Biology Reference
In-Depth Information
For some plants, such as corn (
Zea mays
), pollen containing Bt endo-
toxin may be carried by wind to areas outside cropland, where impacts on
nontarget species might occur. At least 206 nontarget species of lepi-
doptera in the United States belonging to 20 families have been recorded
from plants growing near enough to corn fields to be affected by corn
pollen (Losey et al. 2002).The species of greatest concern is the monarch
butterfly (
Danaus plexippus
), a migratory species that winters in very local-
ized areas, some of which are becoming subject to severe human impacts.
Monarch larvae feed on various species of milkweeds (
Asclepias
spp.),
many of which tend to occur in weedy fields and fencerows, often near
cornfields. Pollen of Bt corn deposited on milkweed leaves has been
shown experimentally to increase the mortality of monarch larvae (see,
e.g., Jesse and Obrycki 2000). Whether the risk from this factor exceeds
that from the use of insecticides that are an alternative for control of corn
pests, however, is uncertain.
Transgene effects may also influence ecological processes at higher
trophic levels. Shifts may occur in insect pest or disease problems in crops
carrying genes for specific resistance to pest organisms of these types.
Genes that encode Bt endotoxins, for example, could affect predatory
insects.The endotoxins produced in crop plant tissues may be transferred
through food chains to foliage-eating insects and to soil organisms
(Hilbeck 2001; Groot and Dicke 2002). Bt endotoxin taken in by a moth
feeding on transgenic maize, for example, increased mortality and delayed
development in green lacewings (
Chrysoperla carnea
) that fed experimen-
tally on the moth larvae (Dutton et al. 2002).The potential for secondary
effects such as this is not well understood.
Heavier use of herbicides and other pesticides has serious implications
for native biodiversity. Nontarget plants and animals are more likely to be
affected as use of pesticides intensifies. Heavier use of pesticides, com-
bined with increased pest resistance, will make control of alien plants and
animals in natural areas much more difficult.
Transgenes and Counterevolution
Many transgenes considered for introduction to crop plants act against
arthropod pests or plant disease agents. As in the case of chemical pesti-
cides, the potential exists for pests and disease agents to evolve resistance
to these transgenic agents. This possibility has been considered for crops
with the Bt endotoxin transgene. Several crop insect pests have evolved
resistance to Bt endotoxin, in most cases at a fitness cost in an endotoxin-
