By Sara La Jeunesse and John Vogel

Penn State University researchers have come up with yet another reason why you may need to upgrade your herbicide spray drift controls.Dicamba drift affects non-target plants and pollinators

 Even at low application rates, dicamba herbicide drift onto plants growing adjacent to farm fields causes significant delayed and reduced flowering of alfalfa and other plants. That spells fewer pollinator visits to plants produced for seed.

INJURED ALFALFA: Just 0.1% to 1% of expected dicamba field application rates negatively influence alfalfa flowering.

New types of transgenic crops resistant to synthetic-auxin herbicides including dicamba and 2,4-D will be widely planted in coming growing seasons, says John Tooker, Penn State Extension entomologist. "It raises concerns about crop damage from these drift-prone herbicides.

"The expected high rate of adoption of the new transgenic crops will increase dicamba and 2,4-D use by four to eight times. Our results suggest that widespread non-target damage from these herbicides may adversely affect pollinator communities."

The research team, including cooperators from the Pennsylvania Association for Sustainable Agriculture, examined alfalfa which requires insect pollination for seed production. They also studied common boneset, a native species that's highly attractive to a wide range of pollinators.

Note: Little commercial alfalfa seed production is done in the East. Most is produced in the West and Northwest.

The findings
A range of sub-lethal dicamba doses – not spray drift – were applied directly to the alfalfa and boneset. Then, flowering and floral visitation by insect species was tracked. Pollen quality was also analyzed to determine if herbicide exposure altered pollen quality.

The scientists found that exposure to drift-level doses of herbicide reduced flowering in both. Common boneset experienced significantly reduced visitations by insect species, including honeybees pollinators and syrphid flies.

"Both plant species are susceptible to very low rates of dicamba," points out Tooker. "Just 0.1% to 1% of the expected field application rate can negatively influence flowering. By extension, we expect that other broadleaf plant species are similarly susceptible to this sort of damage from drift-level doses."

Extended seasonal risks
Synthetic-auxin herbicides, such as dicamba and/or 2,4-D, are usually used early in the growing season. "But with the new transgenic crop varieties coming on the market, these herbicides will be used later when temperatures are warmer and more plant species are leafed out," notes David Mortensen, Penn State plant scientist.

"This'll lead to higher levels of non-target damage to susceptible crop plants and native, wild vegetation," he adds. "For susceptible crop varieties, this sort of damage could reduce yield. For non-crop plant species, this drift-induced damage could significantly decrease the pollinator and natural enemy communities that these plants can support."

The team currently is working to determine the proportion of non-crop habitat in the corn/soybean belt that could be degraded for pollinator communities by drift. "This is particularly relevant because the great majority of floristic diversity in agro-ecosystems resides in small fragments of semi-natural habitat around arable fields," he explains. "The quality of these fragments may be substantially degraded by repeat exposure of dicamba or 2,4-D drift."

La Jeunesse writes for Penn State University