Both herbicide and pesticide-resistant crops have an effect on local pollinator populations. Herbicide-resistant crops are modified for immunity to specific herbicides to allow for broad spectrum spraying without crop loss. There is evidence that this practice leads to no flowering weeds within and adjacent to the treated area. This results in a decrease in pollinator habitat and leads to notable habitat fragmentation where pollinator friendly green spaces are separated by large flowerless expanses. Broad spectrum application also removes any flowering populations from row mows, barrier strips, guard and fell rows, and any adjacent natural or unmanaged areas.
Insect resistant crops such as those transformed with genes from Bacillus thurigiensis (Bt) produce their own chemical defense. Bt corns mode of action ruptures the gut lining in the target species of the corn borer and corn earworm.
Current research suggests that there is no threat to monarch butterfly larvae from Bt corn pollen when considering the level at which pollen toxicity occurs and the level in which Bt corn pollen may be present on neighboring milkweed.
The main effect it will have is that some pest insects are becoming resistant to Bt , which is a very non toxic control method for some insects. This may led to farmers having to resort to relying on more toxic substitutes and foliar sprays.
Case Study: Bt Corn pollen causes decline in Monarchs. Fact or Fiction?
Research published in 1999 as a note in Nature Journal by Cornell researches indicated that Bt corn pollen had a detrimental effect on monarch larvae. During the study the caterpillars were given no choice but to feed on milkweed leaves heavily dusted with Bt corn pollen. After publication this article gained traction in the media and soon became a linchpin in the debate against genetically modified organisms. However the research was considered a trial and many scientists and entomologists had speculated on the less than desirable experimental design (lack of control) and replication (small number of subjects tested) in the experiment. However findings warranted further research.
After this research was conducted and published a more comprehensive study was carried out by a co-operative research group formed by the Agricultural Research Services (ARS) for the United States Department of Agriculture (USDA) . They had posed two major questions from the 1999 study. Firstly how much pollen does it take for Monarch caterpillars to receive a toxic concentration of the protein that ruptures the gut lining and leads to mortality?. Secondly how likely is it for a caterpillar to ingest such concentrations from neighboring milkweed communities? The findings of the research can be found here.
They concluded that the risks of Bt corn pollen on monarch butterfly caterpillars is negligible This study is more widely respected for its design and findings than the first study.