Evaluation of supplemental flowering plant strips for sustainable enhancement of beneficial insects

Evaluation of supplemental flowering plant strips for sustainable enhancement of beneficial insects

Summary

We compared blueberry fields with native perennial flower plantings on their perimeters to fields without flower perimeters in order to determine the impact of this conservation strategy on beneficial insects in crop fields. We found significantly more pollinators and natural enemies of crop pests in the fields adjacent to wildflower plantings. Natural enemies (mostly parasitoid wasps) were especially impacted by the presence of flowers, and were found in greater abundance in fields with flower planting even 120 ft. inside fields. Also, a late-season recovery of natural enemies following pre-harvest insecticide applications was seen only in fields adjacent to wildflower plantings.

Objectives/Performance Targets

Determine the impact of a flowering perennial strip on the abundance of natural enemies in the adjacent blueberry field.

Determine the effect of a flowering perennial strip on abundance and diversity of native bees in the adjacent blueberry field.

Determine the effect of native bees on blueberry pollination.

Determine effect of the flowering strips on the nesting activity of native cavity-nesting bees.

Accomplishments/Milestones

Un-baited yellow sticky traps were deployed for one week intervals from June to August 2007 and 2008. At each of our study fields (n=8), traps were placed along three transects, in four positions: field perimeter, first blueberry bush (0 ft.), 60 ft, and 120 ft. Sampling dates in 2008 were selected to match the growing degree day accumulations (42°F base) for the 2007 samples. Natural enemies and pests were counted in the lab using a magnifier lamp and/or dissecting scope as needed. Statistical analysis was performed in SAS v.9.1 (PROC MIXED) using log(x+1) transformed data.

In two years of sampling we caught a total of 20,961 natural enemies. Of these, the majority were parasitoid wasps (91% and 93% in 2007 and 2008, respectively). At all in-field sampling distances during both years, we caught more natural enemies on traps near flower strips (2007: 0 ft., F1,141=4.46, P=0.036; 60 ft., F1,140=6.17, P=0.0142; 120 ft., F1,141=4.34, P=0.039; 2008: 0 ft., F1,141=13.46, P<0.001; 60 ft., F1,142=6.37, P=0.013; 120 ft., F1,142=5.71, P=0.0182). In the perimeters, we caught more natural enemies near control fields in 2007, but this difference was not significant (F1, 141=3.59, P=0.06). In 2008, we caught more natural enemies in the flower perimeters than in the controls (F1, 142=10.22, P=0.0017).

In the early half of the season, we caught more natural enemies near flower strips on the first sample of 2008 only (Jul. 2, F1, 94=10.21, P=0.0019). In the later samples during both seasons, we caught significantly more natural enemies in the fields adjacent to flowering plant strips (2007: Aug. 10, F1, 93=5.39, P=0.02; Aug. 17, F1, 93=7.97, P=0.006; 2008: Aug. 13, F1, 94=4.36, P=0.039; Aug. 20, F1, 94=10.10, P=0.002; Aug. 27, F1, 94=12.90, P=0.001). This late-season surge in natural enemy abundance near wildflower strips suggests that the flowering strips may be acting as a source for beneficial insects following pre-harvest insecticide applications in late July.

We counted native bees pollinating blueberry flowers during peak bloom in 2007 and 2008. Native bees observed pollinating blueberry flowers were primarily soil-nesting bees (Andrenidae, Halictidae), although bumble bees (Bombus spp.) and cavity-nesting bees (Megachilidae) were also observed. Unfortunately, diversity analysis could not be performed because of the extreme rarity of native bees in our samples (0.26 native bees per 5 min. sample in 576 samples). In 2007, we counted an average of 0.39 and 0.22 native bees per 5 min. sample in flower and control fields respectively. In 2008, we counted an average of 0.26 and 0.15 native bees per 5 min. sample in flower and control fields respectively.

A non-parametric analysis revealed that the higher native bee abundance in fields with flowering plant strips during both years was statistically significant (α=0.05).

In order to measure the effect of flower strips on the pollination services provided by wild bees, we placed potted blueberry (c.v. bluecrop) bushes in our study fields. These plants were induced to bloom earlier than field plants and placed in the field before growers brought in their honey bee hives. Using this method, we were able to measure proportion fruit set and other yield measures on bushes that had been primarily pollinated by wild bees.

Average berry weight from the potted blueberry plants was statistically indistinguishable between the flower and control fields in both years. In 2007, average berry weight was 0.65 g in fields with flower strips and 0.65 g in control fields. In 2008, average berry weight was 0.49 g in fields with flower strips and 0.48 g in control fields. Proportion fruit set was also essentially the same between the two types of fields in both years. In 2007, average proportion fruit set was 0.29 in fields with flower strips and 0.34 in control fields. In 2008, average proportion fruit set was 0.41 in fields with flower strips and 0.40 in control fields.

To measure the contribution of wild bees to blueberry yield, we placed mesh bags over flower clusters on the potted plants and compared the average berry weight on bagged clusters to clusters that had been left open to pollinators. In 2007, the average difference in berry weight between bagged and open clusters was 0.26 g, and in 2008 the average difference was 0.11 g. In other words, about 40% and 22% of the weight of the average berry in 2007 and 2008 respectively, could be attributed to the effect of wild pollinators in the absence of commercial honey bee hives.

Nest boxes designed to capture cavity-nesting bees were deployed throughout the seasons of 2006, 20007, and 2008 at all field sites with flowering plant strips. Nesting activity was evaluated weekly by counting the number of finished nests visible from the front of each box. At the end of each season the contents of the nest boxes was removed and samples of developing insects were reared for identification. In the first two years (2006-2007) only cavity nesting wasps (Eumenus spp. and Isodontia spp.) and one type of leaf cutter bee (Megachile sp.) were found nesting in these boxes. However, in 2008, for the first time, we found a yet to be identified orchard mason bee (Osmia sp.) at one of our field sites, as well as several individuals of Megachile pugnata nesting at several of our field sites. This suggests that native populations in 2008 may have been benefiting from the pollen resources provided by the flowering plant strips, which (planted in 2006) provided their first full season of bloom in 2007.

Impacts and Contributions/Outcomes

The information gathered during this multi-year study has been presented to growers in the North Central region through a variety of outlets. Mr. Walton presented talks about farm management to enhance pollination at two workshops, one entitled, “IPM and Nutrient Management Planning: Addressing Farm Production and Resource Conservation”, and the other, “Increasing Your Yields by Managing Your pollination.”

Also, the results from objective 4 were presented in a poster display at the 2008 Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, MI. A similar poster entitled, “Conservation strips of native perennials to enhance beneficial insect abundance in commercial blueberry fields,” was presented at the 2008 Entomological Society of America annual meeting in Reno, NV.

Finally, we held a meeting at one of our field sites September of 2008, attended by about 20 growers, where growers were able to see a flower planting in bloom, and hear presentations by representatives of USDA-NRCS, the Xerces society, Dr. Isaacs, and Mr. Walton, covering topics such as native pollinator conservation and how growers can receive incentives from NRCS to plant wildlife habitat on their farms.

Collaborators: