Whether the weedkillers are causing depression "is not clear," said Marc Weisskopf, the study's lead author and an associate professor at the Harvard School of Public Health. "But (the result) suggests we should not be ignoring herbicides just because they're targeting plants."
Earlier research on depression and pesticides has focused on insecticides, particularly organophosphates, which are known to be toxic to nerve cells, said Weisskopf.
Monocrotophos, the insecticide that killed 23 school children in India this month, is an organophosphate, for example.
The use of pesticides has also been linked to Parkinson's disease among farmers (see Reuters Health story of May 28, 2013 here:).
As part of a study on Parkinson's disease, Weisskopf and his colleagues assessed the risks for depression with exposure to any kind of pesticide by surveying 567 French farmers about their use of fungicides, insecticides and herbicides.
The team conducted home visits to get a detailed assessment of chemical exposures, including going over bills for pesticide purchases, looking through farming calendars and inspecting old pesticide containers.
They also asked the farmers whether they had ever been treated for depression.
Weisskopf's group reports in the American Journal of Epidemiology that 83 farmers, about 15 percent, said they had been treated for depression. Forty-seven of them had never used pesticides, while 36 had.
Among the farmers without Parkinson's disease, 37 who had never used herbicides and 20 who had used the weedkillers reported being treated for depression.