Introduction:

Pollinator insects are essential to produce many economically and nutritionally important crops grown in the western USA. These crops include blueberries, almonds, sunflowers, cucurbits, and many others. Almond pollination in California plays a vital role in the apiary industry, driving beekeepers to haul huge numbers of bee colonies to California for the few weeks in late winter when almonds bloom. Bees are selective of the pollen and nectar they forage, and diverse floral resources can allow bees to forage according to their nutritional needs (Leponiemi et al. 2023). Planting pollinator habitat in natural areas, gardens, and agricultural land is one method of supporting bee health. Irrigated agricultural land in the western USA can be an excellent resource for bees during the dry summer when flowers are rarer. However, the resident weeds in these settings are often not of high nutritional quality for hungry pollinators. To make matters worse, pollinator habitat in agricultural fields can be choked out by competition from weeds. Our control plots from these studies (Figure 1) demonstrate that point effectively.

Figure 1: Control plots at all three experimental sites were extremely weedy. This sometimes meant that none of the planted species could grow, as seen in the pictures above.

Objective:

The studies described here attempt to use herbicides to improve the chances for success in pollinator habitat establishment.

Methods:

Three locations in Oregon's Willamette Valley were selected for studies. Two were hazelnut orchards watered with drip irrigation, and one was a field plot set up for sprinkler irrigation. Each location received different soil preparation. The first orchard location (Corvallis) was not tilled, and soil compaction issues were present. The second orchard location (Amity) was power-harrowed, so the top two inches of soil were loosened. The third location (Lewis-Brown Research Farm) was plowed and disked.

All three locations were seeded in the fall with a set of flowering species with potential for pollinator habitat (Table 1).

These species were planted in rows, and herbicide treatments were applied over the top perpendicular to planting rows (Table 2). Four herbicides were applied post-emergence, and the rest were applied one day after planting (pre-emergence). Glyphosate treatments were only included in the orchard trials. Experimental plots were set up as a randomized complete block design with four replicates, and each species was treated as a separate experiment. A crop oil concentrate at 1% v/v was included for Motiff (mesotrione) and Basagran (bentazon), while a nonionic surfactant at 0.25% was included for Matrix (rimsulfuron) and Quinstar 4L (quinclorac). All post-emergent treatments (and glyphosate) included ammonium sulfate (AMsol 1% v/v).

In Amity, competition from perennial grasses resulted in poor stand establishment. A grass-selective herbicide (clethodim) was used, and the site was reseeded six months after the initial planting when soil conditions were appropriate.

Results and discussion:

Site differences.

Drastic differences were seen between sites. Table 3 shows how crop coverage differed between the three sites for each species.

Coverage at the Corvallis site was deficient for all species except hairy vetch.

Several species did very well at the Amity location. Phacelia in the glyphosate plots was exceptionally well established due to glyphosate's good control of perennial grasses that were not killed by the power harrow.

Lewis-Brown (LB) plots had the best crop establishment initially. However, this location had intense pressure from perennial weeds, so the initial crop establishment did not translate to superior pollinator habitat. The plots at LB where Alion was applied produced a good stand of Canada thistle (Cirsium arvense) by the end of the trial, which the bees loved.

Pre-emergent treatments

Pre-emergent herbicides often had inconsistent pollinator species safety; however, several combinations seemed safe. Napropamide was safe for Phacelia, Gilia, Clarkia, and Lobularia, while flumioxazin and pendimethalin were safe for poppy (Table 5). All five species only had adequate crop establishment at two of the three locations. Hairy vetch establishment was improved by simazine applications at all three locations, but crop coverage was not significantly different from the untreated control for this species (Table 5). Figure 2 shows the treatment by species combinations that were sometimes safe versus the combinations that were consistently safe for the planted species.

At the two orchard sites, glyphosate treatments were the best for Gilia, Phacelia, and poppy establishment (Table 5).

All three trials were conducted on fine soils with organic matter content ranging from 2-7% (USDA-NCSS soil survey). The safety of pre-emergent herbicides for pollinator species establishment may vary depending on soil characteristics.

Post-emergent treatments

Post-emergent (POST) applications were challenging to evaluate for safety. Weed control efficacy was inadequate, and so often, crop establishment was not good enough to confidently assess crop injury.

One exception was hairy vetch. This species exhibited good tolerance to a post-emergent application of Basagran, a result seen at all three locations. The results from two trials suggest that Clarkia tolerated POST applications of Quinstar. Not enough data were collected to conclude the other four species. See Table 4 for crop coverage data.

Conclusions

Site preparation was an essential consideration in our study. Soil compaction and perennial weed pressure must be addressed to have a successful pollinator habitat planting. It was also clear that pre-emergent herbicides can improve habitat establishment, but safety must be adequately established. This is especially true of different soil types and environments. In California's Central Valley, pendimethalin has been seen to occasionally cause injury in poppy plantings, which is in contrast with this study. 

Figure 2: Crop coverage pictures from two months after planting the Lewis-Brown research farm show that the planted species (rows) tolerated several pre-emergent herbicides (columns). A black outline surrounds successful combinations seen in at least one of the other two trials. Combinations that were never seen to be successful again are surrounded by a red outline.

References:

Leponiemi, M., Freitak, D., Moreno-Torres, M. et al. (2023). Honeybees' foraging choices for nectar and pollen revealed by DNA metabarcoding. Sci Rep 13, 14753. https://doi.org/10.1038/s41598-023-42102-4

Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Web Soil Survey. Available online at the following link: http://websoilsurvey.sc.egov.usda.gov/.