12
May
Ecological Mystery Unravels, With Toxic Pesticide Use at the Center
(Beyond Pesticides, May 12, 2021) Earlier this year, a team of scientists solved an ecological mystery that had persisted for decades. Throughout the southeastern United States, bald eagles and other top-level avian predators were experiencing mass deaths from a disease known as vacuolar myelinopathy (VM), a neurological ailment that causes lesions in affected animal’s brains. Scientists identified the source of the exposure as a cyanobacteria growing on an invasive weed, but up until now, did not know how the bacterium caused disease. Now, scientists have determined that the chemical bromine, likely introduced by brominated herbicides in attempts to manage the invasive species, is the trigger for the production of the cyanobacteria’s neurotoxin.
In the mid-1990s, over 70 bald eagles died in Arkansas’s DeGray Lake over the course of two years. The event was the largest mass mortality of eagles recorded. Scientists identified the disease as vacuolar myelinopathy, and through the course of several years were able to determine that the disease generally affected birds in the built environment, near artificial bodies of water with high levels of aquatic plant life. Waterfowl and other bird species were found to develop lesions in lakes where there was an ongoing VM outbreak.
Evidence built that hydrilla (Hydrilla verticullata) a common invasive submerged aquatic plant found throughout the U.S. was part of the problem. In particular, scientists were concerned with a cyanobacteria called Aetokthonos hydrillicola, found to be growing on the backside of the plant. To determine how the bacterium caused the disease, researchers cultured it in the lab (after significant trial and error), and exposed birds under controlled conditions. But they found that exposed birds did not develop the lesions characteristic of VM. Scientists subsequently considered that it may be an environmental factor resulting in the changes to the cyanobateria when submerged in water.
A. hydrillicola was then observed under an advanced imaging process, which detected the presence of metabolites containing bromine atoms. The presence of bromine was the key to the mystery – the process scientists had used to culture the bacteria in the lab did not contain bromine. When they added it to the medium, A. hydrillicola began to produce a potent neurotoxin.
Researchers have subsequently found evidence that the toxin produced by the bacteria can lodge itself in an animal’s gut, and move up the food chain. This bioaccumulation poses significant hazards– not only to birds of prey, but potentially many animals up the food chain.
Most sources of bromine in a freshwater ecosystem are likely to be added by humans. Flame retardants and other industrial compounds like coal and fracking pollution can contain bromine. But one of the most likely sources is the pesticide diquat dibromide. Pesticide products containing diquat dibromide are often applied in attempts to manage invasive hydrilla. However, the quick spreading nature the plant is almost certain to leave many behind to recolonize man-made freshwater lakes and ponds.
As the study suggests, “Benefits and risks of using any bromide-containing chemical control agents within VM reservoir watersheds need to be reassessed.” The mystery is a posterchild for how counterproductive and difficult it can be to determine non-target effects of pesticide use. Beyond Pesticides has covered a range of ways in which pesticides can cause unintended effects that spread across ecosystems with deleterious trophic cascades. The use of a bromide-based product, intended to kill a plant that harbors a bacterium that, in the presence of bromine, produces a lethal neurotoxin is far beyond the scope of any risk assessment conducted by pesticide regulatory agencies.
While it may be tempting to excuse regulators for that oversight, this is simply another instance in a long line of “big mysteries” that are ultimately linked to pesticide exposure – from pollinator declines to the death of coniferous trees throughout the U.S.
The findings highlight to importance of integrating alternative assessments into pesticide regulation. There are non-toxic, biological controls for hydrilla that are working well and do not required a brominated pesticide. Triploid sterile grass carp, which have a lower risk of escape and reproduction, are being used successfully to control hydrilla in many VM reservoirs, according to the study. When viable alternatives exist for a given use that do not necessitate the application of hazardous pesticides, an alternative assessment would reject the registrations of these risky and unnecessary products in favor of safer practices.
We can change the way toxic pesticides are allowed onto market and manage pest problems without unnecessary hazards through dedication and education. Join Beyond Pesticides for a discussion around many of these themes at the upcoming, first ever virtual National Pesticide Forum, starting May 24th and 25th, and running every subsequent Tuesday until June 15th. Registration options are available for every budget. See here for a complete program and schedule of events.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source: Science (article), Science (peer reviewed study)
