The Effects of Tillage, Irrigation and Landscape Structure on Bee Abundance in Pumpkin

There has been much concern in recent years regarding the decline of both managed honey bee colonies and wild pollinator populations. Several documented cases of pollen limitation and depressed fruit set and yields have been attributed to pollinator declines and thus support this concern. In the case of pumpkin and other Cucurbita spp. crops there is the potential to decrease dependence on honey bees for crop pollination by promoting wild pollinator species. Both landscape level factors, such as the availability of alternate forage and nest sites, and local level factors such as farm management practices may potentially influence the abundance of wild pollinators within a crop. The primary goal of this study was to look at the effects of no-till practices in pumpkin on squash bee (Peponapis pruinosa) abundance at flowers. In an effort to look for a potential mechanism linking tillage practice to bee populations, I also asked if P. pruinosa nests preferentially in crop or in edge areas. As I expected to find Bombus spp., another important pumpkin pollinator, present at my study sites, I looked for any effects of tillage on their abundance as well. In addition, I addressed the question of whether landscape level factors, such as the availability of resource rich space, and/or local level factors such as irrigation practices, predict bee abundance and diversity in pumpkin crops. My study confirms that Peponapis pruinosa and Bombus impatiens contribute significantly to the pollination of pumpkin crops in the mid-Atlantic region of United States. I present evidence suggesting that Peponapis pruinosa prefers to nest within crop areas below Cucurbita spp. vines, where they are potentially susceptible to nest site destruction through tilling. I did not however, find a significant effect of tillage practices 2 on the abundance of these bees. It is possible, though unlikely, that tilling does not destroy nests; I discuss other possibilities that include alternative nesting sites on the property, asynchrony between Peponapis emergence and pumpkin flowering, and a source-sink dynamic caused by this asynchrony in which good nesting sites with late flowers may export their bees to regional sites with early flowers. Factors that did prove to be important in predicting bee abundance at farms include irrigation practices in the case of Peponpapis pruinosa and the proportion of land devoted to orchards in the case of Bombus impatiens. I found irrigation to be predictive of Peponapis pruinosa abundance in a previous data set as well. Knowledge of these effects may be useful to growers interested in promoting wild pollinators on their farms.

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