Search

Simple Summary: Orb-weaving spiders can learn to avoid eating unpalatable prey, but what happens when they encounter one that they have never seen before? In the United States, a spider from East Asia has become established (the "jorō spider") in recent years, and their webs are most prominent during the late-summer and fall, which is also when migrating monarch butterflies are in abundance. Since monarch butterflies are unpalatable (because of developing on toxic milkweeds), we wondered if jorō spiders would recognize this and avoid eating them, given that these species do not coexist in East Asia. Using field observations of butterflies deposited into jorō spider webs, we found that these spiders do avoid eating monarchs, and even remove them from their webs in some cases. They readily attack other butterflies, even those that are similarly colored to monarchs. This raises many questions about how they perceive the "distastefulness" of monarchs, even without tasting the butterflies first. An invasive spider from East Asia has established in the U.S. southeast (the "jorō spider," Trichonephila clavata) and is rapidly expanding its range. Studies assessing the impact of this species are needed, including how expansive its diet is. An open question is whether monarch butterflies, Danaus plexippus, are a potential prey item for this spider, given that jorō spiders do not coexist with monarchs in their native range. Since monarch larvae feed on milkweed, they sequester cardiac glycosides into their adult tissues, rendering them unpalatable to many predators. At sites within northeast Georgia, we staged a series of trials (n = 61) where we tossed monarchs into jorō spider webs and, for comparison, performed similar trials with another aposematic species, gulf fritillary (Agraulis vanilla), and a palatable species, tiger swallowtail (Papilio glaucus). We recorded the outcome of the trials, which included whether the spider attacked or did not attack the prey. We also conducted a visual survey during the same fall season to look for evidence of jorō spiders consuming monarchs naturally. Our findings revealed that jorō spiders avoided eating monarchs; spiders only attacked monarchs 20% of the time, which was significantly less than the attack rates of similarly sized or larger butterflies: 86% for gulf fritillaries and 58% for tiger swallowtails. Some jorō spiders even removed monarchs from their webs. From our visual surveys of the surrounding area, we found no evidence of natural monarch consumption and, in general, butterflies made up only a fraction of the jorō spider diet. We conclude that jorō spiders appear to recognize monarch butterflies as being unpalatable, even without having a prior history with the species. This invokes questions about how these spiders can immediately recognize their unpalatability without touching the butterflies. [ABSTRACT FROM AUTHOR]

Insect–pathogen dynamics can show seasonal and inter‐annual variations that covary with fluctuations in insect abundance and climate. Long‐term analyses are especially needed to track parasite dynamics in migratory insects, in part because their vast habitat ranges and high mobility might dampen local effects of density and climate on infection prevalence.Monarch butterflies Danaus plexippus are commonly infected with the protozoan Ophryocystis elektroscirrha (OE). Because this parasite lowers monarch survival and flight performance, and because migratory monarchs have experienced declines in recent decades, it is important to understand the patterns and drivers of infection.Here we compiled data on OE infection spanning 50 years, from wild monarchs sampled in the United States, Canada and Mexico during summer breeding, fall migrating and overwintering periods. We examined eastern versus western North American monarchs separately, to ask how abundance estimates, resource availability, climate and breeding season length impact infection trends. We further assessed the intensity of migratory culling, which occurs when infected individuals are removed from the population during migration.Average infection prevalence was four times higher in western compared to eastern subpopulations. In eastern North America, the proportion of infected monarchs increased threefold since the mid‐2000s. In the western region, the proportion of infected monarchs declined sharply from 2000 to 2015, and increased thereafter. For both eastern and western subpopulations, years with greater summer adult abundance predicted greater infection prevalence, indicating that transmission increases with host breeding density. Environmental variables (temperature and NDVI) were not associated with changes in the proportion of infected adults. We found evidence for migratory culling of infected butterflies, based on declines in parasitism during fall migration. We estimated that tens of millions fewer monarchs reach overwintering sites in Mexico as a result of OE, highlighting the need to consider the parasite as a potential threat to the monarch population.Increases in infection among eastern North American monarchs post‐2002 suggest that changes to the host's ecology or environment have intensified parasite transmission. Further work is needed to examine the degree to which human practices, such as mass caterpillar rearing and the widespread planting of exotic milkweed, have contributed to this trend. [ABSTRACT FROM AUTHOR]

Humans have an especially close connection with the monarch butterfly, Danaus plexippus, and this connection often involves direct handling of larvae, pupae or adults. Monarchs are frequently handled in citizen science programs, education and outreach activities, classroom demonstrations, and when rearing larvae in captivity. A pertinent question is whether monarchs perceive this human handling in the same way as being attacked by a predator, i.e. does it trigger a physiological fight-or-flight reaction? Here, I investigated if brief periods of handling monarch larvae, pupae, or adults elevates their heart rate (a central component of the insect fight-or-flight response), using non-destructive methods: either microscopy or an electronic heart monitor designed for invertebrates. Results are presented from separate investigations on each life stage. Three minutes of gentle handling caused a significant elevation (20%) in heart rates of 30 fifth-instar larvae. Mild physical disturbance for 20 seconds to a collection of 57 pupae elicited brief bursts of heart contractions (∼88 beats/min), a rate which is three times higher than typical contraction periods at this stage. Adult monarchs captured during fall migration (n=72) showed minor heart rate elevations (6%) after 3 minutes of gentle handling for tagging purposes. The differences in reactions across life stages found here is helpful for elucidating the relative sensitivity of each stage to handling. More importantly, these results elucidate the unseen physiological impact of direct human contact on developing monarchs (larvae and pupae); elevation in heart rate is an integral part of the fight-or-flight reaction in animals and a universal sign that an animal perceives an imminent threat. In this sense then, developing monarchs appear to perceive human handling as a threat, which may alter our (human) perception of this practice. [ABSTRACT FROM AUTHOR]

Annual estimates of population size are important for tracking long-term population trajectories. There is concern that monarch butterflies (Danaus plexippus L.) in eastern North America are declining because the Mexican overwintering colonies have been shrinking over the past 20yr. We examined a I9-yr data set of fall monarch censuses at a site in northern Michigan (Peninsula Point) to determine if the number of monarchs counted here has changed over time. At this site, volunteers walk a standardized transect daily during the fell to record migratory monarchs. Our secondary goal was to evaluate the relationship between the annual numbers at Peninsula Point and the size of the subsequent overwintering colonies. A statistical model that included multiple predictors of 1,793 census counts revealed a small but significant positive effect of year on counts, but the mean annual count number was not significantly correlated with year. In either case, we found no evidence for declines in monarch abundance. We also found no relationship between annual estimates of abundance at Peninsula Point and annual estimates of overwintering colony size. We interpret the disparity between long-term patterns here versus those in Mexico to be the result of mortality during migration or other factors that reduce migration success. Given this conclusion, we argue that preserving migratory habitats and resources, as well as reducing anthropogenic activities that hinder migration, should be the highest priorities for conserving monarchs in eastern North America. [ABSTRACT FROM AUTHOR]

Declines in overwintering colonies of monarch butterflies (Danaus plexxipus) in Mexico raise questions about other life cycle phases, such as spring migration, where monarchs recolonize their breeding range in the United States and Canada with sequential generations. We used data from a longterm citizen science program, "Journey North" (now with 18 yr), to identify possible changes to the recolonization. This program asks people to report the date and location when they see the first adult monarch annually, and this database now contains >11,000 records. We examined sighting dates and migration range size, the latter based on the number of 2-degree latitude-longitude grid squares with monarch sightings, to look for evidence of change in either of these two parameters over the 18yr. Our analyses used regression models that accounted for increasing volunteer participation over the years. We found monarchs are being sighted later at a rate of 1 d later every 4 yr. This does not appear to be related to later emergence of milkweed, based on examination of milkweed reports. Later sightings could be interpreted as a sign of reductions in monarch abundance (it takes longer to see the first monarch of the year). We also found a potential decline in the geographic range of the initial spring migration wave (a decline of 9% over 18 yr). However we detected no change in the continental area encompassed at the end of recolonization, indicating monarchs are still successfully f illing their traditional breeding range in eastern North America. [ABSTRACT FROM AUTHOR]

Recent declines in the size of overwintering colonies of monarch butterflies ( Danaus plexippus) in eastern North America have stimulated calls for greater conservation efforts of the migratory phenomenon. Conservation decisions, however, should be guided by sound science, and the migratory phase of this population is the least-studied part of its life cycle., Data from large-scale citizen science programs can help address this knowledge gap. For over 20 years, volunteers have been tagging migrating monarchs with numbered stickers from MonarchWatch (), who oversees the management of tagging and recovery records. These data have the potential to vastly improve scientific understanding of the migratory phase (such as identifying spatial and temporal trends in mortality) and help target conservation efforts, but this potential has not yet been fully realised, based on a review of published studies in the last 20 years., Since citizen science programs are often understaffed and operate on minimal budgets, data analysis may not be a high priority for project staff. To stimulate scientific investigations using tagging data, there are alternative solutions that could be implemented including making data publicly available or soliciting assistance from external scientists. Such efforts would not only benefit research into monarch biology, but would lend scientific credibility to tagging activities., Tagging monarchs is a popular activity with clear educational value. In my opinion these data could be, however, better utilised to improve the conservation of the migratory phenomenon itself. [ABSTRACT FROM AUTHOR]

The physiological basis for pigment synthesis in lepidopteran wing scales is well-studied, although less is known about the reasons why individuals of the same species vary in pigmentation. Monarch butterflies (Danaus plexippus L.) show subtle variations in the shade of orange on their wings and this is known to predict flight ability and mating success. The present study tests the possibility that the shade of orange is associated with the amount of residual energy reserves carried over from the larval stage. Using monarchs reared in captivity under identical conditions (n = 207), the residuals of a regression of wing size and mass at eclosion, which indicate larval energy reserves, are obtained. This measure is positively related to adult longevity without feeding, indicating the importance of this reserve to the monarchs, as well as the value of the measure for this investigation. The shade of orange (i.e. hue) is determined on scanned wings using image analysis. Importantly, orange hue is predicted significantly by residual mass at eclosion (individuals with more mass are redder). The linkage between these traits may explain previous findings whereby redder monarchs fly further and mate more because both behaviours would be enhanced with greater energy stores. The findings of the present study add to a growing body of work showing how intraspecific variation in pigmentation has biological significance to monarchs, and possibly other butterflies. Although much remains to be investigated regarding the physiological underpinnings of this variation, the results of the present study indicate that future efforts should be rewarding. [ABSTRACT FROM AUTHOR]

Monarch butterflies (Danaus plexippus) throughout the world are commonly infected by the specialist pathogen Ophryocystis elektroscirrha (OE). This protozoan is transmitted when larvae ingest infectious stages (spores) scattered onto host plant leaves by infected adults. Parasites replicate internally during larval and pupal stages, and adult monarchs emerge covered with millions of dormant spores on the outsides of their bodies. Across multiple monarch populations, OE varies in prevalence and virulence. Here, we examined geographic and genetic variation in OE spore morphology using clonal parasite lineages derived from each of four host populations (eastern and western North America, South Florida and Hawaii). Spores were harvested from experimentally inoculated, captive-reared adult monarchs. Using light microscopy and digital image analysis, we measured the size, shape and color of 30 replicate spores per host. Analyses examined predictors of spore morphology, including parasite source population and clone, parasite load, and the following host traits: family line, sex, wing area, and wing color (orange and black pigmentation). Results showed significant differences in spore size and shape among parasite clones, suggesting genetic determinants of morphological variation. Spore size also increased with monarch wing size, and monarchs with larger and darker orange wings tended to have darker colored spores, consistent with the idea that parasite development depends on variation in host quality and resources. We found no evidence for effects of source population on variation in spore morphology. Collectively, these results provide support for heritable variation in spore morphology and a role for host traits in affecting parasite development. [ABSTRACT FROM AUTHOR]

The demands of long-distance flight represent an important evolutionary force operating on the traits of migratory species. Monarchs are widespread butterflies known for their annual migrations in North America. We examined divergence in wing morphology among migratory monarchs from eastern and western N. America, and nonmigratory monarchs in S. Florida, Puerto Rico, Costa Rica, and Hawaii. For the three N. American populations, we also examined monarchs reared in four common environment experiments. We used image analysis to measure multiple traits including forewing area and aspect ratio; for laboratory-reared monarchs we also quantified body area and wing loading. Results showed wild monarchs from all nonmigratory populations were smaller than those from migratory populations. Wild and captive-reared eastern monarchs had the largest and most elongated forewings, whereas monarchs from Puerto Rico and Costa Rica had the smallest and roundest forewings. Eastern monarchs also had the largest bodies and high measures of wing loading, whereas western and S. Florida monarchs had less elongated forewings and smaller bodies. Among captive-reared butterflies, family-level effects provided evidence that genetic factors contributed to variation in wing traits. Collectively, these results support evolutionary responses to long-distance flight in monarchs, with implications for the conservation of phenotypically distinct wild populations. [ABSTRACT FROM AUTHOR]

Heart rates of insects reflect the current level of activity and stress individuals are experiencing, and therefore this information can be useful from a research perspective. In Lepidoptera, as with most insects, the 'heart' consists of a transverse longitudinal tube that runs along the abdomen, just under the abdominal tergites, which pumps or contracts rhythmically to distribute hemolymph. Here, we describe a simple method we developed to observe heart contractions in adult monarch butterflies, Danaus plexippus. The procedure involves stabilizing the live specimen in a pre-prepared plastic bag, while positioning the abdomen in such a way so that the beating heart can be seen (with magnification) through the intersegmental membranes. With this procedure, no harm comes to the specimen, making the technique useful in both lab and field studies. The technique also requires little equipment, except for a dissecting microscope (or other magnifier). Moreover, the procedure should be useful for monitoring other similarly-sized Lepidoptera, and we confirmed this with a Papilio glaucus specimen. Using this method on 10 male and 10 female monarch specimens from captive-reared stock, we found the resting heart rate was 63 beats/min on average (range: 35-86). This information will be useful for comparative purposes, or as a reference point for future studies of monarch physiology. [ABSTRACT FROM AUTHOR]

Simple Summary: Neonicotinoids are the most widely used insecticides in North America and many studies document the negative effects of neonicotinoids on bees. Monarch butterflies are famous for their long-distance migrations, and for their ability to sequester toxins from their milkweed host plants. The neonicotinoids imidacloprid and clothianidin were suggested to correlate with declines in North American monarchs. We examined how monarch development, survival, and flight were affected by exposure to neonicotinoids, and how these effects depend on milkweed host plant species that differ in their cardenolide toxins. Monarch survival and flight were unaffected by low and intermediate neonicotinoid doses. At the highest dose, neonicotinoids negatively affected monarch pupation and survival, for caterpillars that fed on the least toxic milkweed. Monarchs fed milkweed of intermediate toxicity experienced moderate negative effects of high insecticide doses. Monarchs fed the most toxic milkweed species had no negative consequences associated with neonicotinoid treatment. Our work shows that monarchs tolerate low neonicotinoid doses, but experience detrimental effects at higher doses, depending on milkweed species. To our knowledge, this is the first study to show that host plant species potentially reduce the residue of neonicotinoid insecticides on the leaf surface, and this phenomenon warrants further investigation. Neonicotinoids are the most widely used insecticides in North America. Numerous studies document the negative effects of neonicotinoids on bees, and it remains crucial to demonstrate if neonicotinoids affect other non-target insects, such as butterflies. Here we examine how two neonicotinoids (imidacloprid and clothianidin) affect the development, survival, and flight of monarch butterflies, and how these chemicals interact with the monarch's milkweed host plant. We first fed caterpillars field-relevant low doses (0.075 and 0.225 ng/g) of neonicotinoids applied to milkweed leaves (Asclepias incarnata), and found no significant reductions in larval development rate, pre-adult survival, or adult flight performance. We next fed larvae higher neonicotinoid doses (4–70 ng/g) and reared them on milkweed species known to produce low, moderate, or high levels of secondary toxins (cardenolides). Monarchs exposed to the highest dose of clothianidin (51–70 ng/g) experienced pupal deformity, low survival to eclosion, smaller body size, and weaker adult grip strength. This effect was most evident for monarchs reared on the lowest cardenolide milkweed (A. incarnata), whereas monarchs reared on the high-cardenolide A. curassavica showed no significant reductions in any variable measured. Our results indicate that monarchs are tolerant to low doses of neonicotinoid, and that negative impacts of neonicotinoids depend on host plant type. Plant toxins may confer protective effects or leaf physical properties may affect chemical retention. Although neonicotinoid residues are ubiquitous on milkweeds in agricultural and ornamental settings, commonly encountered doses below 50 ng/g are unlikely to cause substantial declines in monarch survival or migratory performance. [ABSTRACT FROM AUTHOR]

Predictors of male monarch butterfly mating success have eluded researchers for years. Although it has long been known that there is variation in male mating success in this species, the source of this variation remains unclear. We used digital image analysis techniques to measure fine-scale variation in three components of the orange color (hue, saturation and brightness of the orange) of the forewings of 174 males at a level of detail that has not been possible until recently, and we compared this information to the mating success of the males in captivity. A second experiment involved addition of orange coloration to the wings of 93 males before mating trials. Our results indicate that one component of the orange color (saturation) correlated with mating success in our first experiment. Furthermore, wing color manipulations had no effect, but the original saturation values of the males in the second experiment were directly related to mating success. Our results provide the first evidence that wing color in monarch butterflies relates to mating success, and they serve as an important starting point for future research. [ABSTRACT FROM AUTHOR]

Each year in eastern North America, monarch butterflies, Danaus plexippus (L.) (Lepidoptera: Nymphalidae), undergo an annual migration to wintering sites in central Mexico. We used monarch migration census data from Cape May, NJ, over a 13-yr period (1992-2004) to test for annual and within-season variation in the numbers of monarchs seen during daily censuses and to examine the timing and patterns of migration waves. Across all years, the total number of monarchs counted over the 60-d season ranged from 452 to 15,751 with a 13-yr average of 3,490 monarchs [yr.sup.-1]. There was significant annual, diurnal, and within-season temporal variation in the census counts. Within seasons, monarch numbers increased during September up until early October, and gradually declined thereafter. Comparison of season averages across years, which we consider indices of population size in the northeastern United States, indicated a highly fluctuating population size with the lowest year on record in 2004. We also found that greater than average daily counts, which we termed "notable migration days," were reported for an average of 19 d per season. On average, seven "migration waves" occurred each year, defined as a period of one or more notable migration days separated by below average days. Waves lasted an average of 3 d and were separated from others by ≈6 d. There was no significant interannual variation in wave duration or time between waves. Our 13-yr study is the longest standardized census of the monarch's fall migration, and we believe its continued operation can provide insights into the population trends of monarchs in the northeastern United States, which may reflect long-term trends from other populations in North America. [ABSTRACT FROM AUTHOR]

Abstract: [1.] As ectotherms, insects often experience varying temperatures throughout their life cycle, and some respond by becoming more or less melanistic (dark coloring) during development to increase or decrease thermal energy absorption as larvae or adults. [2.] Monarch butterflies (Danaus plexippus) breed in temperate and tropical environments worldwide and are exposed to different average and extreme temperatures in different parts of their geographic range. In this study, we compared variation in thermally induced melanism among monarch butterflies from eastern and western North America and from South Florida. [3.] We raised the progeny of wild-captured adult butterflies from these populations in a common garden experiment, rearing individuals in cold (19°C), moderate (26°C), and hot (32°C) temperatures to examine population variation in larval and adult pigmentation. [4.] Across all populations, monarch larvae developed the darkest coloration in the cold treatment and were lightest when reared in hot temperatures. Similar results were observed for measures of adult wing melanism, with the exception of adult females, which developed darker colored wings in warmer temperatures. [5.] Significant population-level differences in average measures of melanism among larvae and adult butterflies were observed. Larvae from the eastern population became substantially darker in colder temperatures than S. Florida or western larvae. Western larvae were lightest overall, which might be adaptive to high temperatures experienced throughout portions of their summer breeding range. S. Florida larvae showed a lower response to cold temperatures relative to monarchs from either migratory population. [6.] Population level differences were also observed for thermal responses in wing melanism, particularly among adult females. Moreover, we found significant family level effects for each measure of larval and adult melanism, pointing to a genetic basis or strong maternal effects influencing these traits in monarch butterflies. [Copyright &y& Elsevier]

The article presents a study on the sex ratios of the North American population of monarch butterflies. Data from Monarch butterflies migrating to overwintering sites in central Mexico show a gradual decline in proportion of females over time. The findings revealed that sampling location did not influence the trend and that breeding monarchs may be facing a number of threats, including the protozoan parasite, Ophryocystis elektroscirrha, which reportedly affects females more than males, habitat alterations and insecticides.