Dr. Eric C. Mussen, Extension Apiculturist, UC Davis; Gene Brandi, Commercial Beekeeper, Los Banos, Calif.


Honey bees are an essential component of modern agriculture as their pollination efforts are necessary for production of about one-third of the crops we produce in this country. Exposure to pesticides has produced negative effects on individual bees and their colonies for nearly a century. Historically, dead or dying bees on the hive bottom boards and on the ground in front of the hives, demonstrated to beekeepers the negative effects of such interactions. The losses can be pretty spectacular and piles of dead bees very voluminous. In addition, there also can be negative effects on queens, drones, developing brood, and bee behavior that eventually result in weakened or dead colonies.

Over the decades there has been a succession of insecticides, acaricides, fungicides, and herbicides as new chemistries were developed and older chemicals were retired, often due to resistance in the target pests. More recently-developed insecticides and acaricides are fast-acting, killing most contaminated honey bees before they can accomplish many foraging flights. Some of those foraging bees die in the field. However, before other bees succumb to a toxic dose, or after the pesticide residues have broken down to sublethal levels, contaminated pollen can get carried into the hives. That pollen is transferred to housekeeping bees that pack portions of it into cells. The stored pollen undergoes microbial, partial digestion and becomes preserved with lactic acid for con-sumption some time during the next half year or so.

Fresh and stored pollens are mainly consumed by so-called “nurse bees” whose head glands extract nutrients from their blood and convert it into brood food. Brood food is a protein-rich, gelatinous secretion fed to the queen, so that she can lay 1,000 to 2,000 eggs a day if required. Brood food is fed to developing worker and drone larvae, and to adult drones and worker bees. When fed to queen larvae, additional proteins and sugar are added to the brood food and we call it “royal jelly.” Nurse bees that consume contaminated pollens will produce contaminated brood food and royal jelly.

In addition to pollen contamination, honey bees can become contaminated by drinking field water which contains chemical residues. While irrigation that leads to tail water puddles or ponds may be declining, the use of chemigation is increasing. Honey bees can drink chemigated water from emitters, leaks in the system, or even from moist soil that contains water from chemigation. Often honey bee colonies show serious decline following imbibition of that water.

Additionally, in recent years, insecticides are being formulated as systemic compounds that move throughout the plant tissues. If and when the treated plants bloom, the pesticide is delivered to the bees in the pollen and nectar. In tree crops, those pesticide levels in blossoms can be surprisingly high, even as much as a year following the initial application.