Introduction

Bees may be exposed to the pesticide through various routes, depending on the type of bee (honeybees, other social bees, solitary bees) and the type of pesticide (applied as foliar spray, systemic insecticide applied to soil or seed).

The route of exposure will determine the risk assessment procedure that needs to be applied as well as the toxicity data that are required.

Below, different possible routes of exposure are shown:

Honey bees – Systemic pesticide applied to the soil or as seed dressing

The systemic pesticide is applied to the soil or as a coating to seeds planted in the soil. The primary routes of exposure are assumed to be via residues in pollen and nectar; however, other routes of exposure such as ingestion of residues in surface water, plant exudates (e.g., guttation fluid), and abraded seed dust may sometimes also play a role. Primary routes of residue transfer are indicated by thick arrows and lesser routes by thin arrows.

Forager worker bees may be exposed by both contact and oral ingestion; however, since the chemical is applied to the soil, the potential for contact exposure is assumed to be limited. For this group of pesticides, the main route of exposure for worker bees is likely to be the oral route, particularly the ingestion of nectar, since nectar is the primary food consumed by forager worker bees. Pollen are also collected on hairs on the worker (forager) bees' bodies, or in small pouches (pollen baskets) on their hind legs. The nectar and pollen collected by worker bees are brought back to the hive where they are incorporated into the food stores, consumed by hive bees, and in turn used to produce food for the queen and the developing brood. If the pesticide concentration is high enough, toxic effects on forager bees, hive bees, bee brood or the queen may result in reduced queen fecundity, brood development success, or survival of adult bees. If these effects are severe enough or last long enough, a significant effect on colony strength may result. [Source: Chapter 6 in: Fischer & Moriarty (2014)]

Honey bees – Non-systemic pesticide applied as a foliar spray

A non-systemic pesticide is applied as foliar spray to crop plants. The primary routes of exposure are assumed to be via contact of foraging bees with the spray as it is applied, or with freshly deposited residues on plant surfaces. For flowers open during spraying, residues may occur in pollen and nectar, and these materials may be brought back into the hive, processed and stored as food that is later utilized by hive bees, bee brood, and the queen. Another possible route of exposure is via surface water (e.g. puddles) that are over-sprayed and used by bees as a source of drinking water. Primary routes of residue transfer are indicated by thick arrows, lesser routes by thin arrows.

Greatest exposure is expected for forager bees that may be exposed via contact with spray droplets and residues on plant surfaces, and via ingestion of residues in water and nectar. If the exposure level is high enough, then forager bees may be killed to the extent that colony strength is reduced (e.g. large bee kill event).

Bees in the hive can also be exposed, but the exposure levels are not expected to be as high as for forager bees, unless the hive is inadvertently over-sprayed during application. However, if pesticide residues in the forage area are high, then other bees may be exposed to these high residues during social grooming. In addition, if concentrations in pollen and nectar brought into the hive are high enough, toxic effects on hive bees, bee brood, or the queen may result. If these effects are severe enough or last long enough, a significant adverse effect on the colony strength may result. [Source: Chapter 6 in: Fischer & Moriarty (2014)]

Reference

Further guidance on different exposure routes of bees to pesticides, as well as their importance:

• D Fischer & T Moriarty (eds.) (2014) Pesticide risk assessment for pollinators. Society of Environmental Toxicology and Chemistry (SETAC) & Wiley Blackwell.

The executive summary of this book is available free of charge.

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 Toolkit reference [A09-03-01-X]