April 2016 Plant Health

Bees and Pesticides: An Overview By Raymond A. Cloyd

The issues associated with pesticides, bees and other pollinators have always been controversial. Pesticides, such as insecticides, are designed to kill insects, and bees, including the honeybees and bumblebees. As such, concerns regarding the effects of pesticides on bees are not new. However, recent issues affiliated with the impact of neonicotinoid systemic insecticides on bees (and other pollinators) have resulted in a “reassessment” regarding the impact of pesticides on bees.

Although the emphasis is primarily on the effects of neonicotinoid systemic insecticides, there are a number of non-pesticide related stress factors that are having short- and long- term effects on bee populations worldwide including parasites, diseases, habitat loss, fragmentation and habitat alteration. In some ways, these factors are more likely to have a greater impact than the effects of pesticides. In fact, habitat alteration, especially on farms, may be the most critical factor responsible for bee decline. In addition, chronic or cumulative exposure to multiple interacting stress factors may be responsible for bee losses and causing substantial reductions in wild or native pollinator populations.

The impact of pesticides on bees, however, seems to be the main issue due
to the importance of bees as pollinators. The European honey bee, Apis mellifera, is not native to the United States; nonetheless, this species is an important pollinator of most agricultural crops, pollinating approximately 130 different crop types valued at $15 to $20 billion in the United States. Moreover, there are additional pollinators including butterflies, moths and native wild bees. In fact, nearly 75 percent of food crops worldwide depend on pollinators for pollination. Therefore, this article will provide an overview of the effects of pesticides on bees with insights on the complex factors and interactions affiliated with how pesticides affect bees. The topics of discussion include: 1) factors associated with bee behavior, 2) factors influencing pesticide exposure and bee toxicity, 3) laboratory vs. field conditions, 4) systemic insecticides, 5) neonicotinoid systemic insecticides, 6) synergism, 7) metabolites, and 8) miscellaneous pesticide interactions.

1. Bee Behavior

Bees collect pollen and nectar to feed their young (larvae), with pollen and nectar being a major source of nutrition. Adult bees tend to consume more nectar than pollen, whereas larvae prefer to feed on pollen. Bees collect pollen and nectar from multiple sources, which may dilute the effects of foraging on plants treated with insecticides. However, since bees only forage so far away from the hive, there may not be any dilution effects in large agricultural cropping systems.

Most of the pollen and/or nectar in the stomach of a foraging bee is not metabolized. Therefore, bees may only be exposed to a small portion of the insecticide contents. Afterward, nearly all of the pollen and nectar is transported back to the hive. As such, the social interactions among bees need to be considered when evaluating exposure to insecticides.

Bee age also may impact insecticide susceptibility. Moreover, body size may have a direct effect on bee sensitivity to insecticides. Larger bees are more tolerant of insecticides than smaller bees because smaller bees have a greater surface-to-volume ratio.

2. Pesticide Exposure and Bee Toxicity

The demand for bees for pollination has increased three-fold, enhancing the chances of bees (e.g., honeybees and bumblebees) being exposed to pesticides. There are two types of exposure associated with pesticides (in this case, insecticides) and bees: direct and indirect.

Direct exposure is affiliated with spray residues that result in mortality of the adults. Indirect exposure occurs when dried residues on leaves and/or flowers cause direct mortality or affect behavior. A pesticide’s toxicity may vary depending on the route of entry. In most instances, contact (dermal) exposure is less toxic than ingestion (oral) exposure. Although the primary focus has been on neonicotinoid systemic insecticides, other pesticides, including fungicides, can result in direct and/or indirect effects on bees.

The primary route of exposure to bees and subsequent bee poisoning occurs when workers forage on treated crops with open flowers contaminated with insecticide residues either through direct spray applications or via pollen and/or nectar that contains concentrations of the active ingredient of systemic insecticides that have been applied to the soil/growing medium.

Furthermore, any drift during spray applications of insecticides onto weeds that
bees forage on may directly affect bees. Bees acquire insecticide residues when foraging on contaminated flowers, which may result in bee death. Consequently, contamination of both pollen and/or nectar is the main source of poisoning affecting honeybee populations (colonies).

Plants not in flower are less of a problem due to the absence of bees. Also, the number or density of flowers associated with a crop governs the number of bees that will visit, which influences the number of bees that may be subsequently affected. Nevertheless, residues in or on pollen and/or nectar may vary in their effects on bees. Insecticide- contaminated pollen and/or nectar returned to the hive and then fed upon by workers or immatures (larvae) may result in a decline of  the colony. However, insecticide residues may not be present in pollen and/or nectar, and bee behavior (described above) may help to avoid contamination of honey. In addition to direct mortality, there may be sublethal effects after exposure to insecticide residues that result in negative effects on foraging behavior, reproduction, memory/learning ability, overwintering success, colony interactions, pollination and colony vigor.

Residue levels and subsequent toxicity may diminish over time as the insecticide degrades due to environmental parameters such as sunlight (ultra-violet degradation), rainfall and temperature. In fact, temperature can have a significant effect on exposure time, due to the relationship with bee activity. Temperature may impact the exposure of bees to insecticides either in or on pollen and/or nectar as bee activity is greater at temperatures greater than 55° F. However, honeybees do not leave the hive to forage when temperatures are less than 50° F, and they do not forage when night temperatures are greater than 55° F.

Temperature also may affect an insecticide’s residual activity. For instance, the residual activity may vary depending on temperature as applications made during low temperatures or cool weather may result in residues remaining longer on plants, consequently increasing the potential for harmful effects to bees. Higher temperatures, however, typically result in less residual activity due to degradation of the insecticide residues, thus decreasing any potential harmful effects to bees.

There are a number of factors associated with insecticides that may impact toxicity to bees and other pollinators including formulation. An insecticide formulated as an emulsifiable concentrate may be less toxic to bees than the same insecticide in a different formulation (soluble powder). For instance, dust formulations of carbaryl (Sevin) can contaminate pollen and subsequently kill bees when insecticide residues are stored in combs or fed to immatures (larvae). However, newer formulations of carbaryl (e.g. Sevin XLR) tend to be less toxic to bees.

One of the most harmful formulations to bees is microencapsulated. Microencapsulated formulations are more toxic to bees because of electrostatic charges resulting in a strong  affinity to adhere to the body of bees. The particles of microencapsulated formulations are about the same size as pollen grains, resulting in increased adhesion to the bee body. The plastic capsules may be stored in frames, possibly leading to declines in bee colonies. It also should be noted that the inert ingredients associated with formulations may be more toxic to bees than the actual active ingredient.

Time of day when an insecticide is applied can directly influence the potential effects to bees. Recommendations are always to apply insecticides in the early morning or late evening when bees are less active. In addition, avoid applying insecticides to “bee attractive” plants that are in flower. Moreover, duration of exposure is critical and varies depending on seasonality and flower type, which is primarily based on flower morphology. For example, bees have been observed to spend less time foraging on the flowers of lavender (Lavandula spp.) compared to apple (Malus spp.). Contact with floral parts is more frequent when bees visit flowers, although this may vary with flower type, which can affect duration of exposure. Differences in foraging among different flower types also may influence the cumulative effects of insecticides. Furthermore, the concentration of an active ingredient in the pollen and/or nectar may vary based on flower type. One issue is what constitutes a “field-realistic dose” in the pollen and/or nectar, which can vary depending on plant and flower type or species and flower age.