A leafcutter bee of alfalfa, used by UC Riverside researchers to study the effects on pesticides and water levels. Credit: David Rankin/UCR
A new study by UC Riverside shows that an insecticide designed for commercial plant nurseries can be harmful to the common honeybee, even when used at a lower rate than the label rate.
Today's publication of the study appeared in Proceedings of the Royal Society B: Biological Sciences.
Neonicotinoids, which are chemically similar to nicotine in their action, are insecticides that protect plants-consuming insects such as aphids but also harm beneficial insects like bees. These insecticides are used extensively by commercial growers.
Many studies have focused on their use as food crops such as canola. They are usually applied in low doses. This study, however, is the first to investigate neonicotinoid use in potted ornamental crops. These plants can be more severe and acute sources of exposure for the bees.
Jacob Cecala, UCR entomologist, and lead author of the study, explained that "Neonicotinoids can be used on food crops as seed treatments." For aesthetic purposes, they are usually used in greater amounts on ornamental plants. No matter how well the plants are watered, they can cause death.
Cecala stated that he was surprised at the result because neonicotinoids can dissolve in water. Cecala assumed that bees would suffer less damage if they were given more water. Researchers were also interested in whether bees could benefit from increased watering, despite being exposed to insecticides by increasing the quality or quantity of nectar provided by the plants.
The researchers tested these assumptions by raising bees on native flowering plants in pots that received either a lot or very little watering. The selection of plants was based on their popularity in nurseries, drought tolerance so that they could bloom even without water and their appeal to bees. Half of the plants were also treated with insecticide.
Although the effectiveness of pesticides in nectar was reduced by increasing water, bees still suffered from their negative effects.
Cecala stated that the bees reproduced at a lower rate with low and high levels of irrigation.
This study also examines neonicotinoid effects via ornamental plant on solitary honeybees. These bee species make up over 90% of North American native bee species and even more in California.
Alfalfa leafcutter female bees build nests from individual cells like these. Credit: Jacob Cecala/UCR
Solitary bees do not refer to bees that have left their hive and are now living alone. This bee is an individual that does not live with a queen and doesn't produce honey. They are not aggressive because they don't have honey to protect them.
Erin Rankin, UCR entomologist, said that "solitary bees are more representative" of the ecosystem and are therefore more susceptible to pesticides.
It won't affect the longevity or health of a worker bee, which is usually social like the honeybee. A solitary bee's death will end the lineage of its offspring.
The researchers used alfalfa Leafcutter Bees to study the phenomenon. These bees build their nests in tunnels and then lay eggs one by one. They look very similar to California's native solitary bees, and they are part of a worldwide genus.
Cecala, Rankin and others tried the experiment for the first time. They used the recommended insecticide concentration on the product label. In a matter of days, all the bees died.
They used only a third of what was recommended for the next experiment and still had negative effects on reproduction, bees' ability to feed themselves and overall health. Cecala stated that the dose almost eliminated them.
Although this study was done with a neonicotinoid product meant for nurseries, similar products that are suitable for home gardeners can also be found in high concentration.
The area of plants in nurseries and residential backyards is smaller than that of food plant fields such as corn or soy. High-potency neonicotinoid product can still have an impact even in small areas. A massive bumblebee death was caused by neonicotinoids being applied to flowering trees at a Oregon retail parking lot.
Researchers recommend that nurseries measure the amount of pesticides entering flowers based on their watering and pesticide regimens. They also suggest that they consider other management methods that will reduce the harm to bees as well as the ecosystems that depend on them.
Cecala stated that "it's not as easy as saying "don't use insecticides" sometimes they are necessary. People can choose to use a different type of insecticide or look at biological pest control methods.
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More information: Jacob M. Cecala et.al, Pollinators & Plant Nurseries: How irrigation and pesticide treatments of native ornamental plant plants impact solitary honeybees, Proceedings of the Royal Society, B: Biological Sciences (2021). Information from the Journal: Proceedings of the Royal Society, B Jacob M. Cecala and co, Pollinators, and Plant Nurseries: How irrigation and pesticide treatments of native ornamental plants affect solitary honeybees, (2021). DOI: 10.1098/rspb.2021.1287