Introduction
The niche breadth of polyphagous insect pests can be expansive due to a
number of biological and abiotic factors including the ability to
exploit diverse host types in heterogenous environments and the capacity
to respond to changing conditions over time (Kennedy & Storer 2000,
Sakai et al. 2001, Little et al. 2020). Species with broad host ranges
also tend to have an outsized impact on crops as compared to monophagous
or oligophagous insect species (Ward and Spalding 1993). It is
reasonable to assume these substantial impacts may also occur in
non-crop areas. Considerable research has been conducted in semi-natural
lands adjacent to or near affected crops, as these are the areas thought
to be highly influential to agroecosystem dynamics (Kennedy and Storer
2000, Rand et al. 2006, Mazzi and Dorn 2012).
For polyphagous pests, non-crop host plants occur throughout the
landscape, including places far removed from agriculture. These areas,
such as forests, are rarely assessed for the presence of agricultural
insect pests unless they are also considered a forest pest, such as pear
thrips, Taeniothrips inconsequens (Uzel) (Teulon et al. 1998), or
the spotted lanternfly, Lycorma deliculata White (Barringer and
Ciafré 2020). Nevertheless, studying non-forest crop pests in remote
areas might be important for a number of reasons. First, these insects
may impact forest food web dynamics through resource use competition of
common host plants. Second, if these remote locations host established
populations of agricultural pests, then they may be a source for
seasonal migrants into cultivated areas. Third, understanding pest
behavior outside of the agroecosystem may yield novel insights into pest
behavior and ecology that may not be evident in highly human-influenced
agricultural areas. Finally, such insights can then be used to improve
modeling predictions for current and future range expansions.
Distribution modeling is a common way to model invasive species and is
based on known life history traits and occurrence. However, ground
truthing to inform or verify model-based inferences (e.g., likelihood of
occurrence or density) often fails to venture outside of areas where the
pest is causing direct economic damage, namely cropland in this case.
Failure to fully verify these distribution models limits their
usefulness and insight (Wright et al. 2006, Fitzpatrick et al. 2007,
Sarquis et al. 2018). Furthermore, niche divergence may occur more
readily in areas with more diverse habitat and can be indicative of
ecological changes such as invasive species establishment, food web
disruption, or climate change (Wright et al. 2006).
Drosophila suzukii (Matsumura) is a highly cosmopolitan
agricultural pest of berry crops. Native to East Asia, D. suzukiiwas limited in spread until 2008 when accidental introductions led to a
range expansion into Europe and continental North America, and in
subsequent years to South America, Western Asia and most recently in
Africa (Hauser et al. 2011, Calabria et al. 2012, Deprá et al. 2014,
Parchami-Araghi et al. 2015, Hassani et al. 2020). Ripe fruit from
cultivated berry crops and wild-growing native and non-native plant
species serve as oviposition sites for female D. suzukii and
nutritional resources for all life stages. The presence and movement of
this fly has been well-studied in croplands and nearby disturbed or
wooded areas that serve as potential refuge sites and often contain
susceptible hosts (Bellamy et al. 2013, Lee et al. 2015, Klick et al.
2016, Elsensohn and Loeb 2018, Santoiemma et al. 2018). Some host plant
species are regionally common and can be found well outside
agroecosystems, including in backyards, roadsides, woods, and fields,
among other places (e.g., Mitsui et al. 2010, Poyet et al. 2014, Ballman
and Drummond 2017).
Several ecological models were created to assess the current and future
distribution of D. suzukii around the world (Gutierrez et al.
2016, dos Santos et al. 2017, Fraimout and Monnet 2018, de la Vega and
Corley 2019, Ørsted and Ørsted 2019). One species distribution model
using global occurrence data indicated a higher likelihood of occurrence
in the southern Appalachian Mountains of the eastern United States than
in surrounding areas (Ørsted and Ørsted 2019). Contrastingly, a
physiologically-based demographic model estimated a lower D.
suzukii density in the same area (Gutierrez et al. 2016). Much of this
region of the Appalachian Mountains, which ranges in elevation from
900-1850 m, is designated as federally protected National Forest land.
No commercial plantings of cultivated D. suzukii hosts are known
to occur within this area, although D. suzukii -susceptibleVaccinium and Rubus spp. native to North America grow well
here (Powell and Searman 1990).
To date, no studies have sought to ground truth model-predicted
occurrence sites sparsely populated by humans as potential population
sources of D. suzukii . In Europe, altitudinal studies
demonstrated an established presence of D. suzukii in high
elevation, mountainous locations (Tait et al. 2018, Santoiemma et al.
2019), and documented recapturing marked adults over 9 km from the
release site (Tait et al. 2018). This distance is suggestive of
weather-assisted movement, as flight mill tests show the flight capacity
of adults is less than 2 km (Wong et al. 2018). Insect dispersal through
wind patterns is documented in several pest species (Hoelscher 1967,
Compton 2002, Moser et al. 2009) and has been postulated as a potential
means of yearly recolonization by D. suzukii at northern U.S.
latitudes after winter temperatures kill the vast majority of
overwintering flies (Rossi-Stacconi et al. 2016, Panel et al. 2018,
Wallingford et al. 2018). Localized D. suzukii movement from
shrubby or wooded landscapes into crop fields is well documented (Klick
et al. 2016, Pelton et al. 2016, Tonina et al. 2018). Uncultivated and
cultivated areas can be exploited concurrently or consecutively
throughout the year, especially in areas where adults are caught
year-round (Ballman and Drummond 2017, Elsensohn and Loeb 2018,
Santoiemma et al. 2018). Uncultivated areas can enlarge or sustain pest
populations that could spill back into crop areas through short or
long-distance movement, and vice versa.
Some non-crop hosts may be preferred oviposition sites for femaleD. suzukii or offer better nutritional resources needed for
larval development. Comparative work exploring oviposition preference
between crop and non-crop host species found that preference depended on
the specific fruit combinations used (Lee et al. 2015, Diepenbrock et
al. 2016). The first direct comparison of D. suzukii oviposition
preference between wild and cultivated fruit of the same crop type found
that females laid more eggs into cultivated than wild blueberries
(Rodriguez-Saona et al. 2019). However, these results may be confounded
by differences in fruit size, weight, and surface area between
domesticated and wild relatives.
Laboratory research into oviposition preference as a factor of fruit
ripeness stage revealed that the ripe stage was the most preferred for
oviposition while progressively under-ripe stages received fewer or no
eggs (Lee et al. 2011, Kamiyama and Guédot 2019). In a field setting,
fewer adults emerged from blackberry fruit infested during under-ripe
stages than fruit infested later at the ripe stage (Swoboda-Bhattarai
and Burrack 2017). These results align with laboratory studies that show
a survival hierarchy with ripe fruit producing the lowest mortality rate
(Lee et al. 2011, Bernardi et al. 2017, Kamiyama and Guédot 2019).
To better understand D. suzukii oviposition preference and
general resource use in areas unaffected by spillover dynamics, we
conducted an elevational gradient study in the southern Appalachian
Mountain region. Here, modeling predictions are uncertain, but non-crop
hosts are common. Over two years, we visited three natural areas and one
roadside tract surrounded by National Forest lands in western North
Carolina to collect wild-growing fruit at different stages of ripeness.
The main objectives of this study were to: 1) document the presence or
absence of D. suzukii in the unpopulated areas; 2) compare
seasonal host use patterns of wild and cultivated blackberry fruits; and
3) assess host preference between wild and crop fruit in a laboratory
setting.