Best management practices that promote sustainable crop pollination: the role of crop rotations and tillage depth

2013 Annual Report for GW13-018

Project Type: Graduate Student
Funds awarded in 2013: $24,954.00
Projected End Date: 12/31/2015
Grant Recipient: University of California, Davis
Region: Western
State: California
Graduate Student:
Major Professor:
Neal Williams
University of California, Davis

Best management practices that promote sustainable crop pollination: the role of crop rotations and tillage depth


Over the past year we made significant progress continuing to explore the role of crop rotations and tillage depth on the squash bee, Peponapis pruinosa. Specifically, we monitored P. pruinosa densities at 95 squash (Cucurbita) fields in Yolo County, California. Additionally, we GPSed those fields in order to continue to build our crop rotation map. We tested the impact of tillage on squash bee survival using a manipulative cage based experiment. We began to develop a spatially explicit simulation model and excavated wild P. pruinosa nests to collect data that will allow us to parameterize that model. In addition to conducting the research project, we began to develop outreach material which we will promote in the upcoming year.

Objectives/Performance Targets

Objective 1: Conduct a manipulative experiment to determine overwintering survival of P. pruinosa under different tillage depth treatments (summer 2013 and summer 2014)


Objective 2: Conduct an observational survey to determine if crop rotations that promote between-year connectivity (i.e. how connected a focal Cucurbita field is to surrounding Cucurbita fields through time) have larger P. pruinosa populations than crop rotations that have low between-year connectivity (summer 2013 and summer 2014)


Objective 3: Build a spatially explicit simulation model that uses crop rotations and tillage practices to predict P. pruinosa abundance and use this model to identify optimal management strategies (fall 2013 – spring 2014)


Objective 4: Validate the model described in Objective 3 with data collected from squash fields in Yolo County (see Objective 1) and grower interviews (summer 2014)


Objective 5: Communicate best-management practices using existing UC Cooperative Extension, Natural Resources Conservation Service and non-profit (e.g. Xerces Society for Invertebrate Conservation) partnerships (fall 2014)


Objective 6: Develop an interactive website that can be used to show growers how crop rotation practices and tillage practices impact squash bee populations (winter 2013 – fall 2014)


Objective 7: Publish the outcomes of Objectives 1-4 in three papers in peer-reviewed journals (fall 2014)


Objective 1:

We established 20 artificial P. pruinosa nesting aggregations, each located under a 10ft x 10ft mesh cage. Cages were planted with Cucurbita spp. Within each cage we released two female and two male P. pruinosa. We then monitored each cage to track bee nesting activity and floral bloom. At the end of the season, after the female P. pruinosa died, we removed the cages. We then randomly selected 10 cages and applied a tillage treatment. The tillage treatment included the following: 6” disk followed by a 16” chisel followed by another 6” disk. This is a typical three-year treatment for the study system (Jeff Mitchell, personal communication). In the spring, we restored the cages to their original location. We then trapped emerging bee offspring in each cage to compare offspring survival in tilled versus untilled conditions. Traps were checked twice a week from May 26 through September 16. Preliminary results suggest that tillage has a negative impact on offspring survival. We will finish processing and analyzing the data from Objective 1 this spring.


Objective 2:

We monitored P. pruinosa density at and GPSed 95 privately owned squash fields in Yolo County, California. Each field was surveyed three times during the flight period of P. pruinosa. At each field we established four 50 meter transects, along which we conducted floral surveys and bee observations. During the 2013 field season, we recorded 4,824 P. pruinosa visits to squash flowers. At the end of each survey, we collected voucher specimens of the bees observed. GPS data was uploaded into ArcGIS for analysis of surrounding landscape characteristics. In the upcoming year, data from this field season will be analyzed to determine the relationship between crop rotation history and patterns of P. pruinosa density on farms in Yolo County.


Objective 3:

We are in the process of developing a spatially explicit simulation model. In order to parameterize the model, we will use data collected from the projects associated with Objectives 1 and 2. In addition, we excavated and collected pollen provisions from wild squash bee nests. This spring we will run those pollen provisions through a particle counter in order to determine the number of pollen grains needed per P. pruinosa offspring. Pollen counts will be used to estimate the carrying capacity of a given field.


Objective 4:

When the model is completed we will validate it using data that we previously collected from Yolo County squash fields. Grower contacts have been made and interviews will be scheduled for the spring/summer 2014 to confirm historical squash field locations.


Objective 5:

We have developed two squash bee identification guides that will be distributed to the UC Cooperative Extension, Natural Resources Conservation Service and non-profits (e.g. Xerces Society for Invertebrate Conservation). We created a “Native Bees” Youtube channel on which we posted three educational videos on P. pruinosa identification and natural history ( In partnership with Rachel Long at the University of California Cooperative Extension, we helped coordinate a crop pollination workshop for Yolo County growers (see Impacts and Contributions/Outcomes below), created a handout for the workshop and presented preliminary results from the tillage study at the workshop.


Objective 6:

We created a website that will host the interactive educational model once it is built. The website ( will host information about this squash bee project and will include posts highlighting research conducted on pollinators in agricultural landscapes. We have invited experts in pollination ecology, climate change, bee diseases, pesticides and habitat restoration to contribute blog posts.


Objective 7:

Objective 7 is scheduled to begin in fall of 2014. However, we have already made significant progress towards this objective; the field data that will form the basis of these publications has been collected. Specimens are being processed and we have begun preliminary analyses.  

Impacts and Contributions/Outcomes

This past year we partnered with 24 squash growers in Yolo County, California. A subset of these growers participated in our fall workshop focused on pollinators of Central Valley crops ( Eight researchers from the University of California Cooperative Extension, University of California, Davis, and University of California, Berkeley presented their work on Apis and non-Apis bees in local agricultural systems. Seventy people attended this workshop including growers, seed industry representatives and master gardeners. The response was positive and a number of people expressed interest in a workshop focused solely on squash. This workshop will take place in 2014, along with a field day. While we have not promoted our Youtube videos at this point, we have had 333 views of videos on our “Native Bees” channel. We are in the process of making the videos available in Spanish. Finally, we have developed and are translating an easy-to-use squash bee identification guide.


Katharina Ullmann
Graduate Student
Dept. of Entomology, UC Davis
One Shields Ave
Davis, CA 95616
Office Phone: 9707080450
Dr. Neal Williams
University of California, Davis
One Shields Ave
Davis, CA 95616
Office Phone: 5307529358