Are sightings of lots of dead bees cause for pesticide concern? It’s ‘hard to tell’

A bee forages in a rose in La Jolla.
A bee forages in a rose in La Jolla.
(Elisabeth Frausto)

After noticing dead bees by the dozens in and around La Jolla, some are raising questions about why the bees died, including whether pesticides had anything to do with it.

According to experts approached by the La Jolla Light, the answer is — maybe, or maybe not.

La Jolla resident Molly Bowman Styles is one who raised the pesticide issue. She said she saw about two dozen dead bees on the sidewalk during her evening walks over a few days in July.

Beekeeper Hilary Kearney, who manages hives all over San Diego County, including two in La Jolla, and provides workshops and other support for beekeepers, said she has noticed that La Jolla, Del Mar and Point Loma “tend to have more pesticide poisonings” of bees.

Beekeeper Hilary Kearney took this photo of a cluster of dead bees in Del Mar.
Beekeeper Hilary Kearney took this photo of a cluster of dead bees in Del Mar. She recommends avoiding pesticides or using them at night to prevent bees from potentially sharing them and poisoning an entire colony.
(Hilary Kearney)

Kearney, who also manages the bees at the San Diego Zoo and San Diego Natural History Museum, said residents or their hired landscapers often “haven’t necessarily made it a priority” to avoid using pesticides.

“When a gardener applies a pesticide improperly or treats flowering plants, that’s when we see a sudden influx of bees dying,” she said. “We can lose the entire colony in one afternoon.

“The bees eat that poisoned nectar and they bring it back to the hive and they share it with their nest mates, and then everyone gets poisoned.”

David Holway, a professor of ecology, behavior and evolution at UC San Diego whose field of study focuses on invasions of social insects such as ants, bees and wasps, said he assumes the observed dead bees are honeybees, which are “among the most abundant bee species in many parts of Southern California.”

Of the more than 600 species of bees in San Diego County, the honeybee is the most common, Holway said. “Sometimes people are alarmed to hear that honeybees are declining, but it is worth noting that in North America, they’re a non-native species,” he said.

“It’s normal to see some dead bees around,” said Bodil Cass, an agricultural scientist with San Diego County who directs the county’s honeybee protection program that works with commercial and “backyard” beekeepers.

Each honeybee queen produces hundreds to a thousand eggs a day, Cass said, and bees that are flying around foraging for pollen and nectar are older ones “at the end of their life.”

“We’re not concerned if it’s a dozen [dead] bees,” Cass said.

A dead bee in La Jolla
“It’s very hard to tell whether the honeybees that people are seeing on the sidewalk or on the beach are succumbing to pesticide exposure or some other cause,” says UC San Diego professor David Holway.
(Elisabeth Frausto)

Periodic collapses in populations of western honeybees have been reported since the late 19th century. Earlier this century, such a die-off, called “colony collapse disorder,” created headlines in North America.

Several possible causes have been proposed, including pesticides, pathogens transmitted by mites, malnutrition, genetics, immunodeficiencies, loss of habitat, changing beekeeping practices or a combination. But no one cause has been settled on among scientists.

“It’s very hard to tell whether the honeybees that people are seeing on the sidewalk or on the beach are succumbing to pesticide exposure or some other cause,” Holway said.

“Indiscriminate use of pesticides is an important problem affecting insect populations all over the world, both native species and non-native species,” he said. However, “the fact that people are seeing [dead bees] is in part just a function of their abundance and isn’t necessarily evidence of some environmental calamity.”

He added that he “would hope that in La Jolla, pesticides are not a large problem given that there’s no large-scale agriculture.”

Kearney said she encourages residents to look for ways other than pesticides to control pests in their yards, such as adding fertilizer, pruning plants or changing their location to adjust sunlight exposure.

“When you have a pest problem, that’s a symptom that there’s something wrong with your plants,” she said.

If people do use pesticides, she recommends doing it in the late evening “so you have the whole nighttime for that pesticide to do its thing and maybe become less and less potent before the pollinators will be accessing it again.”

Kara Roskop-Waters, an agriculture standards inspector in the county division that regulates all pesticide use countywide, said pesticides must be registered with the U.S. Environmental Protection Agency and the California Department of Pesticide Regulation before they’re allowed to be used in the United States and California.

“Part of that process is that they have to test the bees during the product registration to ascertain the effect that pesticide will have on bees,” she said.

“Every pesticide label, if it is toxic to bees, will tell the applicator, whether it’s a homeowner or a growth manager … exactly what you need to do to protect the bees. The most important thing is for the homeowner to read the label.”

She echoed Kearney’s suggestion to use pesticides at night or early in the morning when bees are not foraging, allowing for the pesticides to dry. “As soon as the insecticide is dry, it is considered no longer a risk,” she said.

Roskop-Waters said residents can direct questions about pesticides to the county’s pesticide division at (858) 694-8980.

Cass said the state requires all beekeepers to report the location of their bee colonies, an edict that is managed through the county.

“If any pesticide that’s labeled as toxic to bees is going to be applied in the county when there’s flowering plants around, a beekeeper needs to be notified in advance so they can move their managed hive to a safe location,” Cass said.

She added that the county has a “honeybee hotline,” (800) 200-2337, to report concerns about wild bees, such as swarms and unmanaged hives.

“The main thing we’d encourage anyone who’s concerned about these honeybees and native bees [to do] is to plant flowers in whatever space they have,” Cass said.

Kearney agreed. “The best way to help bees is to plant flowering trees for them, because then you have a long-lasting supply of nectar that will grow into the future and become more abundant,” she said.

For more information about the county’s honeybee protection program, visit