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1. Monarch butterflies reared under autumn‐like conditions have more efficient flight and lower post‐flight metabolism

   https://doi.org/10.1111/een.12828  

   Schroeder, Hayley; Majewska, Ania; Altizer, Sonia (December 2019, Ecological Entomology)

   1. Many migratory animals undergo physiological and behavioural changes to prepare for and sustain long‐distance movements. Because insect migrations are common and diverse, studies that examine how migratory insects meet the energetic demands of long‐distance movements are badly needed. 2. Monarch butterflies (Danaus plexippus) migrate up to 4000 km annually from eastern North America to wintering sites in central Mexico. Autumn generation monarchs undergo physiological and behavioural changes in response to environmental cues to initiate migration. In particular, exposure to cooler temperatures and shorter day lengths in early autumn causes monarchs to enter the hormonally induced state of reproductive diapause. 3. This study examined differences in flight‐associated metabolic rate (MR) and flight performance metrics for monarchs experimentally reared under autumn‐like conditions (typically experienced before the southward migration) relative to monarchs reared under summer‐like conditions. 4. Adult monarchs reared under autumn‐like conditions showed lower post‐flight MRs, greater flight efficiency, and lower measures of reproductive activity relative to monarchs reared under summer‐like conditions. Increases in post‐flight metabolism were associated with monarch body weight, age, and flight velocity. 5. These findings suggest that a trans‐generational shift in flight energetics is an important component of the monarch's complex migratory syndrome, and that physiological changes that accompany reproductive diapause facilitate energy conservation during flight. 

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2. High thermal tolerance in high-elevation species and laboratory-reared colonies of tropical bumble bees

   Gonzalez, V.H.; Oyen, K.; Aguilar, M.L.; Herrea, A.; Martin, R.D.; and Ospina, R. (January 2022, Ecology and evolution)

   Bumble bees are key pollinators with some species reared in captivity at a commercial scale, but with significant evidence of population declines and with alarming predictions of substantial impacts under climate change scenarios. While studies on the thermal biology of temperate bumble bees are still limited, they are entirely absent from the tropics where the effects of climate change are expected to be greater. Herein, we test whether bees' thermal tolerance decreases with elevation and whether the stable optimal conditions used in laboratory-reared colonies reduces their thermal tolerance. We assessed changes in the lower (CTMin) and upper (CTMax) critical thermal limits of four species at two elevations (2600 and 3600 m) in the Colombian Andes, examined the effect of body size, and evaluated the thermal tolerance of wild-caught and laboratory-reared individuals of Bombus pauloensis. We also compiled information on bumble bees' thermal limits and assessed potential predictors for broadscale patterns of variation. We found that CTMin decreased with increasing elevation, while CTMax was similar between elevations. CTMax was slightly higher (0.84°C) in laboratory-reared than in wild-caught bees while CTMin was similar, and CTMin decreased with increasing body size while CTMax did not. Latitude is a good predictor for CTMin while annual mean temperature, maximum and minimum temperatures of the warmest and coldest months are good predictors for both CTMin and CTMax. The stronger response in CTMin with increasing elevation, and similar CTMax, supports Brett's heat-invariant hypothesis, which has been documented in other taxa. Andean bumble bees appear to be about as heat tolerant as those from temperate areas, suggesting that other aspects besides temperature (e.g., water balance) might be more determinant environmental factors for these species. Laboratory-reared colonies are adequate surrogates for addressing questions on thermal tolerance and global warming impacts. 

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3. Exposure to Non-Native Tropical Milkweed Promotes Reproductive Development in Migratory Monarch Butterflies

   https://doi.org/10.3390/insects10080253  

   Majewska, Ania A.; Altizer, Sonia (August 2019, Insects)

   Background: North American monarchs (Danaus plexippus) are well-known for their long-distance migrations; however, some monarchs within the migratory range have adopted a resident lifestyle and breed year-round at sites where tropical milkweed (Asclepias curassavica) is planted in the southern coastal United States. An important question is whether exposure to exotic milkweed alters monarch migratory physiology, particularly the ability to enter and remain in the hormonally-induced state of reproductive diapause, whereby adults delay reproductive maturity. Cued by cooler temperatures and shorter photoperiods, diapause is a component of the monarch’s migratory syndrome that includes directional flight behavior, lipid accumulation, and the exceptional longevity of the migratory generation. Methods: Here, we experimentally test how exposure to tropical milkweed during the larval and adult stages influences monarch reproductive status during fall migration. Caterpillars reared under fall-like conditions were fed tropical versus native milkweed diets, and wild adult migrants were placed in outdoor flight cages with tropical milkweed, native milkweed, or no milkweed. Results: We found that monarchs exposed to tropical milkweed as larvae were more likely to be reproductively active (exhibit mating behavior in males and develop mature eggs in females) compared to monarchs exposed to native milkweed. Among wild-caught fall migrants, females exposed to tropical milkweed showed greater egg development than females exposed to native or no milkweed, although a similar response was not observed for males. Conclusions: Our study provides evidence that exposure to tropical milkweed can increase monarch reproductive activity, which could promote continued residency at year-round breeding sites and decrease monarch migratory propensity. 

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4. Seasonal plasticity in morphology and metabolism differs between migratory North American and resident Costa Rican monarch butterflies

   https://doi.org/10.1002/ece3.9796  

   Tenger‐Trolander, Ayşe; Julick, Cole R.; Lu, Wei; Green, Delbert André; Montooth, Kristi L.; Kronforst, Marcus R. (February 2023, Ecology and Evolution)

   Abstract Environmental heterogeneity in temperate latitudes is expected to maintain seasonally plastic life‐history strategies that include the tuning of morphologies and metabolism that support overwintering. For species that have expanded their ranges into tropical latitudes, it is unclear the extent to which the capacity for plasticity will be maintained or will erode with disuse. The migratory generations of the North American (NA) monarch butterflyDanaus plexippuslead distinctly different lives from their summer generation NA parents and their tropical descendants living in Costa Rica (CR). NA migratory monarchs postpone reproduction, travel thousands of kilometers south to overwinter in Mexico, and subsist on little food for months. Whether recently dispersed populations of monarchs such as those in Costa Rica, which are no longer subject to selection imposed by migration, retain ancestral seasonal plasticity is unclear. To investigate the differences in seasonal plasticity, we reared the NA and CR monarchs in summer and autumn in Illinois, USA, and measured the seasonal reaction norms for aspects of morphology and metabolism related to flight. NA monarchs were seasonally plastic in forewing and thorax size, increasing wing area and thorax to body mass ratio in autumn. While CR monarchs increased thorax mass in autumn, they did not increase the area of the forewing. NA monarchs maintained similar resting and maximal flight metabolic rates across seasons. However, CR monarchs had elevated metabolic rates in autumn. Our findings suggest that the recent expansion of monarchs into habitats that support year‐round breeding may be accompanied by (1) the loss of some aspects of morphological plasticity as well as (2) the underlying physiological mechanisms that maintain metabolic homeostasis in the face of temperature heterogeneity. 

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5. Parasite dynamics in North American monarchs predicted by host density and seasonal migratory culling

   https://doi.org/10.1111/1365-2656.13678  

   Majewska, Ania A.; Davis, Andrew K.; Altizer, Sonia; de Roode, Jacobus C. (March 2022, Journal of Animal Ecology)

   Abstract 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 butterfliesDanaus plexippusare commonly infected with the protozoanOphryocystis 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 onOEinfection 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 ofOE, 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. 

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