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Bt Pollen Deposition & Monarch Larvae |
Richard L. Hellmich
USDA-ARS, Corn Insects and Crop Genetics Research Unit
and Department of Entomology, Iowa State University
Ames, Iowa
Bt Pollen Effects on Monarch Larvae. Richard L. Hellmich and Leslie C. Lewis, USDA-ARS, Corn Insects and Crop Genetics Research Unit, and Department of Entomology, Iowa State University, Ames, Iowa; and John M. Pleasants, Department of Zoology and Genetics, Iowa State University, Ames Iowa.
Laboratory tests were conducted to determine whether monarch butterfly larvae, Danaus plexippus, prefer or avoid milkweed leaves with Bt pollen, and to determine whether Bt pollen influences larval weight and survival. Each larva was presented with two leaf disks from common milkweed, Aesclepias syriaca, in a small petri dish with an agar substrate. Pollen was suspended in water and applied to leaves with a thin layer chromatography sprayer. Three types of Bt pollen (MON810, CBH351, and 176) were tested with two controls: no pollen and nonBt pollen. The five treatments resulted in 15 pair-wise comparisons which were replicated six times. Leaf choice, leaf feeding (mm2), larval weight (mg), and larval survival were determined. Several trials were conducted with pollen densities in three categories: ~ 600 pollen grains/cm2 (very high, uncommon amount found in our field studies); ~ 150 pollen grains/cm2 (high: found infrequently, only within 1 meter of field edge during pollen shed); and ~ 60 pollen grains/cm2 (moderate: found within 2 meters of field edge during pollen shed). Choice and feeding results suggest that monarch larvae are influenced by the presence of pollen. When presented with leaves with no pollen and very high amounts (~ 600 grains/cm2) of pollen, more larvae were found on the leaves with no pollen. Weight and survival data indicate that larvae feeding on milkweed leaves with high (~150 grains/cm2) and moderate (~ 60 grains/cm2) amounts of pollen from the MON810 and CBH351 are not affected. At the very high levels of pollen (~ 600 grains/cm2) these same larvae were smaller than larvae from the control treatments, but survival was not different from that of the controls. Larvae feeding on leaves with pollen from the 176 type of corn were affected at each of the pollen density levels. Bt 176 corn represents about 2.5% of total corn planted in the United States.
Pollen deposition on milkweed leaves under natural conditions. John M. Pleasants, Department of Zoology and Genetics, Iowa State University, Ames, Iowa; and Richard L. Hellmich and Leslie C. Lewis, USDA-ARS, Corn Insects and Crop Genetics Research Unit, and Department of Entomology, Iowa State University, Ames, Iowa.
This study was designed to determine the Bt pollen density on milkweed leaves to which monarch larvae might be exposed under natural conditions. Transects were set up at 7 different nonBt corn fields from the edge of the field up to 200 meters, in all 4 compass directions. At points along the transect, pollen deposition was sampled using portions of milkweed stems bearing 2 leaves, potted milkweed plants, and slides coated with glycerin. The sampling period was about 4 days, covering peak anthesis. Leaves and slides were then examined under a microscope and counts made of the number of corn pollen grains per unit area. These field-sampling and counting methods proved to be very reliable in assessing pollen deposition. The pollen in the air at a given distance, as determined from amounts on glycerin slides, decreased rapidly with distance from the edge of the downwind sides of the fields. Pollen amounts became negligible beyond 60 meters. The amounts of pollen deposited on milkweed leaves was much less than on slides. On average, milkweed leaves captured only 30% of the pollen available. Our feeding trials with monarch larvae and 2 types of Bt pollen indicated that leaves with pollen densities below 150 grains/cm2 had no effect on larval weight or survivorship. Over all 7 fields and compass directions, the proportion of leaves with pollen densities below 150 grains/cm2 was 80% for leaves at the edge of a corn field (0 meters), 88% at 1 meter, and 100% for 2 meters and beyond. Beyond 8 meters the pollen density on leaves was essentially zero. These results are for pollen deposition under non-rainy conditions. Comparisons of leaves before and after rains indicated that heavy rains reduced the amount of pollen on leaves by about 90%.
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