Asclepias

About: Asclepias is a research topic. Over the lifetime, 276 publications have been published within this topic receiving 7920 citations. The topic is also known as: Milkweed.

Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population

Abstract: The size of the Mexican overwintering population of monarch butter- flies has decreased over the last decade. Approximately half of these butterflies come from the U.S. Midwest where larvae feed on common milkweed. There has been a large decline in milkweed in agricultural fields in the Midwest over the last decade. This loss is coincident with the increased use of glyphosate herbicide in conjunction with increased planting of genetically modified (GM) glyphosate-tolerant corn (maize) and soybeans (soya). 2. We investigate whether the decline in the size of the overwintering population can be attributed to a decline in monarch production owing to a loss of milkweeds in agricultural fields in the Midwest. We estimate Midwest annual monarch production using data on the number of monarch eggs per milkweed plant for milkweeds in dif- ferent habitats, the density of milkweeds in different habitats, and the area occupied by those habitats on the landscape. 3. We estimate that there has been a 58% decline in milkweeds on the Midwest landscape and an 81% decline in monarch production in the Midwest from 1999 to 2010. Monarch production in the Midwest each year was positively correlated with the size of the subsequent overwintering population in Mexico. Taken together, these results strongly suggest that a loss of agricultural milkweeds is a major contributor to the decline in the monarch population. 4. The smaller monarch population size that has become the norm will make the species more vulnerable to other conservation threats.

Toxic cardenolides: chemical ecology and coevolution of specialized plant–herbivore interactions

Abstract: Cardenolides are remarkable steroidal toxins that have become model systems, critical in the development of theories for chemical ecology and coevolution. Because cardenolides inhibit the ubiquitous and essential animal enzyme Na⁺/K⁺-ATPase, most insects that feed on cardenolide-containing plants are highly specialized. With a huge diversity of chemical forms, these secondary metabolites are sporadically distributed across 12 botanical families, but dominate the Apocynaceae where they are found in > 30 genera. Studies over the past decade have demonstrated patterns in the distribution of cardenolides among plant organs, including all tissue types, and across broad geographic gradients within and across species. Cardenolide production has a genetic basis and is subject to natural selection by herbivores. In addition, there is strong evidence for phenotypic plasticity, with the biotic and abiotic environment predictably impacting cardenolide production. Mounting evidence indicates a high degree of specificity in herbivore-induced cardenolides in Asclepias. While herbivores of cardenolide-containing plants often sequester the toxins, are aposematic, and possess several physiological adaptations (including target site insensitivity), there is strong evidence that these specialists are nonetheless negatively impacted by cardenolides. While reviewing both the mechanisms and evolutionary ecology of cardenolide-mediated interactions, we advance novel hypotheses and suggest directions for future work.

Diversity and temporal change in the effective pollinators of Asclepias tuberosa

Abstract: Although pollination effectiveness is a central process underlying the evo- lution of plant and pollinator traits, it is difficult to measure and has rarely been reported for a diverse spectrum of visitors under natural conditions. We measured the effectiveness of all common flower visitors to Asclepias tuberosa (butterfly weed) at a site in southeastern Arizona, in terms of visitation rate, per-visit rate of pollinia removal and insertion, and pollinia load. Bombus and Apis (Hymenoptera) were the most effective pollinators, counter to predictions that A. tuberosa is butterfly-pollinated. We also documented large differences between 2 yr in the pollination effectiveness of visitors, primarily due to changes in vis- itation rate. Bombus were the most frequent and effective pollinators in 1992. In 1993, Apis were equivalent to Bombus. Battus (Lepidoptera) were the second most effective pollinators in 1992, but were scarce in 1993. Thus, conclusions about the identity of effective pollinators based on floral traits, casual observations of visitation, or even precise measurement of effectiveness in a single season are all potentially suspect. We compare our results to those of previous studies of Asclepias pollination.

Stable isotopes (δD and δ 13 C) are geographic indicators of natal origins of monarch butterflies in eastern North America

Abstract: Wing membranes of laboratory and field-reared monarch butterflies (Danaus plexippus) were analyzed for their stable-hydrogen (δD) and carbon (δ13C) isotope ratios to determine whether this technique could be used to identify their natal origins. We hypothesized that the hydrogen isotopic composition of monarch butterfly wing keratin would reflect the hydrogen isotope patterns of rainfall in areas of natal origin where wings were formed. Monarchs were reared in the laboratory on milkweed plants (Asclepias sp.) grown with water of known deuterium content, and, with the assistance of volunteers, on native milkweeds throughout eastern North America. The results show that the stable hydrogen isotopic composition of monarch butterflies is highly correlated with the isotopic composition of the milkweed host plants, which in turn corresponds closely with the long-term geographic patterns of deuterium in rainfall. Stable-carbon isotope values in milkweed host plants were similarly correlated with those values in monarch butterflies and showed a general pattern of enrichment along a southwest to northeast gradient bisecting the Great Lakes. These findings indicate that natal origins of migratory and wintering monarchs in Mexico can be inferred from the combined δD and δ13C isotopic signatures in their wings. This relationship establishes that analysis of hydrogen and carbon isotopes can be used to answer questions concerning the biology of migratory monarch butterflies and provides a new approach to tracking similar migratory movements of other organisms.