Interactive effects of pesticides, drought, and pathogens on the common eastern bumble bee Bombus impatiens.

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Commodities

• Agronomic: cotton, sunflower
• Vegetables: cucurbits

Practices

• Crop Production: drought tolerance, greenhouses, pollinator health
• Pest Management: chemical control

Pesticides and pathogens are both implicated in patterns of worldwide bee declines. Systemic pesticides applied to seeds can be transported to other tissues including pollen and nectar, resulting in pollinator exposure during foraging. Pesticide exposure makes bees more susceptible to pathogen infection, with implications for survival and reproduction. However, the behavioral mechanisms driving the effects of pesticide exposure on pathogen transmission remain unexplored.

Unprecedented drought events in recent years are also negatively affecting floral resources. A recent study showed that drought stress increased pollen pesticide concentrations in sunflower, suggesting an interaction that may have direct consequences for pollinator health. However, the full implications of drought for bee exposure to pesticides are largely unknown. I will manipulate water availability in three crops (sunflower, squash, and cotton) to assess drought effects on systemic pesticide concentrations in pollen.

I will then test the effects of pesticide exposure on pathogen transmission in common eastern bumble bee (Bombus impatiens) colonies. I will assign colonies to be exposed to none, low or high chronic pesticide concentrations, and add individuals infected with the gut pathogen Crithidia bombi to mimic conditions of a newly acquired infection in the colony. Using novel technology, I will track interactions between bees and assess whether pesticides alter behavioral mechanisms driving pathogen transmission.

By assessing how drought affects exposure to pesticides in crops and the cascading effects of pesticide exposure on pathogen transmission within colonies, my project will provide information for farmers to make informed decisions about using seed-treated crops.

Objective 1. I will determine how drought stress affects pesticide concentration in pollen of three major US agricultural crops whose seeds are treated with systemic pesticides.

Hypothesis 1. Drought stress will increase pesticide concentrations in pollen from seed-treated crops.

This experiment has already been initiated, greatly increasing its chances of success on a short timeline. However, pesticide analysis is expensive, and funds from NE SARE would allow me to substantially increase the sample size and therefore my ability to detect treatment effects. I have already begun growing three crops that are seed-treated with pesticides: sunflower, cotton, and squash. Plants of each crop will be randomly assigned to one of three drought treatments (well-watered, moderate water, and drought).  I will collect pollen samples throughout the summer and send them to the Cornell Chemical Ecology Core Facility for pesticide testing. If I find that drought increases pesticide concentrations in pollen, this will shed light on how climate change can aggravate the risk of exposure for bees to pesticides used in major crops. Even if the drought does not affect pesticide concentrations, this project will quantify ranges of pesticide residues across different crops, providing more accurate assessments of potential pollinator exposure.

Objective 2. I will assess the effects of chronic exposure to neonicotinoids on pathogen (Crithidia bombi) transmission dynamics in bumble bee (Bombus impatiens) colonies. 

Hypothesis 2. I predict that bees exposed to pesticides will interact less in the colony, potentially leading to lower transmission rates of C. bombi. However, individuals who become infected will have a high infection intensity due to a weakened immune system in response to pesticides.

I will conduct this laboratory experiment during the 2021-22 academic year, using an automated tracking system that allows me to record interactions between bees in the colony, after which I will assess pathogen prevalence and infection intensity. If exposure to pesticides reduces interaction rates by disrupting social behavior³⁵, bees may be less likely to acquire pathogens from their nestmates. However, if exposure to pesticides increases individual bee susceptibility to pathogens, the transmission may still increase even if contact rates are decreased or unaffected. Previous studies have shown that pesticides weaken bee immune systems^12,36, making them more susceptible to pathogens³⁷. However, there is little information about pathogen transmission patterns inside the colony, which is critical to address.