Quantifying the Risks of Pesticide Exposure to Squash Bee Behavior and Pollination Services

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Commodities

• Animals: bees

Practices

• Animal Production: animal protection and health, behavior, pollination services, pesticide impacts

Pesticides are widely used to prevent the loss of crop yields to insect pests. However, pesticides can also have detrimental effects on beneficial insects that contribute to successful crop pollination. While there has been much recent interest in the effects of pesticides on bees, the majority of this research has focused on honeybees (Apis mellifera) and bumblebees (Bombus spp.), with experiments largely conducted under lab conditions. However, other bee species are also critical pollinators to many crops. The majority of the 4,000 bee species in North America are solitary and ground nesting, and in Cucurbita crops (pumpkin, squash, zucchini) the specialist ground nesting squash bee (Eucera pruinosa) provides the majority of the pollination services. Recent studies have shown that insecticide exposure in semi-field experiments can reduce the reproductive success of squash bees, but how pesticide exposure impacts their foraging behavior and ultimately, population numbers, is not known. If foraging behavior or abundance is negatively impacted by exposure this could have major consequences to pumpkin production. In this study I will assess the influence of pesticide exposure on squash bee communities across sites in central Texas. Specifically, I will address how pesticides impact squash bee abundance, the contributions that individual squash bees make to pollination services in terms of pollination efficiency, and if pesticide exposure predisposes bees to be increasingly susceptible to further exposure by the two commonly used insecticides: thiamethoxam and bifenthrin.

Aim 1: Assess how pesticide exposure and landscape composition impact squash bee abundance . It is critical to understand exposure across the community, but especially for the key pollinator, E. pruinosa; this will be done To understand variation in pesticide exposure across sites we will  compare the pesticide residues detected in the bees themselves to the residues detected in the pollen and nectar from the plants, and soil from each site. Texas pumpkin farms off a useful natural laboratory for addressing questions about pesticide exposure since there is a wide range of management practices employed, from small organic farms to very large scale farms with conventional management that includes the use of traditional insecticides. Pollinator surveys on cultivated pumpkins will be conducted at 12 farms across central Texas that vary in their usage of pesticides at three time points during the blooming period. Specifically, we will compare how bee abundance is predicted by a traditional hazard assessment that only considers the concentrations of pesticides in the crop nectar versus a hazard assessment that additionally considers exposure through pollen and soil (Chan et al., 2019). This will allow us to identify the most important routes of exposure by comparing these two different hazard assessments to our measures of pesticide residues in individual bees, to see what routes of exposure are most important to consider to prevent population declines.  

Aim 2: Determine to what extent pollination efficiency is affected by pesticide exposure and landscape composition. By directly measuring if the individual contribution a squash bee makes to fruit set correlates with pesticide exposure at a site we can have a more accurate representation of potential losses. 

Understanding the abundance of the pollinators across sites in addition how much each of those pollinators contributes to crop production allows us to quantify if the bees are negatively impacted by pesticide exposure at what point we would expect to see losses to yield.  

Aim 3: Experimentally test if exposure of two commonly used insecticides have lethal or sub-lethal effects on squash bees. The effects of insecticides on pollinators are typically tested using honey bees and bumblebees; we have adapted established protocols from those species to test how field realistic exposure of two commonly used insecticides affects squash bees. Specifically we will address is propensity to forage is negatively impacted by field realistic exposure to thiamethoxam and bifenthrin with a proboscis extension response assay that can be used on wild caught bees. This will allow us to test whether insecticide effects on these responses are generalizable beyond models species, and if prior pesticide exposure in the environment leads to greater susceptibility to these effects.