et al. 2010). Our study attempted to address the uncertainty
related to the potential of a species to adapt to a cold environment and highlighted
the geographic consequences if adaptation to cold is not considered. However, other
sources of uncertainty still remain and are important future directions of this work. For
example, the time of year may influence the effectiveness of selection on E. postvittana,
assuming multi-voltinism and a randomly mating population. Selection is only likely
to increase cold tolerance when there is a strong pressure (winter). But if there is any

154 Amy C. Morey et al. / NeoBiota 18: 151-156 (2013)

Figure 2. The frequency of years (2003-2012) in which temperatures fell below the threshold required

to cause 50% mortality (LT’,,) of E. postvittana in North America: a without selection, and b after nine
generations of selection for increased freeze tolerance. Dark blue indicates where the LT,, was reached
in 9-10 of the 10 years, dark green indicates where the LT’, was never reached during the 10-yr period.

Could natural selection change the geographic range limits of light brown apple moth... 155

trade-off between increased cold tolerance and fitness (see Watson and Hoffman 1996,
Huey 2010), cold adapted individuals may be selected against during months when
selection pressure is reduced (summer). Similarly, particularly for a highly polyphagous
insect like E. postvittana, host plant variability could also affect cold tolerance measures
(e.g., Liu et al. 2009). Understanding the relationship between cold tolerance and
these additional factors will undoubtedly continue to further enhance the accuracy and
ultimate utility of pest risk mapping tools.

Acknowledgements

We thank Glen Fowler, Tracy Twine, and Tim Kurtti for comments on an earlier draft.
We are also grateful for the assistance of Erica Nystrom, Laurel Mosca, Alex Sloane,
and Kiley Friedrich in data collection, and to the MAES/MDA BSL-2 staff. This pro-
ject was supported in part by a U.S. National Science Foundation (NSF)-Integrative
Graduate Education and Research Traineeship Fellowship and the USDA Forest Ser-
vice. We also thank NSF-Research Coordination Networks-FORECAST for funding
A.C. Morey’s participation in the sixth annual meeting of the International Pest Risk
Mapping Workgroup.

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