Mechanisms and Consequences of Partial Migration in Insects

Abstract

Partial migration, where a proportion of a population migrates, while other individuals remain resident, is widespread across most migratory lineages. However, the mechanisms driving individual differences in migratory tendency are still relatively poorly understood in most taxa, but may be influenced by morphological, physiological, and behavioral traits, controlled by phenotypic plasticity and the underlying genetic complex. Insects differ from vertebrates in that partial migration is often associated with pronounced morphological differences between migratory and resident phenotypes, such as wing presence or length. In contrast, the mechanisms influencing migratory tendency in wing-monomorphic insects is less clear. Insects are the most abundant and diverse group of terrestrial migrants, with trillions of animals moving across the globe annually, and understanding the drivers and extent of partial migration across populations will have considerable implications for ecosystem services, such as the management of pests and the conservation of threatened or beneficial species. Here, we present an overview of our current but incomplete knowledge of partial migration in insects. We discuss the factors that lead to the maintenance of partial migration within populations, and the conditions that may influence individual decision making, particularly in the context of individual fitness and reproductive tradeoffs. Finally, we highlight current gaps in knowledge and areas of future research that should prove fruitful in understanding the ecological and evolutionary drivers, and consequences of partial migration in insects.

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MINI REVIEW
published: 24 October 2019
doi: 10.3389/fevo.2019.00403
Frontiers in Ecology and Evolution | www.frontiersin.org 1October 2019 | Volume 7 | Article 403
Edited by:
Brett K. Sandercock,
Norwegian Institute for Nature
Research (NINA), Norway
Reviewed by:
Stephen Baillie Malcolm,
Western Michigan University,
United States
Michael T. Hallworth,
Northeast Climate Adaptation Science
Center, United States
*Correspondence:
Myles H. M. Menz
mmenz@ab.mpg.de
Specialty section:
This article was submitted to
Behavioral and Evolutionary Ecology,
a section of the journal
Frontiers in Ecology and Evolution
Received: 29 May 2019
Accepted: 08 October 2019
Published: 24 October 2019
Citation:
Menz MHM, Reynolds DR, Gao B,
Hu G, Chapman JW and Wotton KR
(2019) Mechanisms and
Consequences of Partial Migration in
Insects. Front. Ecol. Evol. 7:403.
doi: 10.3389/fevo.2019.00403
Mechanisms and Consequences of
Partial Migration in Insects
Myles H. M. Menz 1,2,3, 4
*, Don R. Reynolds 5,6 , Boya Gao 7,8, Gao Hu 8, Jason W. Chapman 7,8, 9
and Karl R. Wotton 7
1Department of Migration, Max Planck Institute of Animal Behavior, Radolfzell, Germany, 2Centre for the Advanced Study of
Collective Behaviour, University of Konstanz, Konstanz, Germany, 3Department of Biology, University of Konstanz, Konstanz,
Germany, 4School of Biological Sciences, The University of Western Australia, Crawley, WA, Australia, 5Natural Resources
Institute, University of Greenwich, Chatham, United Kingdom, 6Rothamsted Research, Harpenden, United Kingdom,
7Centre for Ecology and Conservation, University of Exeter, Cornwall Campus, Penryn, United Kingdom, 8College of Plant
Protection, Nanjing Agricultural University, Nanjing, China, 9Environment and Sustainability Institute, University of Exeter,
Cornwall Campus, Penryn, United Kingdom
Partial migration, where a proportion of a population migrates, while other individuals
remain resident, is widespread across most migratory lineages. However, the
mechanisms driving individual differences in migratory tendency are still relatively poorly
understood in most taxa, but may be influenced by morphological, physiological,
and behavioral traits, controlled by phenotypic plasticity and the underlying genetic
complex. Insects differ from vertebrates in that partial migration is often associated
with pronounced morphological differences between migratory and resident phenotypes,
such as wing presence or length. In contrast, the mechanisms influencing migratory
tendency in wing-monomorphic insects is less clear. Insects are the most abundant and
diverse group of terrestrial migrants, with trillions of animals moving across the globe
annually, and understanding the drivers and extent of partial migration across populations
will have considerable implications for ecosystem services, such as the management of
pests and the conservation of threatened or beneficial species. Here, we present an
overview of our current but incomplete knowledge of partial migration in insects. We
discuss the factors that lead to the maintenance of partial migration within populations,
and the conditions that may influence individual decision making, particularly in the
context of individual fitness and reproductive tradeoffs. Finally, we highlight current gaps
in knowledge and areas of future research that should prove fruitful in understanding the
ecological and evolutionary drivers, and consequences of partial migration in insects.
Keywords: animal migration, flight capacity, intraspecific variation, insect migration, migratory potential,
movement ecology, wing polymorphism
INTRODUCTION
Vast numbers of animals migrate seasonally across large geographic scales, usually due to shifts in
resource availability—indeed, the importance of habitat ephemerality as a primary driver of insect
migration has long been recognized (Southwood, 1962; Denno et al., 1991; Dingle, 2014)—and also
in response to increased predation, parasitism and pathogen pressure (Altizer et al., 2011; Chapman
et al., 2015). Migrants connect habitats and populations through their annual movements, but also
have profound effects on ecosystem processes such as nutrient fluxes and the provision of ecosystem

Menz et al. Partial Migration in Insects
services (Bauer and Hoye, 2014; Bauer et al., 2017; Wotton
et al., 2019). There is no universally accepted definition of
migration, and many authors take a restricted “vertebrate-
centric” view and define migration as round-trip movements
between discrete “breeding” and “non-breeding” locations, which
inevitably excludes most insect examples from this definition.
In our review, we adopt a broader view of migration, based
on the behavioral definition of Kennedy and Dingle, defined
as any movements which are persistent and straightened-out,
and characterized by some (temporary) inhibition of behaviors
associated with feeding or reproduction (Dingle, 1996, 2014;
Dingle and Drake, 2007; Chapman and Drake, 2019). The
function of migratory movements is, of course, spatial relocation,
but this shift to new habitats is best viewed as a population-level
outcome of the individual behaviors. In other words, migration is
defined as a behavioral process, with the consequences explained
at the ecological or evolutionary level. Other movement ecology
researchers might categorize some of the examples we provide in
our review as dispersal instead of migration, but we adopt this
broad view in order to discuss insect examples in the context of
the established framework for partial migration.
“Partial migration,” whereby part of a population remains
resident while the rest migrates, is a common phenomenon
among migratory species (Lack, 1943; Lundberg, 1988; Dingle,
1996, 2014; Chapman et al., 2011; Kokko, 2011; Shaw and Levin,
2011), and has been reported from a wide range of taxa such
as fish (Chapman et al., 2012), birds (Nilsson et al., 2011), and
mammals (Mysterud et al., 2011; Berg et al., 2019). However,
the term has been little used in studies of insects and other
invertebrates (but see Hansson and Hylander, 2009; Attisano
et al., 2013; Slager and Malcolm, 2015; Dällenbach et al., 2018;
Ruiz Vargas et al., 2018; Vander Zanden et al., 2018). Partial
migration arises through intra-population variation in migratory
tendency, may be driven by physiological, morphological, or
behavioral variation (Chapman et al., 2011), and has been
proposed to be an early evolutionary stage in the transition
to full migration (Berthold, 2001) but, in insects, it could also
mark a reversion to residency. Frequency distributions of insect
flight duration are often sharply skewed, with short flights
significantly more common than long flights (Davis, 1980).
Therefore, if short migratory flights become adaptive because
overwintering in situ in temperate areas becomes favorable due
to warming conditions, short fliers could swiftly replace long-
distance migrants in the population. Changes in the frequency
of morphs indicates that there must be strong selection for long-
distance insect migration to be maintained in the face of the
higher mortality rates, physiological costs, and delays to breeding
associated with migration (Roff and Fairbairn, 1991; Zera and
Denno, 1997; Fox and Dennis, 2010; Bonte et al., 2012; Chapman
et al., 2015).
The mechanisms influencing the incidence of partial
migration within populations are not well-understood. Three
types of partial migration are often recognized in the literature,
“breeding,” where a population remains together during
the non-breeding season, but migrants and residents breed
separately, “non-breeding” where a population breeds in the
same habitat, but migrants and residents spend the non-breeding
season separately and “skipped-breeding” where a population
spends the non-breeding season in one location, but part of
the population remains and does not breed, while the other
migrates to breed (Chapman et al., 2011; Shaw and Levin, 2011;
Dingle, 2014). However, these definitions are based on organisms
with separate breeding and non-breeding areas, which is often
inapplicable to migratory insects, many of which continuously
breed year-round with several generations required to complete
the migratory cycle (Flockhart et al., 2013; Stefanescu et al., 2013;
Chapman et al., 2015). Furthermore, in contrast to vertebrates,
migratory insects can show extreme morphological variation
between generations, with the production of macropterous
morphs, which are long-winged and can undertake migratory
flights, brachypterous or micropterous morphs which are
short-winged and sedentary, and apterous morphs which are
wingless. Short-winged and wingless morphs are unable to
migrate and are hereafter referred to collectively as short-winged
forms (Johnson, 1969; Roff and Fairbairn, 1991, 2007; Gatehouse
and Zhang, 1995; Zera and Denno, 1997; Dingle, 2014). In other
cases, the ability to migrate may depend on traits other than
wing-length, such as size of the flight muscles or fuel reserves.
Thus, whether an individual is migratory or not may come from
a “decision” based upon the context in which it finds itself or
be pre-determined, for example maternally, as can occur in
Hemiptera (Gatehouse, 1994; Vellichirammal et al., 2017).
Here we present an overview of what is known about
the incidence and maintenance of partial migration, which
is widespread in insects. We contrast the phenomenon in
insects and vertebrates, and examine the current terminology
used to define the types of partial migration. Knowledge
gaps, and fruitful areas for future research, are highlighted.
Finally, we argue that insects, with their developmental
plasticity and short generation times, provide excellent
subjects for investigating the mechanisms that influence
migratory decisions.
PARTIAL MIGRATION IN INSECTS
Insect immature stages (eggs, larvae, nymphs, and pupae)
are typically comparatively sedentary compared to adults,
so inter-individual differences in migration propensity are
generally a feature of the adult stage. Partial migration has
been described in a number of insect species from a broad
range of orders, such as Hemiptera, Orthoptera, Lepidoptera,
Diptera, and Odonata (Figure 1), but much of the work
on variation in migratory potential has focused on wing-
dimorphic hemipterans (Johnson, 1969; Gatehouse and
Zhang, 1995; Zera and Denno, 1997; Roff and Fairbairn,
2007; Dingle, 2014). In all cases, it is assumed that an
individual will either migrate or remain more-or-less
sedentary in one or another life stage in order to increase
its overall fitness.
In contrast to most vertebrates, migrant insects are relatively
short-lived and usually undergo multiple generations within
a year (Chapman et al., 2015). Consequently, defining partial
migration into the three main types developed primarily for
vertebrates (Chapman et al., 2011; Shaw and Levin, 2011)
is inappropriate for insects, particularly due to their short
generation times. Some authors have adapted the existing
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Menz et al. Partial Migration in Insects
FIGURE 1 | Examples of insect species where partial migration has been studied. In all cases presented here, the migratory cycle consists of a number of generations
annually and the proportion of migrants and non-migrants may change between generations. Images: N. lugens, Y. He; O. fasciatus, J. Gallagher (CC BY 2.0); Gryllus
firmus, D. Roff; D. erippus, G. Ruellan (CC BY 3.0); E. balteatus, W. Hawkes; A. junius, M. Ostrowski (CC BY-SA 2.0).
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Menz et al. Partial Migration in Insects
FIGURE 2 | Partial migration in insects. (A) Migrants (colored purple) enter a
breeding ground from a previous breeding, overwintering, or aestivation
ground. In a northern temperate system this ground may be at lower latitudes.
The migrants oviposit creating generation 1 that consists of a varying degree
of migrants (purple) and non-migrants (green) depending on the conditions
encountered (photoperiod, temperature, resources, population density).
Non-migrants act as resident breeders (for example summer generations of
monarch butterflies and migratory hoverflies or various flightless morphs of
polymorphic species), producing additional generations in the breeding ground
that may also consist of varying amounts of migrants or non-migrants. In
contrast, the migrants move away from the breeding area becoming
temporally separated from the non-migrants, a situation termed sequential
partial migration (Ruiz Vargas et al., 2018). (B) Migration to breeding grounds
(Class I; Johnson, 1969). Migrants may enter a second breeding ground and
the process depicted in (A) continues (and may do so over multiple additional
areas). Separate breeding grounds may vary through latitudinal or altitudinal
and seasonal clines, and the relative fitness of each morph may vary between
successive areas depending on conditions. Migration with multiple phases
typically consists of relatively short-lived insects, or morphs of a particular
species. Continuously breeding species such as the painted lady butterfly may
cycle through this system, while other species may only undertake part of it,
for example, aphids, planthoppers and spring migrations of hoverflies (see text
for other examples). (C) In some cases, a species may switch to migration to
overwintering or aestivation grounds (Class III; Johnson, 1969). Insects with
long-distance migration are often relatively long lived, examples include
autumn morphs of migrant hoverflies, monarch butterflies and bogong moths.
However, overwintering may also take place within the breeding grounds
without migration (colored black), such as for migratory hoverflies and the
green darner dragonfly. Typically, migration or breeding continues again in the
spring.
definitions to suit insects, coining terms such as “sequential
partial migration,” where migratory and non-migratory animals
are separated temporally, rather than spatially (Ruiz Vargas
et al., 2018), or “alternate migration,” to reflect that some
migratory individuals switch from a migratory to a non-
migratory strategy upon encountering a resident population
(Vander Zanden et al., 2018). In many cases, sequential
partial migration appears apt, as the proportion of migratory
and non-migratory individuals change between generations
and this definition reflects the multi-generational aspect of
insect migration (Figure 2). Broad definitions, such as that a
population with 1–99% migrants can be considered as partially
migratory (Chapman et al., 2011), will obviously promote
the inclusion of insect taxa. A number of hypotheses have
been raised to understand the mechanisms driving individual
variability in migratory tendency, and these are discussed
further below.
Morphological Variation Between Migrants
and Non-migrants
In comparison to vertebrates, insects can show extreme
wing polymorphisms between migratory and non-migratory
phenotypes. Consequently, partial migration in insects
needs to be considered in terms of the contrast between
wing-monomorphic and wing-polymorphic species, as
there are likely to be different mechanisms and selection
pressures acting on these two fundamentally different
types. As most work on the trade-offs between migration
and residency has been conducted on wing-polymorphic
species, comparing migratory and sedentary phenotypes
in wing-monomorphic insects may prove useful for
elucidating the underlying mechanisms, but such studies are
rare (Tigreros and Davidowitz, 2019).
In birds, there are many examples of differences in body
size between migrants and residents, with the latter often being
larger, possibly due to larger-bodied individuals having a greater
physiological tolerance to overwintering (Ketterson and Nolan,
1976; Belthoff and Gauthreaux, 1991) or the ability to endure
periods of low resource availability (Boyle, 2008; Jahn et al., 2010;
Chapman et al., 2011). In insects, migrants are often larger than
non-migrants (Roff and Fairbairn, 2007), a pattern that has been
demonstrated for wing-dimorphic species such as the milkweed
bug (Oncopeltus fasciatus) (Hegmann and Dingle, 1982), and
gerrid (water-strider) bugs (Fairbairn, 1992), as well as wing-
monomorphic species (Altizer and Davis, 2010). Differences in
wing loading and morphology have also been reported between
migratory and non-migratory monarch (Danaus plexippus) and
southern monarch (D. erippus) butterflies, with migrants having
larger, more pointed wings and higher wing loads than residents
(Dockx, 2007; Altizer and Davis, 2010; Slager and Malcolm, 2015;
Vander Zanden et al., 2018), which should result in more fuel-
efficient flight (Roff and Fairbairn, 1991; Rankin and Burchsted,
1992). Interestingly, no differences in wing morphology were
reported between overwintering adults and migrants of the
marmalade hoverfly (Episyrphus balteatus;Raymond et al.,
2014b). There was also no difference in resting metabolic rate
between sexes in E. balteatus, but the smaller females were shown
to have higher evaporative water loss than the larger males
(Tomlinson and Menz, 2015).
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Menz et al. Partial Migration in Insects
Reproduction or Migration?
The costs of migration in relation to reproductive fitness differ
between the sexes such that some authors consider that males
and females should be considered separately (Johnson, 1969;
Gatehouse and Zhang, 1995); here we primarily discuss the
relationship as it relates to females. Insect migration is often
considered in the context of the “oogenesis-flight syndrome,”
which posits a trade-off between migration and reproduction
(Johnson, 1969; Gatehouse and Zhang, 1995; Dingle, 1996).
Development of flight muscles, and migratory flight itself, are
energetically costly (Dudley, 1995; Dingle, 2014) and, whereas
non-migrants can immediately allocate resources to breeding,
migrating individuals will often spend time in reproductive
diapause (Johnson, 1969; Rankin and Burchsted, 1992).
Migration often occurs pre-reproductively (Gatehouse, 1994;
Gatehouse and Zhang, 1995), with reproductive maturity being
linked to the cessation of migration, or even the termination
of diapause following a period of aestivation or overwintering
(Johnson, 1969). However, there is sometimes a more nuanced
relationship between reproduction and development of the
flight apparatus in wing-monomorphic insects (Rankin et al.,
1986; Sappington and Showers, 1992), with some species even
migrating with fully-developed oocytes (May et al., 2017;
Tigreros and Davidowitz, 2019).
The trade-off between migration and reproduction can be
modulated by resource availability in both wing-monomorphic
and dimorphic species (Roff and Fairbairn, 2007; Ruiz Vargas
et al., 2018). In wing-dimorphic species, the production
of macropterous individuals is often determined in early
developmental stages or even maternally (Gatehouse, 1994;
Wilson, 1995; Ogawa and Miura, 2014; Vellichirammal et al.,
2017). Host quality strongly influences wing-morph in brown
planthoppers (Nilaparvata lugens); upon colonization of a new
resource patch, there is an increased proportion of short-winged
individuals, which are unable to migrate but have a greater
reproductive potential than the macropterous morph (Lin et al.,
2018). As the rice crop matures there is an increase in the
proportion of the macropterous form, which can migrate to
colonize new rice fields, but the proportion of long-winged
individuals within a population can vary between seasons and
years (Hu et al., 2017). In aphids, the production of winged
morphs may be influenced by environmental conditions such as
crowding, decreasing food quality, or the presence of predators
(Müller et al., 2001). In wing-monomorphic species, or in
long-winged individuals of dimorphic species, the ability to
respond to changes in resource availability and switch between a
migratory and non-migratory state or vice versa may be driven
by differences in physiology, such as the ability to reallocate
nutrients from flight to reproduction. Indeed, Attisano et al.
(2013) demonstrated that resident female milkweed bugs showed
a higher level of oosorption (where females resorb nutrients
from developing oocytes thus favoring survival over current
reproduction) than did migrants.
Density Dependence
It has been predicted that an increased proportion of migrants
should occur in populations at higher densities (Chapman
et al., 2011). In insects, partial migration may allow individuals
that move to breed to avoid the negative consequences of
resource competition (Taylor and Taylor, 1983; Dingle, 1996). For
example, in the planthoppers N. lugens and Sogatella furcifera,
an increased proportion of long-winged individuals may be
produced at high densities (Matsumura, 1996; Lin et al., 2018).
Similarly, crowding can promote the production of winged
offspring in aphids (Johnson, 1969; Müller et al., 2001). The lower
fecundity typically found in winged forms typically is an example
of the tradeoff between the colonization of new habitats and
reproductive output.
Predation and Parasitism Risk
Partial migration may confer some reduction in the risk of
predation or parasitism, by movement into an enemy free space,
resulting in improved survival for migrants. However, the role
of trophic interactions has received relatively little attention
in the partial migration literature (Chapman et al., 2011) and
has rarely been studied in migratory insects (Altizer et al.,
2011; Chapman et al., 2015). Nonetheless, there is evidence
that migration can reduce the prevalence of infection from
the protozoan parasite, Ophryocystis elektroscirrha in monarchs
(Bartel et al., 2011; Altizer et al., 2015; Flockhart et al., 2018), with
resident populations having higher infection rates than migrant
populations (Satterfield et al., 2015, 2016, 2018), providing
evidence of “migratory escape” (Altizer et al., 2011) from
contaminated environments.
The Evolution, Expression, and
Maintenance of Partial Migration
Migratory flight tendency has been shown to be heritable in
a broad range of insect species, indicating a strong genetic
component to migratory behavior (Wilson, 1995; Dingle, 1996,
2014; May et al., 2017; Dällenbach et al., 2018). The capacity
of insects to form migrants or non-migrants from within the
same population could potentially be determined by genetic
polymorphisms, for example alleles that influence flight or timing
(Niitepõld et al., 2009; Hut et al., 2013; Zhan et al., 2014) and/or
the expression of environmentally-induced phenotypic plasticity.
While evidence for a solely genetically determined difference
is lacking for partial migration, phenotypically plastic pathways
are a widespread feature of insect life histories (Nijhout, 1999)
and are likely to provide the predominant mechanisms allowing
migrants to switch forms, an idea strengthened by the low level
of genetic differentiation and phylogeographic structuring found
within many partial migrant populations (Mun et al., 1999;
Freeland et al., 2003; Raymond et al., 2013; Zhan et al., 2014).
How discrete migratory states within a population are
maintained is unclear, but two hypotheses have been proposed
(Chapman et al., 2011). One possibility is the attainment of
an evolutionary stable state, where the fitness of each form is
balanced by frequency-dependent selection. For example, in wing
dimorphic insects where the more fecund flightless form is
balanced by the colonizing abilities of the migrant morph (Roff,
1994; Zera and Denno, 1997). Alternatively, the fitness benefits of
either morph may occur as a result of conditional strategies, were
the decision to migrate is based upon gaining the highest fitness
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Menz et al. Partial Migration in Insects
possible under certain circumstances and a balancing of fitness
is not necessary (Chapman et al., 2011). The generally short life
span of insect migrants and their higher reliance on favorable
meteorological conditions for migration (Alerstam et al., 2011;
Hu et al., 2016) highlights the importance for selecting the
optimal strategy in any given situation. Migratory hoverflies,
such as E. balteatus, for example, may migrate south to warmer
climes (Wotton et al., 2019) but are also capable of sedentary
overwintering behavior as adults, larvae, or pupae (Raymond
et al., 2014a), an adaptation that presumably increases their
fitness over attempting to migrate in unfavorable conditions (also
see Vander Zanden et al., 2018).
The inheritance and phenotypic expression of migratory states
has been investigated in both wing polymorphic (Fairbairn and
Yadlowski, 1997; Roff et al., 1997) and monomorphic (Kent
and Rankin, 2001) insects and interpreted in the context of
the “threshold model”: a quantitative genetic model for the
evolution of polygenic, dichotomous traits (Roff, 1996). Under
this model, a normally distributed trait, called the liability,
underlies the expression of the migratory dimorphism and a
threshold determines the developmental trajectory—in this case
migrant or non-migrant. If the liability exceeds the threshold
then the individual takes one path, say migration, if not it
becomes sedentary. In the case of wing polymorphism, it is
hypothesized that the liability for wing production may be
governed by hormone profiles at a particular larval stage: in
larvae where levels exceed the threshold (conceivably controlled
by levels of hormone receptors among other factors) the
flightless morph is formed (Oostra et al., 2011; Roff, 2011).
An additional consideration is that threshold traits also vary
with environmental factors such as temperature, photoperiod,
and density (Hondelmann and Poehling, 2007; Guerra and
Reppert, 2013). A more realistic model—the environmental
threshold model—allows for both genetic variation, and for
individual or environmental conditions to modify the threshold
and the liability (Roff, 1994; Wikelski et al., 2006; Hallworth
et al., 2018; see Pulido, 2011 for a full consideration of
the model and its implications for partial migration) and
therefore has the potential to provide a comprehensive
framework for a deeper understanding of partial migration
in insects.
Ecological Implications of Partial Migration
in Insects
Insects are the most abundant and speciose terrestrial migrants,
with trillions of individuals undertaking movements annually
(Holland et al., 2006; Chapman et al., 2015; Hu et al., 2016).
Additionally, many migratory insect species are important
agricultural pests (Drake and Gatehouse, 1995), or are
beneficial—as pollinators or natural enemies (Wotton et al.,
2019) or as food for other animals (Krauel et al., 2015; Warrant
et al., 2016). Consequently, understanding the incidence and
mechanisms involved in the regulation of partial migration in
insect populations has significant implications for ecosystem
functioning and species management. Models based on predator-
prey dynamics and interactions with environmental conditions
have been developed to study the ecosystem effects of partial
migration in fish (Brodersen et al., 2008, 2011), and similar
approaches may be considered for insects, particularly in the
context of nutrient transfer between trophic levels and across
landscapes. Furthermore, understanding the factors influencing
the level of migration within populations may allow for the
implementation of more realistic species management strategies.
Future Directions and Gaps in Knowledge
Despite the deficiency of research investigating the mechanisms
driving partial migration in insects, the phenomenon evidently
occurs in numerous species, and there are exciting opportunities
for research into the evolution and ecology of the phenomenon.
Insects are excellent model systems; they are relatively small,
easily maintained, and can be manipulated in a laboratory
environment. The opportunity for identifying new partial
migration study systems will be facilitated by the huge diversity
of migratory insect species and their broad range of life histories.
Little is known about the influence of anthropogenic
landscape change on partial migration in insect populations.
There is evidence that landscape alterations can readily lead
to an increase in the propensity for residency in migratory
insects, usually in response to favorable conditions, such as the
availability of food resources. For example, increased planting
of tropical milkweed (Asclepias curassavica) in Florida has led
to an increase in residency in monarchs, but residents suffer
from increased parasitism compared to migrants (Satterfield
et al., 2015). Urbanization can also increase the propensity for
residency or overwintering through the provision of winter
refugia or foraging resources, such as garden flowers. Luder
et al. (2018) demonstrated that migratory hoverflies appeared
earlier in the season in urban areas compared to agricultural
areas, indicating that cities may provide favorable conditions
for overwintering. Warming temperatures have also led to an
increase in overwintering of migratory species in the UK,
such as the red admiral butterfly (Vanessa atalanta), although
much of the population still immigrates to the UK each
spring (Sparks et al., 2005; Fox and Dennis, 2010). Fairly
simple laboratory experiments could be used to shed light
on whether warming or constant temperatures, or increased
food constancy, influences the migratory propensity in wing-
monomorphic insects.
Tracking the migratory behavior of insects in the field is
difficult, primarily due to their small size and sheer numbers.
Individual tracking of insects to determine migratory decisions
has been hindered because the majority of species fall well below
the body size required to carry active transmitters (Wikelski et al.,
2006; Kissling et al., 2014; Knight et al., 2019). Consequently,
many studies investigating insect migratory behavior which
may be relevant to partial migration have been conducted in
the laboratory, using proxy measures for migratory potential,
such as flight duration and activity (Minter et al., 2018).
Tethered flight experiments have proven useful for determining
migratory tendency in a range of insect species (Dällenbach
et al., 2018; Minter et al., 2018; Naranjo, 2019). However, the
further miniaturization of individual tracking technology will
provide exciting opportunities to understand the drivers of
partial migration and the mechanisms that influence individual
decision-making. The use of intrinsic markers, such as stable
isotopes, has proven useful for elucidating the origin of migratory
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Menz et al. Partial Migration in Insects
insects (Hobson et al., 2012; Flockhart et al., 2013; Hallworth
et al., 2018) and is applicable to a range of species. Recent
advances in molecular techniques, including metabarcoding of
pollen carried on the bodies of insects also shows great promise
(Suchan et al., 2019). Techniques using intrinsic markers, where
the utility is not limited by the size of the insect, will likely
prove key in understanding patterns of partial migration in many
insect taxa.
AUTHOR CONTRIBUTIONS
MM and KW wrote the first draft of the manuscript. All
authors contributed to the development of ideas and writing of
the manuscript.
FUNDING
This project has received funding from the European
Union’s Horizon 2020 research and innovation programme
under the Marie Skłodowska-Curie Grant Agreement
No. 795568 (awarded to MM). Funding to KW was
provided by the Royal Society through a University
Research Fellowship (UF150126). Rothamsted Research
receives grant-aided support from the United Kingdom
Biotechnology and Biological Sciences Research Council
(BBSRC). Funding to GH and BG were provided by
the National Natural Science Foundation of China
(31822043) and the Natural Science Foundation of Jiangsu
Province (BK20170026).
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Conflict of Interest: The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could be construed as a
potential conflict of interest.
Copyright © 2019 Menz, Reynolds, Gao, Hu, Chapman and Wotton. This is an
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Frontiers in Ecology and Evolution | www.frontiersin.org 9October 2019 | Volume 7 | Article 403