Fungicide recommendations suggest that fungicides should not be applied to fruit and nut trees when honey bees or blue orchard bees are flying or when pollen is being shed.
However, some fungicides alter the foraging behavior of bees and can be toxic to adults or larvae. Fungicides can interfere with pollen tube germination and damage the stigmas of flowers. This can result in fewer cross-pollinated flowers and reduced yields.
During the 2008 almond bloom, scientists at the USDA-ARS Carl Hayden Bee Research Center, Tucson, Ariz., studied the effects of supplemental protein feeding on honey bee colonies during almond pollination in the Bakersfield, Calif., area.
Just past full bloom one grower sprayed Propiconazole, a broad spectrum fungicide, on every other row during daylight hours. Applications were repeated for several days since the fungicide was not applied prior to bud break due to poor weather.
The bees stopped foraging along the sprayed rows. The week after the spraying, given the weather conditions and number of open blossoms, only about half of the honey bees that should have been foraging on the almond blossoms were actually on the blossoms. In addition, dead bees were found outside of several colonies which suggested the sprays were toxic to the bees.
Other studies have reported adult bee toxicity from fungicide applications. Scientists working with the blue orchard bee in a commercial cherry orchard in California's Central Valley reported decreased foraging after applications of fungicides, surfactants, and foliar fertilizers. This prompted investigations on the toxicity of several fungicides on honey bees and blue orchard bees.
The scientists found that Propiconazole use was toxic and reduced the survivorship of adult bees in both species. However, recommended field application rates of Propiconazole are considered too low to kill a substantial number of bees, but when mixed with surfactants, fertilizers, or residual amounts of insecticides left in tank sprayers, the solutions could be more toxic to bees than each compound alone.
One study found that Propiconazole mixed with a pyrethroid insecticide was 16.2 times more toxic to bees than the insecticide alone. Similarly, research has shown that feeding larval honey bees pollen contaminated with fungicides can lead to increased mortality. Exposure to pollen containing captan, ziram, or iprodione led to 100 percent mortality of larvae.
One possible reason is when honey bees collect pollen contaminated with fungicides the levels of the compounds become higher in the stored pollen than in the pollen brought back to the hive by the foragers.
High levels of fungicides in stored pollen might also inhibit the growth of certain strains of fungus that are necessary to convert pollen into bee bread. The loss of the beneficial fungus could reduce the nutritional value of the pollen to bees.
Aside from potentially harming and deterring bees from visiting blossoms, fungicides can also reduce fruit or nut set even though pollination has occurred. For the blossom to set, pollen deposited on the stigma must germinate and grow a tube down the length of the style to the ovary where ovule fertilization occurs.
In laboratory experiments, most almond pollen exposed to fungicides failed to germinate. When germination occurred, the pollen tubes frequently didn't reach the ovary for fertilization. A fungicide application to the stigma can further prevent fertilization due to stigmatic surface damage. This reduces the pollen reaching the stigma and germination.
Other experiments suggest that negative fungicide effects on pollen germination and pollen tube growth varies by the crop and only last while the fungicide is wet. This can reduce pollination success on the day of the compound application.
Brown rot and other diseases can significantly harm a crop, but the recommended cure can reduce pollination potential and may harm the bees.
So what should growers do? First, make sure the tank sprayer is clean and free of insecticide residue. Apply fungicides at the end of bloom. If this is not possible, apply several days before the honey bees are brought to the orchard.
In almonds and apples, cross-pollination and nut set cannot occur until the pollen is available from at least two compatible cultivars so there is no need to bring in the bees early. This provides more time to safely spray before the bees arrive; reducing the likelihood of exposing bees and open flowers to fungicides. Applying fungicides in the evening or at night after the blossom has shed its pollen might reduce the harmful effects on bees. However, even nighttime applications might keep bees from foraging on trees for several days. Nighttime applications might still be potentially damaging to the stigmas in open blossoms.
Not every commercial fungicide has been tested for its impact on bees, pollen, or stigmas. Of those tested, not all are equally harmful.
It's best to assume however, that all fungicides might present some potential hazards and care should be taken in product use.