Organic Field Bindweed Management: Case Studies and Research Findings

Researchers on a NIFA ORG project about organic management of field bindweed investigated field bindweed moth (Tyta luctuosa) larvae as a management strategy. We engaged Willamette Valley producers of multiple sizes filling diverse production and market niches. We also gathered data at an Oregon State University research farm.

We worked with a small market producer, who struggles to manage field bindweed in his greenhouse. In the greenhouse, field bindweed competes with vegetable starts for fertilizer and water. Transplanting starts into the field risks spreading bindweed if its seeds or roots have contaminated the potting mix. Our two commercial collaborators were blueberry producers. Blueberries are mulched for weed control with sawdust and various types of weed mats. Both commercial farms were using NOP-approved insecticides.

Increasingly, commercial blueberry operations have adopted zippered plastic weed mats, that can be opened to facilitate fertilization and repair or replacement of drip irrigation components. In commercial blueberry production, bindweed grows into the bushes through the openings in the mat through which the bushes grow. Field bindweed interferes with the machine picking widely adopted in commercial blueberry operations. It also grows around the edges of the mat and climbs into the bushes.

Here we summarize the key findings and challenges identified on four participant farms as we investigated the value of Tyta luctuosa herbivory as a field bindweed management tactic.

Research Site 1 was a small, diversified vegetable and specialty seed farm, implementing organic methods, although they are not organically certified. The farm includes 33 acres, 22 of which are in cropland and another 11 in riparian forest. Of the 22 acres of cropland, 3 acres are intensively managed vegetables for farmers market sales.

Weed control is accomplished through cultivation, hoeing, hand pulling, and seed load management. Field bindweed was chiefly a problem in the greenhouse where starts are produced. The owner had been pulling bindweed by hand whenever he saw it. Because the greenhouse was not an ideal setting for the release of T. luctuosa larvae, we identified five quarter-meter quadrats outdoors in uncontrolled habitat that was not in production, to serve as an establishment nursery. Ten larvae were released on June 13 onto each quadrat (50 larvae total) and monitored through July 24.

Key findings at Research Site 1:

• Herbivory – evidence of larval feeding – was noted 30 days after the release of the larvae in each treated plot, but was low at the final evaluation, 41 days after release. Possible reasons for this include that the larvae stopped feeding and had pupated; predation or parasitization of the larvae by unknown insects, birds or animals; or bindweed outgrew the impact of the larvae and herbivory was more difficult to detect.
• Flowering was reduced 60% in treated plots vs. the control at the final evaluation which supported the grower’s weed seed load management strategy.  However, at 30 days after the release of the larvae, flowering was higher in treated plots. This could represent a compensatory mechanism by the plant in response to herbivory.
• No notable difference in shoot length was observed between treated and untreated quadrats.

Challenges at Research Site 1:

• Emmelina monodactyla (plume moth) is another moth whose larvae feed on field bindweed. Its presence on Research Site 1 was identified after monitoring began. E. monodactyla is not host-specific enough to be considered as a biocontrol agent. We are less familiar with the type of leaf damage it causes and may have attributed its feeding to T. luctuosa feeding. Thus, initial confusion in identifying herbivory may have confounded the evaluations. By the end of the monitoring period, the five quadrats had coalesced into a single patch of field bindweed.
• The owners of Research Site 1 stayed with the project a full year but withdrew because of their fears that field bindweed would become unmanageable on their farm. The grower’s primary concern - the greenhouse – was not suitable for releasing larvae and will continue to be managed by hand pulling.

Research Site 2 was 375 acres of commercial blueberries. At the time of the study, about one quarter of that was under transition to organic production. Their berries are distributed for fresh markets, with some of the crop going to processing.

At research site two, our trials were limited to the organic-transition block. Larvae were enclosed in 8” x 10” mesh bags with field bindweed shoots to limit larval predation and parasitism. Each blueberry bush (n=6) was treated with a mesh bag containing 5 or 10 larvae with a zero larvae control. This bagging method was only implemented at Site 2. Bagged larvae were placed in the field with shoots on July 8 and monitored through Aug 16.

Key findings at Research Site 2:

• We did not find any difference between herbivory rates in mesh bags containing 5 larvae or 10 larvae.
• Data were insufficient to evaluate flowering reduction or shoot length. Future efforts will incorporate more spatial area and limit releases to 5 larvae per bush.
• A sufficient number of lab-reared larvae is crucial to field experiments.

Challenges at Research Site 2:

• T. luctuosa larvae will prey on one another when their density rises above an optimum. It may be that the larvae self-reduced their density in the mesh bags to an acceptable level as the cause of the lack of a treatment difference between 5 larvae and 10.

• Data recording difficulties reduced the accuracy of data at Research Site 2.  We believe the counts and other monitoring protocols were correct, but there was confusion in the labeling of the plots. We are unsure which monitoring dates belong together.
• The T. luctuosa treatment block was treated with Entrust (insecticide) one day after larvae were placed. Compatibility of T. luctuosa with NOP-approved pesticides needs to be examined.

Research Site 3 This cooperating farm is owned by one of the largest producers of organic blueberries in the western US.  The cooperating corporation manages over 400 acres of organic blueberries. The manager of Research Site 3 is well-known to members of our research team and has collaborated on other pest management projects. Research Site 3 incorporates zippered weed mat and picks by machine, and benefits from a dedicated weeding crew.

Key findings at Research Site 3:

• Initially, flowering was reduced by about half, although this benefit decreased over time.
• No reduction in shoot biomass was noted between treated and untreated plots.
• Moths were found in an adjacent hazelnut orchard in year 2, but not in the blueberries. We believe that adult Tyta found in the hazelnuts were due to larval releases in the neighboring blueberries the year before. We hypothesize that anything being sprayed on the berries encouraged the moths to move into a neighboring perennial crop.
• Tyta moths are known to disperse by at least 2 miles, but intentional releases have been limited in the Willamette Valley.

Challenges at Research Site 3:

• Although plots were sufficiently flagged, and we had communicated with thefield manager, bindweed plots were taken out by the weeding crews, the field was mown, and flags removed.
• Larvae were released in other blueberry rows at the same site within 2 weeks, but timing may have been insufficient to detect differences in herbivory.

Research Site 4 was a university research farm. The farm includes 170 acres, of which some 120 acres are plot ground. This farm is that it is under experienced research management, ensuring that bindweed trials could continue uninterrupted. T. luctuosa had been released on this site in 2014, in an adjacent unmanaged plot, so it is likely that a that population is now established there. An experiment was established in an out of production hazelnut/wheat production area within the research farm. Plots (n=12) had varying initial percent cover offield bindweed within each block. Treated plots were augmented with 10 larvae on AUG 13. Monitoring is ongoing at this site. One of our team conducted trials with pheromones and semiochemicals as part of a dissertation project. Once the dissertation is defended and published, more specific information about semiochemical findings will be made available.

Key findings at Research Site 4:

• Herbivory was evident in all plots but was more noticeable in those that had been augmented with additional larvae.
• Percent change in bindweed cover in the middle of each plot was evaluated SEPT 11. Average cover was 32% lower (than the initial measurement) where pheromones and larvae had been applied and 21% lower when floral-based semiochemicals and larvae were applied.
• Interestingly, applying larvae without pheromones or semiochemicals was associated with an increase in field bindweed cover by 16 and 22%, respectively.
• Percent change in flower production was monitored from JUL 9 to SEPT 10. The greatest reduction in number of flowers was found in plots that received larvae and a semiochemical or pheromone treatment. Flowering increase was greatest when treatments were applied without larvae (~45% increase). These findings are similar to what was noted with bindweed cover as the evaluation metric (above).
• All female moths trapped in wing traps had been drawn to a specific pheromone-semiochemical blend. That blend attracted more moths than the pheromone alone.
• It is important to note that untreated plots at this site are not true checks. Rather, they represent effects due to the existing, endemic population at the research farm. This was intentional, but future efforts will incorporate insecticide as an exclusion tactic to better determine herbivory effects in response to semiochemical treatments.

For more information on Integrated Management of Field Bindweed. See also:

• Integrated Management of Field Bindweed: Ecological Management Planning

Find more information about the USDA NIFA ORG project Harnessing the Voracity of the Biocontrol Tyta luctuosa to Improve Management of Field Bindweed During Transition to Organic and Beyond at https://www.eorganic.info/bindweed.