Dicamba—a drift-prone herbicide linked to millions of acres of off-target crop damage across in 17 states—destroys mostly everything in its path except the crops that are genetically engineered to resist it. It's so damaging that several states, including Arkansas, Tennessee and Missouri have introduced temporary bans on the weedkiller.
There's now another reason to worry about the controversial chemical. It's particularly harmful to milkweed, the only host plant for the iconic and already at-risk monarch buttery.
In a new report publisehd Thursday, researchers with the Center for Biological Diversity warn that the expanded use of dicamba, which is projected to increase by nearly 100-fold on cotton and soybean fields, will put more than 60 million acres of monarch habitat at risk by 2019.
Here are some key findings from the paper:
- Accelerating harm: In addition to 61 million acres of monarch habitat being directly sprayed with dicamba, an additional 9 million acres could be harmed by drift of the pesticide.
- Deadly timing: The timing and geographical distribution of dicamba use coincides precisely with the presence of monarch eggs and larva on milkweed.
- Double trouble: Dicamba degrades monarch habitat both by harming flowering of plants that provide nectar for adults as they travel south for the winter and by harming milkweed that provides an essential resource for reproduction.
- Greater menace to milkweed: Research has shown that just 1 percent of the minimum dicamba application rate is sufficient to reduce the size of milkweed by 50 percent, indicating it may have a greater impact on milkweed growth than the already widely used pesticide glyphosate.
Populations of the once-common butterfly have plummeted approximately 80 percent in just the last two decades. A major threat to the species is its loss of habitat due to the lack of availability of milkweed through land conversion for agriculture, removal of native plants and the use of pesticides.