1. During the 2009-2010 overwintering season and following a 15-year downward trend, the total area in Mexico occupied by the eastern North American population of overwintering monarch butterflies reached an all-time low. Despite an increase, it remained low in 2010-2011.2. Although the data set is small, the decline in abundance is statistically significant using both linear and exponential regression models.3. Three factors appear to have contributed to reduce monarch abundance: degradation of the forest in the overwintering areas; the loss of breeding habitat in the United States due to the expansion of GM herbicide-resistant crops, with consequent loss of milkweed host plants, as well as continued land development; and severe weather.4. This decline calls into question the long-term survival of the monarchs' migratory phenomenon.Resú men. 1. Durante la temporada invernal 2009-2010, y siguiendo una tendencia a la baja de 15 an˜os, la superficie total ocupada por mariposas monarca en Me´xico, provenientes del este Ame´rica del Norte, llego´a su punto ma´s bajo. A pesar de su incremento, dicha superficie siguio´siendo baja en 2010-2011.2. Aunque que el conjunto de datos disponibles es au´n pequen˜o, esta disminucio´n de la abundancia de mariposas es estadı´sticamente significativa, tanto si se usan modelos de regresio´n lineales como exponenciales.3. Hay tres factores que parecen haber contribuido con esta tendencia de reduc-cio´n del nu´mero de mariposas: la degradacio´n de bosque en las a´reas de invernacio´n en Me´xico; la pe´rdida de ha´bitat de reproduccio´n en los Estados Unidos, debido a la expansio´n de cultivos gene´ticamente modificados resistentes a herbicidas, con la consiguiente pe´rdida de las plantas hospederas de algodoncillo, y por continuos cambios en el uso del suelo no favorables para ellas; y, las recientes condiciones cli-ma´ticas severas.4. Esta disminucio´n hace que nos cuestionemos sobre la posibilidad de supervivencia a largo plazo del feno´meno migratorio de las mariposas monarca.

Summary 1. Monarch butterflies Danaus plexippus (L.) (Lepidoptera: Nymphalidae) are susceptible to infection by the obligate protozoan parasite Ophryocystis elektroscirrha (McLaughlin and Myers) (Apicomplexa: Neogregarinida). Because monarchs form resident and migratory populations in different parts of the world, this host–parasite system provides the opportunity to examine how variation in parasite prevalence relates to host movement patterns. 2. Parasite prevalence was evaluated using 14 790 adult monarchs captured between 1968 and 1997. Comparison of three populations in North America indicated that parasite prevalence is associated negatively with host dispersal distances. A continuously breeding, nonmigratory population in southern Florida showed high prevalence (over 70% heavily infected). The western population migrates moderate distances to overwintering sites on the Pacific Coast and has intermediate prevalence (30% heavily infected). The eastern migratory population, which travels the longest distance to Mexican overwintering sites, has exhibited less than 8% infection throughout the past 30 years. 3. Variation in parasite loads within North American migratory populations was investigated to determine whether the prevalence of heavy infection and average parasite loads declined during migration or overwintering. Average parasite loads of summer‐breeding adults in western North America decreased with increasing distance from overwintering sites. This suggests that heavily infected monarchs are less likely to remigrate long distances in spring. No differences in the frequency of heavily infected adults were found among eastern or western North American monarchs throughout the overwintering period, however, suggesting that this parasite does not affect overwintering mortality. 4. Changes in the prevalence of monarchs with low parasite loads demonstrate that spore transfer occurs during migration and overwintering, possibly when adult butterflies contact each other as a result of their clustering behaviour. 5. This study of geographical and temporal variation in O. elektroscirrha among populations of D. plexippus demonstrates the potential role of seasonal migration in mediating interactions between hosts and parasites, and suggests several mechanisms through which migratory behaviour may influence parasite prevalence.

Summary1. A novel experimental method was developed to study negative physical and chemical effects of latex and cardiac glycosides on first‐instar monarch butterfly larvae in their natural environment in north central Florida. Forceps were used to nibble through the petioles of leaves of the sandhill milkweed Asclepias humistrata, mimicking the behaviour of mature monarch larvae. This notching cut off the supply of latex to the leaves without significantly reducing either their cardiac glycoside concentration or water content.2. The mean cardiac glycoside concentration in larvae that fed on intact leaves was nearly two and a half times greater than in larvae that fed on notched leaves. This was probably because more latex is present in the gut of the larvae that fed on the intact leaves. Supporting this is the fact that the mean concentration of cardiac glycosides in the latex was 34–47 times that in the leaves.3. Wet weights, dry weights, and growth rates of first‐instar larvae that fed on intact leaves over a 72‐h period were less than half those of larvae that fed on notched leaves.4. Mortality due to miring in the latex was 27% on the intact leaves compared with 2% on the notched leaves.5. Latex, cardiac glycosides, and other as yet undetermined plant factors all have a negative effect on first‐instar larval survival.6. Video‐analyses indicated that ingestion of latex caused the larvae to become cataleptic and increased their chances of being mired on the leaf by the setting latex glue. Dysfunction resulting from latex ingestion may lead to the larvae falling off the plant and being killed by invertebrate predators.7. The difficulty of neonate monarch larvae surviving on A. humistrata – one of the principal milkweed species fed on each spring as monarchs remigrate from Mexico into the southern U.S.A. – is evidence that a co‐evolutionary arms race is operating in this plant–herbivore system.

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Monarch butterflies in eastern North America accumulate lipids during their fall migration to central Mexico, and use them as their energy source during a 5 month overwintering period. When and where along their migratory journey the butterflies accumulate these lipids has implications for the importance of fall nectar sources in North America. We analyzed the lipid content of 765 summer breeding and fall migrant monarch butterflies collected at 1 nectaring site in central Virginia over 4 years (1998-2001), and compared them with 16 additional published and unpublished datasets from other sites, dating back to 1941. Virginia migrants store significantly more lipid than summer butterflies, and show significant intraseason and between-year variation. None of the Virginia samples, and none of the historical samples, with one exception, had lipid levels comparable with those found in migrants that had reached Texas and northern Mexico. This evidence suggests that upon reaching Texas, the butterflies undergo a behavioral shift and spend more time nectaring. The one exceptional sample led us to the discovery that monarchs that form roosts along their migratory routes have higher lipid contents than monarchs collected while nectaring at flowers. We propose that for much of their journey monarchs are opportunistic migrants, and the variation within and between samples reflects butterflies' individual experiences. The stored lipids appear to be of less importance as fuel for the butterflies' migration than for their survival during their overwintering period, in part because soaring on favorable winds reduces the energetic cost of flying. The conservation of nectar plants in Texas and northern Mexico is crucial to sustaining the monarch's migratory spectacle, and nectar abundance throughout eastern North America is also important. As generalists in their selection of nectar sources and nectaring habitats, monarchs are unlikely to be affected by small changes in plant communities. Agricultural transformations of natural communities in the eastern United States and Great Plains, however, and especially the extensive planting of genetically modified herbicide-resistant soybeans and corn, may be changing the availability of nectar for monarchs and other pollinators. This new technology is eliminating virtually all forbs in and surrounding agricultural fields, including the monarch's larval hostplants (milkweeds) and native and nonnative nectar sources. To evaluate whether changes in nectar availability are altering the butterflies' ability to accumulate energy, we recommend that monarchs' lipid contents be assayed annually at sites throughout eastern North America.

A new bioassay for comparing the palatability to avian predators of monarch butterflies reared on various asclepiadaceous food plants containing cardiac glycosides indicates a palatability spectrum. The monarchs reared on one plant species are six times as emetic as those fed another, while those raised on an asclepiad which lacks cardiac glycosides are not emetic at all.

No abstract

Addressing population declines of migratory insects requires linking populations across different portions of the annual cycle and understanding the effects of variation in weather and climate on productivity, recruitment, and patterns of long-distance movement. We used stable H and C isotopes and geospatial modeling to estimate the natal origin of monarch butterflies (Danaus plexippus) in eastern North America using over 1000 monarchs collected over almost four decades at Mexican overwintering colonies. Multinomial regression was used to ascertain which climate-related factors best-predicted temporal variation in natal origin across six breeding regions. The region producing the largest proportion of overwintering monarchs was the US Midwest (mean annual proportion = 0.38; 95% CI: 0.36-0.41) followed by the north-central (0.17; 0.14-0.18), northeast (0.15; 0.11-0.16), northwest (0.12; 0.12-0.16), southwest (0.11; 0.08-0.12), and southeast (0.08; 0.07-0.11) regions. There was no evidence of directional shifts in the relative contributions of different natal regions over time, which suggests these regions are comprising the same relative proportion of the overwintering population in recent years as in the mid-1970s. Instead, interannual variation in the proportion of monarchs from each region covaried with climate, as measured by the Southern Oscillation Index and regional-specific daily maximum temperature and precipitation, which together likely dictate larval development rates and food plant condition. Our results provide the first robust long-term analysis of predictors of the natal origins of monarchs overwintering in Mexico. Conservation efforts on the breeding grounds focused on the Midwest region will likely have the greatest benefit to eastern North American migratory monarchs, but the population will likely remain sensitive to regional and stochastic weather patterns.