Corn pollen deposition on milkweeds in and near cornfields

Abstract

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 cm 2 ) inside the cornfield and was progressively lower from the field edge outward, falling to 14.2 grains per cm 2 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 cm 2 and the highest pollen density observed was 1400 grains per cm 2 , 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. In order to accurately interpret results of studies that examine the effects of Bt corn pollen on monarch butterfly larvae it is necessary to know the range and distribution of naturally occurring pollen densities on milkweed leaves. This provides a perspective on both laboratory and field studies in which monarch larvae feed on milkweed leaves with Bt corn pollen (1, 2). It lets us determine how frequently the pollen densities observed in these studies would occur in nature. The studies reported here contribute to the exposure characterization necessary for assessing the risk of Bt corn pollen to monarch butterflies. In particular, this paper describes the densities of corn pollen on milkweed leaves during corn anthesis for a number of geographic locations and under a variety of environmental conditions. We describe the pollen densities (pollen grains per cm 2 ) that were found on leaves of milkweed plants within cornfields as well as near cornfields because corn pollen is wind-dispersed at least 60 m (3) and possibly more than 200 m (4). These data are used in a companion paper (5) on the results of laboratory studies on the responses of monarch larvae fed milkweed leaves with different densities of artificially applied Bt corn pollen. These data are also used in a second companion paper (6) to provide a frame of reference for the Bt pollen densities found in field trials of larvae feeding on milkweed leaves. Finally, these data are used in a summary companion paper (7) that provides a full risk assessment of monarchs and Bt corn pollen. In addition to characterizing naturally occurring pollen densities, we examined several factors that affect pollen deposition on milkweed leaves, including position of a leaf on the plant and rainfall. Materials and MethodsThis article includes the results of several studies conducted at d...

Toxins in transgenic crop byproducts may affect headwater stream ecosystems

“…Crop byproducts from Bt corn contain this toxin (3,4), but until now the effects of Bt corn byproducts on stream organisms have not been examined. This is in sharp contrast to numerous studies examining potential effects on nontarget organisms in the terrestrial environment (4)(5)(6)(7)(8).…”

“…During pollen shed, wind can transport corn pollen from 40 to 60 m away from source fields (10), and rain can dislodge and transport pollen away from crops (6). After harvest, crop byproducts remain on fields and may be transported to adjacent streams via wind and water.…”

The release of genetically modified crops into the environment

“…This study led to considerable debate over the environmental impact and relevance for the potential risks from Bt maize. Follow-up studies to investigate the impact of widespread plantings Bt-maize on the Monarch butterfly essentially concluded that the impact of Bt-maize pollen from current commercial hybrids on Monarch butterfly populations is negligible Oberhauser et al, 2001;Pleasants et al, 2001;Sears et al, 2001;Stanley-Horn et al, 2001;Zangerl et al, 2001). This is based on the low expression of Bt toxin genes in pollen for most maize hybrids, lack of acute toxicity at expected field rates, limited overlap of pollen shed and larval activity, and the limited overlap in distribution of Bt-maize and milkweed.…”

Impact environnemental des cultures transgéniques

“…La toxine Cry1Ac purifiée d'une lignée de coton transgénique, par exemple, s'est avérée bénigne en laboratoire pour une dizaine d'insectes herbivores ou prédateurs non ciblés, mais toxique à fortes doses contre plusieurs insectes lépidoptères apparentés à l'insecte visé (Sims 1995 Zangerl et al 2001). Si des facteurs variés comme le faible taux d'expression de la toxine Cry1Ab dans les lignées de maïs Bt actuellement disponibles, la faible quantité de pollen déposée sur les feuilles d'asclépiade dans les champs ou la préférence des adultes pour des feuilles d'asclépiade sans pollen au moment de la ponte Pleasants et al 2001;Tschenn et al 2001) suggèrent des impacts négligeables de ces hybrides sur le papillon en comparaison aux effets négatifs des mesures phytosanitaires adoptées pour la répression des mauvaises herbes et des insectes en systèmes conventionnels (Gatehouse et al 2002;Sears et al 2001;Shelton et Sears 2001;Wolt et al 2003 Les différentes études menées en laboratoire et en champ concernant l'impact des cultures transgéniques Bt sur la croissance, le développement ou la survie des arthropodes herbivores non ciblés ont conduit à des conclusions variables en fonction : (i) du degré de parenté entre l'insecte cible et l'organisme non ciblé; (ii) de la nature de l'interaction entre l'organisme étudié et la plante modifiée; et (iii) de la lignée végétale utilisée pour les tests (Tableau 1). En lien avec l'action des toxines du Bt contre certains groupes d'insectes, des effets négatifs ont été notés dans les cas où l'espèce considérée était apparentée à l'insecte visé par la toxine exprimée.…”