Does insecticide exposure affect solitary bee nesting behavior?

Methods

Summary

The alfalfa leafcutter bee and other solitary pollinators can get exposed to insecticides during development by feeding on contaminated nectar and pollen provisions or as adults when collecting contaminated nectar and pollen to feed their offspring.

One of the difficulties in assessing the effects of neurotoxic insecticides on bee populations is that their effects vary with the level of exposure. The old adage that the “dose makes the poison” remains true, but there is an interesting complication. At high doses the effects will be lethal, but insecticides can degrade over time, and the majority of beneficial insects are exposed to what are called sublethal doses. At these lower doses, most individuals survive exposure but their behaviors and physiology can be affected in substantial ways. Bees’ memory, capacity to navigate and orient in the field, as well as their immunity to diseases and parasites are some of the many documented sublethal effects of exposure to insecticides.

In order to understand the complex consequences of insecticide exposure on the alfalfa leafcutter bee, we decided to focus on nest building behavior, which is a critical component of this bee’s biology and reproduction. We will focus on a class of insecticide that has been under much scrutiny for its potential toxicity to bees, neonicotinoid insecticides.  We will conduct a two-phase experiment described below.

Phase 1: For how long do the neurotoxic effects of neonicotinoids last in the alfalfa leafcutter bee?

During this phase, we will determine whether a one-time exposure to insecticide alters bee brain chemistry for the entire period necessary for building nests (~1week). This will be done by exposing alfalfa leafcutter bees to field-realistic doses of neonicotinoids and then dosing the activity of an enzyme that is the primary target of neonicotinoid insecticides: acetylcholinesterase.

By completing this first phase we will know if field-realistic doses of neonicotinoids are likely to alter the brain chemistry of alfalfa leafcutter bees long enough to affect their nest building behaviors over the course of a week.

Phase 2: Effects of insecticide exposure on nest building behaviors in the alfalfa leafcutter bee

In this second phase, we will release bees in an alfalfa field after having exposed them to three treatments: no insecticides, low dose of insecticides, and high dose of insecticides. These concentrations will be determined with the data generated from Phase 1. After treating the bees, we will release them in 9 screened enclosures (3 enclosures for each treatment) in an alfalfa field.  We will let the bees naturally build their nests over the course of 1+ week within the cages.  

We will then dissect each nest to compare nest size and structure among our different treatments.

By measuring and comparing these traits we will have a greater understanding of how a key component of solitary bee survival and reproduction – nest construction – is affected by exposure to insecticides.