John M. Pleasants

Impact of Bt corn pollen on monarch butterfly populations: A risk assessment

A collaborative research effort by scientists in several states and in Canada has produced information to develop a formal risk assessment of the impact of Bt corn on monarch butterfly (Danaus plexippus) populations. Information was sought on the acute toxic effects of Bt corn pollen and the degree to which monarch larvae would be exposed to toxic amounts of Bt pollen on its host plant, the common milkweed, Asclepias syriaca, found in and around cornfields. Expression of Cry proteins, the active toxicant found in Bt corn tissues, differed among hybrids, and especially so in the concentrations found in pollen of different events. In most commercial hybrids, Bt expression in pollen is low, and laboratory and field studies show no acute toxic effects at any pollen density that would be encountered in the field. Other factors mitigating exposure of larvae include the variable and limited overlap between pollen shed and larval activity periods, the fact that only a portion of the monarch population utilizes milkweed stands in and near cornfields, and the current adoption rate of Bt corn at 19% of North American corn-growing areas. This 2-year study suggests that the impact of Bt corn pollen from current commercial hybrids on monarch butterfly populations is negligible.

Corn pollen deposition on milkweeds in and near cornfields

The density of corn pollen on leaves of milkweed plants inside and outside of cornfields was measured in several studies from different localities. The purpose was to obtain a representative picture of naturally occurring pollen densities to provide a perspective for laboratory and field studies of monarch larvae feeding on milkweed leaves with Bt corn pollen. Pollen density was highest (average 170.6 grains per cm2) inside the cornfield and was progressively lower from the field edge outward, falling to 14.2 grains per cm2 at 2 m. Inside the cornfield, and for each distance from the field edge, a frequency distribution is presented showing the proportion of leaf samples with different pollen densities. Inside cornfields, 95% of leaf samples had pollen densities below 600 grains per cm2 and the highest pollen density observed was 1400 grains per cm2, which occurred in a study with a rainless anthesis period. All other studies had rainfall events during the anthesis period. A single rain event can remove 54–86% of the pollen on leaves. Leaves on the upper portion of milkweed plants, where young monarch larvae tend to feed, had only 30–50% of the pollen density levels of middle leaves.

Temporal and spatial overlap between monarch larvae and corn pollen

To assess the likelihood that monarch larvae will be exposed to Bacillus thuringiensis (Bt) pollen, we studied milkweed and monarch densities in habitats which comprise much of the land available to breeding monarchs, e.g., cornfields, cornfield edges, other agricultural fields, and nonagricultural areas, in four regions of the monarch breeding range. We found that monarchs use milkweed in cornfields throughout their breeding season, and that per plant densities are as high or higher in agricultural habitats as in nonagricultural habitats. As a result of the prevalence of agricultural land, most of the monarchs produced in the upper Midwest are likely to originate in cornfields or other agricultural habitats. There was a greater temporal overlap between susceptible monarchs and corn anthesis in the northern than the southern part of the summer breeding range, because of earlier pollen shed in the south. The importance of agricultural habitats to monarch production suggests that, regardless of the impact of genetically modified crops, agricultural practices such as weed control and foliar insecticide use could have large impacts on monarch populations.

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

Abstract.  1. The size of the Mexican overwintering population of monarch butterflies 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).

Nectar production rates of Asclepias quadrifolia: causes and consequences of individual variation

The causes and reproductive consequences of individual variation in nectar production rates within a population of Asclepias quadrifolia were investigated. Two parameters were correlated with nectar production rate per flower: the root weight of the plant and the number of flowers in the inflorescence (umbel). Nectar production increased with increasing root weight but levelled off after a root size of about 3 g was reached. Nectar production decreased with increasing umbel size, but only for umbels that were greater than average size. A total of 57% of the variance in individual nectar production could be explained by these two variables with root weight accounting for 67% of the explained variance. Root weight is a good indicator of a plants energetic status, indicating the importance of available energy in determining quantity of nectar produced. About 30% of the energy devoted to flowering is utilized in nectar production. Nectar production was significantly correlated with the male component of reproductive fitness, pollinaria removal, but not with the female component, pollinia insertion. Since pod production is limited by resource availability rather than the number of pollinia insertions, nectar production in A. quadrifolia is most closely associated with the maximization of the male function.

Milkweed loss in agricultural fields because of herbicide use

Abstract.  1. The size of the Mexican overwintering population of monarch butterflies 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).

The distribution of standing crop of nectar: what does it really tell us?

SummaryBrink (1982) characterizes the distribution of standing crop of nectar for Delphinium nelsonii as bonanzablank, based on comparison with a Poisson. He then discusses possible effects of standing crop variability on pollinator foraging behavior. We disagree with the use of the Poisson and the resulting conclusions. The expected distribution should not be based on doling out random amounts of nectar to flowers, but based on random return times to flowers by pollinators (elapsed time=nectar accumulated). When this model is used, standing crop variance does not differ markedly from expectation. What differences do exist can be accounted for by variability in nectar production rates of individual plants. We also take issue with the use of the bonanza-blank terminology. As originally formulated this refers to nectar production differences within a plant rather than standing crop differences among plants.

Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies ( Danaus plexippus )

The Eastern, migratory population of monarch butterflies (Danaus plexippus), an iconic North American insect, has declined by ~80% over the last decade. The monarch’s multi-generational migration between overwintering grounds in central Mexico and the summer breeding grounds in the northern U.S. and southern Canada is celebrated in all three countries and creates shared management responsibilities across North America. Here we present a novel Bayesian multivariate auto-regressive state-space model to assess quasi-extinction risk and aid in the establishment of a target population size for monarch conservation planning. We find that, given a range of plausible quasi-extinction thresholds, the population has a substantial probability of quasi-extinction, from 11–57% over 20 years, although uncertainty in these estimates is large. Exceptionally high population stochasticity, declining numbers, and a small current population size act in concert to drive this risk. An approximately 5-fold increase of the monarch population size (relative to the winter of 2014–15) is necessary to halve the current risk of quasi-extinction across all thresholds considered. Conserving the monarch migration thus requires active management to reverse population declines, and the establishment of an ambitious target population size goal to buffer against future environmentally driven variability.

Milkweed restoration in the Midwest for monarch butterfly recovery: estimates of milkweeds lost, milkweeds remaining and milkweeds that must be added to increase the monarch population

The loss of milkweeds in corn and soybean fields in the Midwest due to herbicide use has been implicated in the decline of the monarch butterfly population. In order to gauge the magnitude of the milkweed restoration effort necessary to compensate for this loss, it is necessary to know how many milkweeds have been lost and how many remain on the landscape. I estimate that since 1999, 850 million milkweeds have been lost from corn and soybean fields. In addition, since 2008, over 11 million milkweeds have been lost from grasslands due to their conversion into cropland, an annual loss rate of about 2 million milkweeds. Of the estimated 2.2 billion milkweeds present on the landscape in the Midwest in 1999, only 1.34 billion remained in 2014, a decline of almost 40%. But because each milkweed stem in an agricultural field averages 3.9 times more monarch eggs than a milkweed stem in non‐agricultural habitats, the potential monarch support capacity loss has been 71%. A conservation goal of 6 ha of overwintering butterflies has been proposed. It is estimated that the current milkweed population could support about 3.2 ha under average weather conditions in the breeding season. A total of 425 million milkweeds would need to be added to increase the monarch support capacity by just one more overwintering hectare and 1.6 billion would be needed to meet the 6 ha conservation goal. Thus, a massive milkweed restoration effort will be needed to produce a resilient monarch population.

Competition among Pollinators: Quantification of Available Resources

In a study of the bee community of short-grass prairie Tepedino and Stanton (1981) attempted to verify several predictions of competition theory by comparing abundances of bees and flowers. Their results were inconclusive suggesting, they said, that bees do not continuously compete for floral resources. We questioned their use of the number of open flowers as a measure of resource availability to bees because it is the nectar contained within those flowers that is the resource actually utilized. In two Rocky Mountain meadows we attempted to correlate bumblebee abundance with both the number of available flowers and the number of flowers multiplied by their 24 hour nectar production rates. Our results demonstrated that merely using floral abundance as an estimate of resource availability can be very misleading. The more time consuming method of quantifying the resources actually used by bees should be performed if meaningful conclusions concerning competition are to be drawn.