Do pesticides in agricultural soils pose a risk to ground-nesting bees?

March 31, 2019



Around the world, insect pollinator populations have been in decline. Out of the multiple interacting environmental stressors contributing to decreases, use of pesticides on agricultural crops is near the top of the list. 


In Ontario and globally, Curcubita crops (e.g., pumpkin and summer squash) are grown for their fruits. On conventional farms in Ontario, crops are treated with pesticides including a class of insecticides called neonicotinoids. Toxicological studies using neonicotinoids (also referred to as “neonics”) provide evidence for their detrimental effects on species of managed bees, including the eusocial honey bee and species of solitary bees.


The hoary squash bee (Peponapis pruinosa) is a solitary ground-nesting species and an important pollinator of Curcubita crops. Females excavate a nest in the soil that is up to 45 cm deep with 3 to 5 chambers for developing larvae. In North America, the squash bee relies almost entirely on Cucurbita flowers for nectar and pollen and will therefore build nests near Curcubita crops. Due to the female, and potentially larvae, coming into direct contact with the ground, they may be at risk of exposure to pesticides found in the soil.


A recent study by Chan et al. (2018) sought to answer four questions associated with the risk of hoary squash bee to pesticides (insecticide, fungicide, or herbicide): 1) Which pesticides are potentially hazardous to hoary squash bees?; 2) Among soil, pollen, and nectar exposure routes, which poses the greatest hazard?; 3) Are adult females or larvae at greatest risk?; and 4) What is the risk to other ground-nesting solitary bees on farms using insecticides? Notably, this is the first study to examine the risk of insecticide exposure for ground-nesting bees through contact with soil.


The study involved collecting soil, nectar, and pollen samples from 18 farms growing Curcubita crops throughout southern Ontario. Collection was in July and August 2016 which coincided with the female squash bee’s nest-building and foraging period. Samples were then analyzed to determine the type and concentration of pesticide. Data from the literature was combined with data from this study to estimate the amount of pollen, nectar, and soil that the squash bee would be exposed to either through physical contact or ingestion. The study focused on female and larval squash bees since data is lacking for male hoary squash bees.


The authors performed hazard and risk assessments of pesticides to hoary squash bee using data obtained on the concentration of pesticides in the samples, estimates of the amount of each matrix (soil, pollen, or nectar) the squash bee would be exposed to, and existing pesticide toxicity values. Their assessments centered on insecticides due to the relatively low risk of other pesticides, such as herbicides and fungicides, to insects. Chan et al (2018) expanded their study to also include the risk of insecticide exposure to other ground-nesting solitary bee species. To do this, they drew on the Ontario Ministry of the Environment and Climate Change’s database on background levels of neonicotinoids in soils from 38 agricultural lands (e.g., corn and soy crops) and used the hoary squash bee as a surrogate to determine risk to other species.


Based on their assessments, Chan et al. (2018) determined that neonicotinoids in soils may be a considerable risk to female hoary squash bees while they construct their nest in the vicinity of Cucurbita crops. They also found that neonicotinoids pose a risk to other bee species that build nests in soil near neonicotinoid-treated crops. These results demonstrate that recognizing and mitigating risks to ground-nesting bees from exposure to neonicotinoids in agricultural soils is critically important to protect these species from harmful effects such as population declines.


Read the full pre-print of this study here (open access):



Chan DSW, Prosser RS, Rodriguez-Gil JL, and Raine NE. 2018. Risk of exposure to systemic insecticides in agricultural soil in Ontario, Canada for the hoary squash bee (Peponapis pruinose) and other ground-nesting bee species. bioRxiv preprint.