The Effect of Alfalfa Perimeter Plantings on Strawberry Pollen Consumption by Syrphidae (Diptera) in Wisconsin

Abstract

Syrphid flies visit flowers to feed on nectar and pollen and provide an important pollination service in many cropping systems. We previously observed that perimeter plantings of alfalfa concentrate syrphid populations in trap-cropped June-bearing strawberry fields, but whether this trap cropping strategy impacts syrphid visitation to strawberry flowers remains unknown. This study examined whether trap cropping influences syrphid visitation to strawberry flowers by comparing the proportion of strawberry pollen in the syrphid digestive tract. We expect that the gut content of syrphids collected from strawberry fields with an alfalfa border will have a higher proportion of strawberry pollen than those from fields with the weedy border. Although syrphids are abundant in alfalfa strips, alfalfa and strawberry flower at different times, whereas weeds present in the field border compete with strawberry. We conducted gut content analysis on syrphids by dissecting syrphids and pulverizing their abdomen. Calberla’s staining solution was added to visualize pollen under the microscope, allowing individual grains to be identified and counted. We found no evidence that alfalfa perimeter plantings influence the average proportion of strawberry pollen found in the digestive tract of syrphids. However, strawberry pollen represented a significantly higher proportion of gut contents of syrphids collected from the perimeter compared to those collected from strawberry fields, and there was a lower proportion of strawberry pollen in syrphids collected in week two compared to week one and three. Differences in strawberry pollen consumption and floral visitation may reflect patterns of syrphid activity over time. Future research including direct observations of syrphid movement in strawberry fields with an alfalfa border, studying the efficiency of syrphids as strawberry pollinators, and analyzing week two samples to determine whether there was a particular type of pollen other than strawberry that was high in proportion to strawberry pollen will provide useful information about this vital, but understudied group of pollinators.