Pesticide hazard for ground nesting solitary bee species associated with North American agroecosystems

About 21% (739/3500) of North American wild bee species are closely associated with agroecosystems, of which, 40% (298/739) are solitary species that nest in the ground. Of great importance in evaluating hazard to all bee species is the toxic load of the crops with which bee species are associated and the extent of that association. When we evaluated 594 agriculturally associated bee species for hazard from exposure to pesticides applied to crops, 47% of the species in the 90^th percentile (i.e., the 10% most at risk) were solitary ground-nesters. Agriculturally associated solitary ground-nesters are more exposed to pesticides than most other bee species because of the solitary nature of their social structure (ie they lack functional redundancy), the extent of pesticide pollution of agricultural soils, the amount of soil they handle during nest construction, and the length of time they spend in close contact with soil during development. The consequences to one agriculturally associated ground-nesting species, the hoary squash bee (Xenoglossa pruinosa) of exposure to neonicotinoid insecticides applied to soil have been demonstrated in field-realistic conditions. Pollen collection, nest building, and offspring production were all strongly negatively impacted, resulting in steeply declining populations over time in some cases. As such, there is a need for ground-nesting solitary bee models within pesticide risk assessment paradigms, especially when highly bee toxic, persistent pesticides are applied directly to soil as drenches or seed treatments.