Researchers find herbicides may not be only factor to affect species diversity.
THE increasing use of chemical herbicides is often blamed for declining plant biodiversity on farms; however, other factors beyond herbicide exposure may be more important to species diversity, according to researchers at Pennsylvania State University.
If herbicides are a key factor in the declining diversity, then thriving species would be more tolerant to widely used herbicides than rare or declining species, according to J. Franklin Egan, a research ecologist with the U.S. Department of Agriculture's Agricultural Research Service.
"Many ecotoxicology studies have tested the response of various wild plant species to low-dose herbicide exposures, but it is difficult to put these findings in context," Egan said. "Our approach was to compare the herbicide tolerances of plant species that are common and plant species that are rare in an intensively farmed region. We found that rare and common plant species had roughly similar tolerances to three commonly used herbicides."
This could mean that herbicides may not have a persistent effect in shaping plant communities.
The researchers, who reported their findings in the online version of the journal Environmental Toxicology & Chemistry, said over the past several decades — the same time frame that the use of herbicides was on the rise — other factors such as the simplification of crop rotations, segregation of crops and livestock and increasing mechanization have also been rapidly evolving. In addition, clearing woodlots, hedgerows, pastures and wetlands to make way for bigger farm fields has continued apace and resulted in habitat loss.
While the findings are preliminary, the approach could be effective in clarifying the implications of herbicide pollution for plant conservation, Egan said.
"These findings are not an invitation to use herbicides recklessly," he said. "There are many good reasons to reduce agriculture's reliance on chemical weed control, but for the objective of plant species conservation, other strategies, like preserving farmland habitats — including woodlots, pastures and riparian buffers — may be more effective than trying to reduce herbicide use."
Farmers should always take extra precautions when using herbicides to avoid unintended consequences like drifting herbicides on neighboring fields and farms.
Researchers have found a range of effects — positive, neutral and negative — when the herbicide dicamba was sprayed on old fields that are no longer used for cultivation and on field edges, according to Egan. He said the effects should be similar for the related 2,4-D compound.
"The general consensus is that the effects of the increased use of these herbicides are going to be variable," Egan said. However, "given that there is really so much uncertainty, we think that taking precautions to prevent herbicide drift is the right way to go."
Farmers are expected to use dicamba and 2,4-D on their fields more often in the near future because biotechnology companies are introducing crops that have been genetically modified to resist those chemicals. In the past, 2,4-D and dicamba were the herbicides most frequently involved in accidental herbicide drift, according to the researchers.
Because the herbicides typically target broadleaf plants, such as wildflowers, they are not as harmful to grasses, Egan said. In the study, the researchers found that grasses eventually dominated the field edge test site that was once a mix of broadleaf plants and grasses. The old field site showed little response to the herbicide treatments.
Herbicide drift was also associated with the declines of three species of herbivores — including pea aphids, spotted alfalfa aphids and potato leaf hoppers — and an increase in a pest called clover root curculio, Egan said. The researchers found more crickets — which are considered beneficial because they eat weed seeds — in the field edge site.
The research findings, reported in a recent issue of Agriculture, Ecosystems & Environment, did not reveal a drop in the number of pollinators, such as bees, in the fields. However, the relatively small size of the research fields limited the researchers' ability to measure the effect on pollinators, according to Egan.
Farmers can cut down on herbicide drift by taking a few precautions, Egan noted. They can spray low-volatility herbicide blends, which are less likely to turn to vapors, and use a nozzle design on the sprayer that produces larger droplets that do not easily drift in the wind. Egan also recommended that farmers follow application restrictions printed on herbicide labels and try to spray on days that aren't so windy, whenever possible.