Pollinator Refuge Establishment and Conservation Value: Impacts of Seedbed Preparations, Seed Mixtures, and Herbicides

in HortScience

On-farm pollinator refuge habitats can supplement floral and nesting resources to support wild and managed pollinator communities. Although the popularity of installing these habitats has grown, and federal programs provide technical and financial advice to participating landowners, recommendations regarding habitat establishment and species composition vary. We examined the effects of seedbed preparation, seed mix composition, and herbicide applications on pollinator refuge establishment and pollinator visitation in a controlled experiment across 2 years. Seedbeds were prepared either by a no-till method with glyphosate herbicide or by repeated conventional inversion tillage. Seed mixes contained either nine annual, biennial, and perennial forbs (mix AP); seven annual, biennial, and perennial forbs that are tolerant to imazapic herbicide (mix IT); or nine perennial forbs (mix P). Mixes AP and P were grown with and without application of the graminicide herbicide sethoxydim and mix IT was grown with application of the herbicide imazapic. Seedbed preparation methodology had a strong impact on pollinator refuge establishment. A no-till approach generated greater wildflower and lower weed cover relative to tillage, leading to a greater number of blooms. In particular, there were more Indian blanket, purple coneflower, slender mountain mint, and wild bergamot blooms following a no-till seedbed preparation, indicating that certain species are more vulnerable to the effects of tillage than others. The AP and IT treatments displayed more wildflower and less weed percent cover than the P treatments during the first year, but in the second year wildflower and weed cover were similar across all mixes grown with and without herbicide. Overall pollinator abundance, which was dominated by native bees, correlated positively with wildflower bloom counts, suggesting that habitat establishment methods that increase wildflower blooms can positively affect the pollinator conservation value of the habitats. This research indicates that establishing on-farm wildflower habitats can be most successful with no-till seedbed preparation, a mixture of annual, biennial, and perennial forb species, and that herbicides applied after planting wildflowers may not be worth the costs of application.

Contributor Notes

We thank Velva Groover, Chris McCullough, and Mike Graham for data collection contributions. This research was supported by USDA Agroecosystem Management Grant 11664133 and start-up funds provided to M.E. O’Rourke by the College of Agriculture and Life Sciences at Virginia Tech.

Corresponding author. E-mail: megorust@vt.edu.

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    Proportions of wildflower and total weed cover by mix in 2016 and 2017. Annual and perennial mix (mix AP-h) includes nine species of annual and perennial wildflowers and was treated with a sethoxydim herbicide, mix AP is the same wildflower mix without herbicide, imazapic-tolerant mix (mix IT) contains a subset of seven AP wildflower species and was treated with imazapic herbicide, perennial-only mix (mix P-h) contains nine perennial wildflowers and was treated with sethoxydim herbicide, and mix P is the same wildflower mix without herbicide. Tukey’s honestly significant difference tests were performed to evaluate significant differences at the P ≤ 0.05 level.

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    Proportions of wildflower and weed cover by no-till and till seedbed preparations in 2016 and 2017. Tukey’s honestly significant difference tests were performed to evaluate significant differences at the P ≤ 0.05 level.

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    Linear regression of total pollinators by total wildflower blooms/m2 in 2017. Total pollinators include the sum of syrphids, wasps, lepidopterans, honey bees, bumblebees, and “other bee” categories of species.

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    Total numbers of pollinators in each mix-herbicide treatment plot. Data are averaged over seven sampling dates from June through Aug. 2017 and represent pollinators observed during 6-min periods. Means are not significantly different when marked by the same letter, as analyzed by Tukey’s honestly significant difference post hoc tests at the P ≤ 0.05 level.

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    Proportions of floral visitations by different pollinator groups to individual wildflower species. Data are averaged over three sampling dates in Aug. 2017. BS = black-eyed Susan; MS = maximilian sunflower; PC = plains coreopsis; PCN = purple coneflower; PP = partridge pea; SB = spotted bee balm. Tukey’s honestly significant difference tests were performed to evaluate significant differences among pollinator groups per wildflower species at the P ≤ 0.05 level.

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    Mean total wildflower blooms in 2016 and 2017 by (A) mix and (B) seedbed preparation. Annual and perennial mix (mix AP-h) includes nine species of annual and perennial wildflowers and was treated with a sethoxydim herbicide, mix AP is the same wildflower mix without herbicide, an imazapic-tolerant mix contains a subset of seven AP wildflower species and was treated with imazapic herbicide, perennial-only mix (mix P-h) contains nine perennial wildflowers and was treated with sethoxydim herbicide, and mix P is the same wildflower mix without herbicide. Pairwise means comparisons were tested across mixes and within each year by Tukey’s honestly significant difference post hoc analysis, and between no-till and till methods within each year with Student’s t tests. Means within the same year are not significantly different when marked by the same letter at the P ≤ 0.05 level.

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    Proportions of broadleaf weed, grass weed, and wildflower cover in different mix-herbicide treatments. Vegetative cover sampled on 8 Sept. 2016 and 11 Sept. 2017. Pairwise means comparisons tested across mixes and within year by the Steel–Dwass nonparametric post hoc analysis in 2016 and by Tukey’s honestly significant difference analysis in 2017. Grass cover in 2017 was log + 1 transformed before analysis. Means within the same year are not significantly different when marked by the same letter at the P ≤ 0.05 level.

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