A new UC Riverside study shows that a type of insecticide made for
commercial plant nurseries is harmful to a typical bee even when applied
well below the label rate.
The study was published on July 28, 2021, in the journal Proceedings of the
Royal Society B: Biological Sciences.
Chemically similar to nicotine, neonicotinoids are insecticides that protect
against plant-consuming insects like aphids, but seriously harm beneficial
insects, like bees. They are widely used by commercial growers.
Much research has focused on their use in food crops like canola, in which
they are typically applied at low doses. However, this study is one of the
few to examine neonicotinoid application in potted ornamental plants, which
can represent more potent, acute sources of exposure to the toxin for bees.
“Neonicotinoids are often used on food crops as a seed treatment,” explained
UCR entomologist and lead study author Jacob Cecala. “But they’re usually
applied in higher amounts to ornamental plants for aesthetic reasons. The
effects are deadly no matter how much the plants are watered.”
Cecala said he was surprised by this result, given that neonicotinoids are
water soluble. Going into the study, he assumed that more water would dilute
the amount of harm they caused the bees. The researchers were also curious
whether increased watering could benefit bees despite insecticide exposure
by increasing the quantity or quality of nectar offered by the plants.
To test these assumptions, the researchers raised bees on flowering native
plants in pots that either received a lot of watering, or a little. Plants
were selected based on their popularity at nurseries, drought tolerance to
ensure blooming even without much water, and their attractiveness to bees.
In addition, half the plants were treated with the insecticide.
Though increased water decreased the pesticide’s potency in the nectar of
the flowers, the negative effects on bees were still observed.
“Unfortunately, we observed a 90% decrease in the bees’ reproduction with
both high and low levels of irrigation,” Cecala said.
This study is also one of the few to examine neonicotinoid effects via
ornamental plants on solitary bees, which make up more than 90% of native
bee species in North America, and an even higher percentage in
California.
Solitary bees are not bees who have left the hive and are now alone. This is
a type of bee that lives alone, does not produce honey, and does not have a
queen or live in a hive. Because they do not have a store of honey to
protect, they are also not aggressive.
“Solitary bees are more representative of the ecosystem here, and they are
potentially more vulnerable to pesticides,” said UCR entomologist and study
co-author Erin Rankin.
If a worker bee that is social — like the honeybee — gets exposed to
insecticide and dies, it won’t necessarily affect the longevity of the hive.
However, if a solitary bee dies, its lineage is cut short.
In this study, the researchers used alfalfa leafcutter bees, which make
their nests in tunnels and lay eggs one at a time. They are very similar to
California’s solitary native bees and are part of a genus that can be found
worldwide.
The first time Cecala and Rankin tried this experiment, they used the
concentration of insecticide recommended on the product label. All the bees
died in a matter of days.
The next time they ran the experiment, they used a third of the recommended
dose and still found negative effects on reproduction, the ability of the
bees to feed themselves, and overall fitness. “It almost completely wiped
them out,” Cecala said.
Though this study used a neonicotinoid product formulated for nurseries,
formulations of similar products for home gardeners also tend to be highly
concentrated.
Plants in nurseries or residential backyards represent a smaller total area
than food plant fields like corn or soy. However, high-potency neonicotinoid
products can have a big effect even in small areas. In 2013, neonicotinoids
applied to flowering trees in a retail parking lot in Oregon caused a
massive bumblebee die off, with more than 25,000 found dead.
The researchers recommend that nurseries quantify the amount of pesticides
that are making their way into flowers given their watering and pesticide
regimes, and consider alternative management practices that reduce harm to
bees and the ecosystems dependent on them.
“It’s not as simple as ‘don’t use pesticides’ — sometimes they’re
necessary,” Cecala said. “However, people can look for a different class of
insecticide, try to apply them on plants that aren’t attractive to bees, or
find biological methods of pest control.”
Reference:
Pollinators and plant nurseries: how irrigation and pesticide treatment of
native ornamental plants impact solitary bees” by Jacob M. Cecala and Erin
E. Wilson Rankin, 28 July 2021, Proceedings of the Royal Society B
Biological Sciences.
DOI: 10.1098/rspb.2021.1287
Tags:
Plants & Animals