Photoperiodism and seasonality in neotropical population of Plutella xylostella L. (Lepidoptera: Yponomeutidae)

Neotropical populations of the diamondback moth Plutella xylostella L. have seasonal cycles of growth and decrease, and moth migration plays a fundamental role in generating such population dynamics. Since the oscillation of these populations is predictable, photoperiod might operate as a signal that triggers the migratory behaviour of the insect. Migration in insects is usually preceded by reproductive diapause, a photoperiodic response that can be characterised by morphological, physiological and behavioural alterations that permit to discriminate between migratory and non-migratory forms. In this study, I tested whether the pre-imaginal and reproductive development of P. xylostella from Minas Gerais (Brazil) is affected by artificial day-lengths that are equivalent to the periods of natural population growth or decrease. No evidence of photoperiodic response was found for the insect reared in laboratory on five different constant photoperiods, from 8h to 16h of light per day. There was no significant variation in survival and duration of egg, larva, and pupa stages or in pupal weight, adult size (forewing length), fecundity, and longevity. Although some species have geographically distinct photoperiodic responses, previous assumptions that cosmopolitan P. xylostella responds to photoperiod in temperate regions was questioned. Migratory and population seasonality among neotropical populations of P. xylostella certainly occurs independently of the photoperiodic announcement of seasonal changes in habitat quality.     

Wing size but not wing shape is related to migratory behavior in a soaring bird

Both wing size and wing shape affect the flight abilities of birds. Intra and inter‐specific studies have revealed a pattern where high aspect ratio and low wing loading favour migratory behaviour. This, however, have not been studied in soaring migrants. We assessed the relationship between the wing size and shape and the characteristics of the migratory habits of the turkey vulture Cathartes aura, an obligate soaring migrant. We compared wing size and shape with migration strategy among three fully migratory, one partially migratory and one non‐migratory (resident) population distributed across the American continent. We calculated the aspect ratio and wing loading using wing tracings to characterize the wing morphology. We used satellite‐tracking data from the migratory populations to calculate distance, duration, speed and altitude during migration. Wing loading, but not aspect ratio, differed among the populations, segregating the resident population from the completely migratory ones. Unlike what has been reported in species using flapping flight during migration, the migratory flight parameters of turkey vultures were not related to the aspect ratio. By contrast, wing loading was related to most flight parameters. Birds with lower wing loading flew farther, faster, and higher during their longer journeys. Our results suggest that wing morphology in this soaring species enables lower‐cost flight, through low wing‐loading, and that differences in the relative sizes of wings may increase extra savings during migration. The possibility that wing shape is influenced by foraging as well as migratory flight is discussed. We conclude that flight efficiency may be improved through different morphological adaptations in birds with different flight mechanisms.     

Migration in butterflies: a global overview

Insect populations including butterflies are declining worldwide, and they are becoming an urgent conservation priority in many regions. Understanding which butterfly species migrate is critical to planning for their conservation, because management actions for migrants need to be coordinated across time and space. Yet, while migration appears to be widespread among butterflies, its prevalence, as well as its taxonomic and geographic distribution are poorly understood. The study of insect migration is hampered by their small size and the difficulty of tracking individuals over long distances. Here we review the literature on migration in butterflies, one of the best-known insect groups. We find that nearly 600 butterfly species show evidence of migratory movements. Indeed, the rate of ‘discovery’ of migratory movements in butterflies suggests that many more species might in fact be migratory. Butterfly migration occurs across all families, in tropical as well as temperate taxa; Nymphalidae has more migratory species than any other family (275 species), and Pieridae has the highest proportion of migrants (13%; 133 species). Some 13 lines of evidence have been used to ascribe migration status in the literature, but only a single line of evidence is available for 92% of the migratory species identified, with four or more lines of evidence available for only 10 species – all from the Pieridae and Nymphalidae. Migratory butterflies occur worldwide, although the geographic distribution of migration in butterflies is poorly resolved, with most data so far coming from Europe, USA, and Australia. Migration is much more widespread in butterflies than previously realised – extending far beyond the well-known examples of the monarch Danaus plexippus and the painted lady Vanessa cardui – and actions to conserve butterflies and insects in general must account for the spatial dependencies introduced by migratory movements.     

Social transmission of migratory knowledge: quantifying the risk of losing migratory behavior

When migration is a learned behavior, small populations have a significant problem of maintaining migratory knowledge across generations. These populations risk losing migratory behavior entirely, which may exacerbate existing stressors on population size. Here we investigated the success of various behavioral, demographic, and social factors towards maintaining migration within small populations. Using a discrete-time probabilistic model to simulate repeated migrations, we found that migratory group size plays an important role in maintaining migratory knowledge within the population. Rare, large groups allow for migratory knowledge to be spread to many individuals at once. When a population learns migration information incrementally, the presence of individuals that can learn quickly, therefore transitioning rapidly into leaders, has a profound impact on migrational persistence. Furthermore, small populations are better able to maintain migratory behavior when groups rely on informed leaders as compared to using collective group knowledge, even when that collective knowledge is heavily weighted towards knowledgeable individuals. Finally, we found that both species with short lifespans and species that migrate with fixed group compositions are at especially high risk of losing their migration behavior at small population sizes.     

Genetic Structure and Demographic History Reveal Migration of the Diamondback Moth Plutella xylostella (Lepidoptera: Plutellidae) from the Southern to Northern Regions of China

The diamondback moth Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae) is one of the most destructive insect pests of cruciferous plants worldwide. Biological, ecological and genetic studies have indicated that this moth is migratory in many regions around the world. Although outbreaks of this pest occur annually in China and cause heavy damage, little is known concerning its migration. To better understand its migration pattern, we investigated the population genetic structure and demographic history of the diamondback moth by analyzing 27 geographical populations across China using four mitochondrial genes and nine microsatellite loci. The results showed that high haplotype diversity and low nucleotide diversity occurred in the diamondback moth populations, a finding that is typical for migratory species. No genetic differentiation among all populations and no correlation between genetic and geographical distance were found. However, pairwise analysis of the mitochondrial genes has indicated that populations from the southern region were more differentiated than those from the northern region. Gene flow analysis revealed that the effective number of migrants per generation into populations of the northern region is very high, whereas that into populations of the southern region is quite low. Neutrality testing, mismatch distribution and Bayesian Skyline Plot analyses based on mitochondrial genes all revealed that deviation from Hardy-Weinberg equilibrium and sudden expansion of the effective population size were present in populations from the northern region but not in those from the southern region. In conclusion, all our analyses strongly demonstrated that the diamondback moth migrates within China from the southern to northern regions with rare effective migration in the reverse direction. Our research provides a successful example of using population genetic approaches to resolve the seasonal migration of insects.     

Lack of genetic differentiation between monarch butterflies with divergent migration destinations

Monarch butterflies are best known for their spectacular annual migration from eastern North America to Mexico. Monarchs also occur in the North American states west of the Rocky Mountains, from where they fly shorter distances to the California Coast. Whether eastern and western North American monarchs form one genetic population or are genetically differentiated remains hotly debated, and resolution of this debate is essential to understand monarch migration patterns and to protect this iconic insect species. We studied the genetic structure of North American migratory monarch populations, as well as nonmigratory populations in Hawaii and New Zealand. Our results show that eastern and western migratory monarchs form one admixed population and that monarchs from Hawaii and New Zealand have genetically diverged from North American butterflies. These findings suggest that eastern and western monarch butterflies maintain their divergent migrations despite genetic mixing. The finding that eastern and western monarchs form one genetic population also suggests that the conservation of overwintering sites in Mexico is crucial for the protection of monarchs in both eastern and western North America.     

Migration costs drive convergence of threshold traits for migratory tactics

Partial migration of some, but not all, members of a population is a common form of migration. We evaluated how migration costs influence which members migrate in 10 populations of two salmonid species. The migratory patterns of both species were evaluated based on the size at maturity for resident males, which is the threshold trait that determines the migratory tactics used within a population. In both species, this size was smaller in males located further from the sea, where migration costs are presumably higher. Moreover, the threshold sizes at maturity in males were correlated between both species. Our results suggest that migration costs are a significant convergent selective force on migratory tactics and life-history traits in nature.     

Synchronized oviposition triggered by migratory flight intensifies larval outbreaks of beet webworm

Identifying the reproductive consequences of insect migration is critical to understanding its ecological and evolutionary significance. However, many empirical studies are seemingly contradictory, making recognition of unifying themes elusive and controversial. The beet webworm, Loxostege sticticalis L. is a long-range migratory pest of many crops in the northern temperate zone from 36 °N to 55 °N, with larval populations often exploding in regions receiving immigrants. In laboratory experiments, we examined (i) the reproductive costs of migratory flight by tethered flight, and (ii) the reproductive traits contributing to larval outbreaks of immigrant populations. Our results suggest that the beet webworm does not initiate migratory flight until the 2nd or 3rd night after emergence. Preoviposition period, lifetime fecundity, mating capacity, and egg hatch rate for adults that experienced prolonged flight after the 2nd night did not differ significantly from unflown moths, suggesting these traits are irrelevant to the severity of beet webworm outbreaks after migration. However, the period of first oviposition, a novel parameter developed in this paper measuring synchrony of first egg-laying by cohorts of post-migratory females, for moths flown on d 3 and 5 of adulthood was shorter than that of unflown moths, indicating a tightened time-window for onset of oviposition after migration. The resulting synchrony of egg-laying will serve to increase egg and subsequent larval densities. A dense population offers potential selective advantages to the individual larvae comprising it, whereas the effect from the human standpoint is intensification of damage by an outbreak population. The strategy of synchronized oviposition may be common in other migratory insect pests, such as locust and armyworm species, and warrants further study.     

昆虫迁飞随气流散布若干问题的商榷

昆虫通过飞翔从一个生境迁移到另一个新的生境,是普遍存在的自然现象,而随气流远距离季节性迁飞是在特定环境条件下产生的,是昆虫与环境在进化过程中的统一。对于迁飞的个体,“从A迁飞到B,并在B获得生境”是随机事件,但对于迁飞的群体则存在统计规律。迁飞昆虫以被动的随气流散布方式,获得对环境的主动适应。     

华北二点委夜蛾种群动态监测及北京北部地区虫源性质分析

二点委夜蛾Proxenus lepigone是玉米生产中的一种新发害虫, 2011年曾在黄淮海夏玉米主产区全面暴发, 对夏玉米生产构成严重威胁。为了明确二点委夜蛾的种群动态, 探讨北京北部二点委夜蛾种群是否为迁入虫源, 2012年, 在华北地区的河北省栾城县、 北京市区和北京延庆县等地, 利用高空探照灯诱虫器、 垂直监测昆虫雷达等对二点委夜蛾成虫进行了监测, 并结合气象资料, 对北京延庆的虫源性质进行了综合分析。结果表明： 二点委夜蛾老熟幼虫作茧后可在北京延庆越冬。2012年, 北京延庆诱集二点委夜蛾累计33 951头, 可划分为3个世代。第1代成虫的诱集数量不符合正态分布, 且在姊妹灯下的数量差异符合迁飞种类的特点。在当地条件不适宜的情况下, 北京延庆监测点第1代成虫日均诱虫数量高于条件相对适宜的河北栾城。在北京延庆, 第1代成虫的逐日诱集数量与空中风向密切相关。雷达监测还表明二点委夜蛾可能是雷达回波的目标。综合以上证据表明, 北京延庆第1代成虫包含从周边迁飞而来的个体, 二点委夜蛾可能是一种兼性迁飞昆虫。本研究可为二点委夜蛾成虫能否远距离迁飞提供例证, 对今后提高其预测预报和防治水平具有重要意义。     

Correlates of alternative migratory strategies in western bluebirds

Partial migration occurs when only some animals in a population migrate. While evidence suggests that migratory strategies are partially controlled by genes, individual and environmental conditions which alter the cost‐benefit trade‐off of migration among individuals are also likely to play a role. Three hypotheses have been advanced to explain condition‐dependent partial migration: the arrival time, dominance and body size hypotheses. In this study, we asked whether these hypotheses explained differences in migratory strategy among individuals in a partially migratory population of western bluebirds Sialia mexicana breeding in southern British Columbia, Canada. We used stable hydrogen isotope signatures in claw tissue to determine migratory strategy of individual bluebirds, and examined patterns of migration at both individual and population levels. The proportion of resident bluebirds varied significantly over the three years of the study, and across study sites. Several migrants switched to the resident strategy between years; however, we found no evidence of strategy switching in the opposite direction. Young birds were significantly more likely to be resident than older birds, a pattern which could arise if early arrival is particularly important for birds obtaining a territory for the first time. Furthermore, young females were the most likely of all sex–age classes to be resident, which may reflect a survival advantage of residency for young females. Finally, birds mated assortatively by migratory strategy and isotopic evidence suggests that members of a pair often wintered in the same place. Our results provided no support for the dominance or body size hypotheses, and only limited support for the arrival time hypothesis in bluebirds. However, taken together, we suggest that our findings indicate that social factors may influence migratory strategies in this system.     

Population dynamics in migratory networks

Migratory animals are comprised of a complex series of interconnected breeding and nonbreeding populations. Because individuals in any given population can arrive from a variety of sites the previous season, predicting how different populations will respond to environmental change can be challenging. In this study, we develop a population model composed of a network of breeding and wintering sites to show how habitat loss affects patterns of connectivity and species abundance. When the costs of migration are evenly distributed, habitat loss at a single site can increase the degree of connectivity (mixing) within the entire network, which then acts to buffer global populations from declines. However, the degree to which populations are buffered depends on where habitat loss occurs within the network: a site that has the potential to receive individuals from multiple populations in the opposite season will lead to smaller declines than a site that is more isolated. In other cases when there are equal costs of migration to two or more sites in the opposite season, habitat loss can result in some populations becoming segregated (disconnected) from the rest of the network. The geographic structure of the network can have a significant influence on relative population sizes of sites in the same season and can also affect the overall degree of mixing in the network, even when sites are of equal intrinsic quality. When a migratory network is widely spaced and migration costs are high, an equivalent habitat loss will lead to a larger decline in global population size than will occur in a network where the overall costs of migration are low. Our model provides an important foundation to test predictions related to habitat loss in real-world migratory networks and demonstrates that migratory networks will likely produce different dynamics from traditional metapopulations. Our results provide strong evidence that estimating population connectivity is a prerequisite for successfully predicting changes in migratory populations.     

Geomagnetic information modulates nocturnal migratory restlessness but not fueling in a long distance migratory songbird

Geomagnetic cues have been shown to influence migratory orientation and migratory fuelling in night‐migratory songbird species. Here, we used captive‐bred northern wheatears Oenanthe oenanthe from the southern Norwegian population to show that other aspects of the birds’ migratory program can be influenced by magnetic cues as well. We observed that the amount of migratory restlessness increased strongly with progression of the migratory season when the birds were kept constantly in the magnetic field of northern Germany, but the amount of migratory restlessness decreased when the magnetic field changed along the birds’ natural flyway are simulated. Thus, the Earth's magnetic field can also act as a ‘signpost’ cue for fine‐tuning the spatio‐temporal course of migration.     

Altitudinal migration: ecological drivers,knowledge gaps,and conservation implications

Animal migration has been the subject of intensive research for more than a century, but most research has focused on long‐distance rather than short‐distance migration. Altitudinal migration is a form of short‐distance migration in which individuals perform seasonal elevational movements. Despite its geographic and taxonomic ubiquity, there is relatively little information about the intrinsic and extrinsic factors that influence altitudinal migratory behaviour. Without this information, it is difficult to predict how rapid environmental changes will affect population viability of altitudinal migrants. To synthesize current knowledge, we compiled literature on altitudinal migration for all studied taxa, and identified the leading hypotheses explaining this behaviour. Studies of animal altitudinal migration cover many taxonomic lineages, with birds being the most commonly studied group. Altitudinal migration occurs in all continents except for Antarctica, but about a third of the literature focused on altitudinal migration in North America. Most research suggests that food and weather are the primary extrinsic drivers of altitudinal migration. In addition, substantial individual‐level variation in migratory propensity exists. Individual characteristics that are associated with sex, dominance rank, and body size explain much of the variation in migratory propensity in partially migratory populations, but individual‐level correlates are poorly known for most taxa. More research is needed to quantify the effects of habitat loss, habitat fragmentation, and climate change on altitudinal migrants. Demographic studies of individually marked populations would be particularly valuable for advancing knowledge of the cascading effects of environmental change on migratory propensity, movement patterns, and population viability. We conclude our review with recommendations for study designs and modelling approaches that could be used to narrow existing knowledge gaps, which currently hinder effective conservation of altitudinal migratory species.     

Home on the range: factors explaining partial migration of African buffalo in a tropical environment

Partial migration (when only some individuals in a population undertake seasonal migrations) is common in many species and geographical contexts. Despite the development of modern statistical methods for analyzing partial migration, there have been no studies on what influences partial migration in tropical environments. We present research on factors affecting partial migration in African buffalo (Syncerus caffer) in northeastern Namibia. Our dataset is derived from 32 satellite tracking collars, spans 4 years and contains over 35,000 locations. We used remotely sensed data to quantify various factors that buffalo experience in the dry season when making decisions on whether and how far to migrate, including potential man-made and natural barriers, as well as spatial and temporal heterogeneity in environmental conditions. Using an information-theoretic, non-linear regression approach, our analyses showed that buffalo in this area can be divided into 4 migratory classes: migrants, non-migrants, dispersers, and a new class that we call "expanders". Multimodel inference from least-squares regressions of wet season movements showed that environmental conditions (rainfall, fires, woodland cover, vegetation biomass), distance to the nearest barrier (river, fence, cultivated area) and social factors (age, size of herd at capture) were all important in explaining variation in migratory behaviour. The relative contributions of these variables to partial migration have not previously been assessed for ungulates in the tropics. Understanding the factors driving migratory decisions of wildlife will lead to better-informed conservation and land-use decisions in this area.     

Spatial and environmental effects on Coho Salmon life history trait variation

Adult size, egg size, fecundity, and mass of gonads are affected by trade‐offs between reproductive investment and environmental conditions shaping the evolution of life history traits among populations for widely distributed species. Coho salmon Oncorhynchus kisutch have a large geographic distribution, and different environmental conditions are experienced by populations throughout their range. We examined the effect of environmental variables on female size, egg size, fecundity, and reproductive investment of populations of Coho Salmon from across British Columbia using an information theoretic approach. Female size increased with latitude and decreased with migration distance from the ocean to spawning locations. Egg size was lowest for intermediate intragravel temperature during incubation, decreased with migration distance, but increased in rivers below lakes. Fecundity increased with latitude, warmer temperature during the spawning period, and river size, but decreased in rivers below lakes compared with rivers with tributary sources. Relative gonad size increased with latitude and decreased with migration distance. Latitude of spawning grounds, migratory distance, and temperatures experienced by a population, but also hydrologic features—river size and headwater source—are influential in shaping patterns of reproductive investment, particularly egg size. Although, relative gonad size varied with latitude and migration distance, how gonadal mass was partitioned gives insight into the trade‐off between egg size and fecundity. The lack of an effect of latitude on egg size suggests that local optima for egg size related to intragravel temperature may drive the variation in fecundity observed among years.     

Dynamics of neural crest-derived cell migration in the embryonic mouse gut

Neural crest-derived cells that form the enteric nervous system undergo an extensive migration from the caudal hindbrain to colonize the entire gastrointestinal tract. Mice in which the expression of GFP is under the control of the Ret promoter were used to visualize neural crest-derived cell migration in the embryonic mouse gut in organ culture. Time-lapse imaging revealed that GFP(+) crest-derived cells formed chains that displayed complicated patterns of migration, with sudden and frequent changes in migratory speed and trajectories. Some of the leading cells and their processes formed a scaffold along which later cells migrated. To examine the effect of population size on migratory behavior, a small number of the most caudal GFP(+) cells were isolated from the remainder of the population. The isolated cells migrated slower than cells in large control populations, suggesting that migratory behavior is influenced by cell number and cell-cell contact. Previous studies have shown that neurons differentiate among the migrating cell population, but it is unclear whether they migrate. The phenotype of migrating cells was examined. Migrating cells expressed the neural crest cell marker, Sox10, but not neuronal markers, indicating that the majority of migratory cells observed did not have a neuronal phenotype.     

Is Mythimna turca (Lepidoptera: Noctuidae) a migrant?

The double line moth, Mythimna turca (Linnaeus, 1761) (Lepidoptera: Noctuidae), is an important widely distributed insect species. Studies on the moth in European countries have focused on population monitoring and in China on taxonomy of Mythimna and distribution surveys of M. turca. However, whether M. turca is a migratory insect is unknown. Here, migration of the moth population was monitored using a searchlight trap from 2004 to 2020 on Beihuang island, Northern China, providing direct evidence that M. turca migrates across the Bohai Strait seasonally, in late May to early September. Population dynamics, ovarian development, wind direction and possible migration trajectories were also analyzed. Most moths migrated in June and at the end of August and the beginning of September, and a few migrated in May, July and October. The average duration of this windborne migratory period of M. turca was 93 d. Migratory trajectories demonstrated that M. turca mainly moved northward in spells of southerly winds from June to August and moved southward in spells of northerly winds during September. This new key information about the ecology of M. turca population will aid the formulation of M. turca management strategies.     

Evolutionary aspects of migratory behavior in European warblers

During the past 20 years, European Sylvia warblers have been used for a model study of the control mechanisms of bird migration and of evolutionary aspects of migratory behavior. Endogenous annual rhythms (‘circannual’ rhythms) and photo-period have proved to be the essential internal and external controlling factors. It is unknown whether this basic system, that also controls migration in other bird species, is currently evolutionarily stable or is instead adapting birds to the present slightly changing environmental conditions. Using the Blackcap, the control of partial migration in a bird species was analyzed. Two-way selective breeding experiments demonstrated a large selection response and high heritability values. These experiments have also indicated that a partially migratory population can become either almost completely migratory, or sedentary, in two to five generations. Hence, genetic influences are very important and presumably dominant over environmental factors in the expression of migratory or sedentary behavior. The large selection response implies a strikingly high evolutionary potential with respect to strong selection pressures. Further, in the Blackcap, migratory orientation behavior (in addition to migratory activity) was immediately transmitted into a F₁-generation when a cross-breeding experiment was performed using birds from a migratory and a resident population. The hybrids displayed their migratory activity along an axis that is used by their migratory parents. Finally, a rapidly developing novel migratory habit (new migratory direction to new wintering areas) in the Blackcap is discussed with respect to a positive feedback-mechanism, possibly including a series of advantages leading to above-average fitness.