The IPM Continuum of Weeds in Snap Bean

Contributed by Aaron Becerra-Alvarez, Department of Horticulture, Oregon State University

Having recently begun my position at Oregon State University, I am being introduced to the diverse agricultural region of the Willamette Valley of Western Oregon. In the Willamette Valley, more than 170 different crops are produced from grass seed crops, perennial crops, fresh, processed vegetables, and vegetable seed. One important vegetable crop is snap beans (Phaseolus vulgaris L.). Today, I want to discuss Integrated Pest Management (IPM) of weeds in snap beans.

First, what is the definition of IPM? If you browse through the internet, you will find various definitions. Some definitions are specific, and some are broad. However, all definitions have similar ideals and objectives to develop effective pest management and minimize risks using various tactics with scientifically based knowledge. IPM can be characterized as a continuum, and practitioners may already be somewhere on the IPM continuum in the Willamette Valley, despite heavy reliance on herbicides.

IPM can be divided into five major components:

• Pest identification
• Field monitoring of pest populations
• Developing control action guidelines
• Preventing pest problems
• Integrating cultural, mechanical, physical, biological, and chemical management tools

In Table 1, common weed management practices are presented on the IPM cumulative continuum grouped by the five major components. The list is not exhaustive but presents some broad ideals and practices for weed management.

Snap bean production in the Willamette Valley is commonly in crop rotations with other vegetables, grass seed or forage crops. Crop rotations help delay or prevent herbicide resistance and control problematic weeds. This may be the reason why there are not many recorded herbicide-resistant weed populations in these systems yet. However, there is evidence of resistance developing in lambsquarters (Chenopodium album L.) to bentazon (Basagran, Group 6)  as mentioned in an earlier WoW blog post and in presentations by Riboldi et al. (2020).

Good record keeping of weeds in annual production systems and their management tactics is useful to learn how and why a certain weed has become a problem. For example, in a 2019 and 2020 project in collaboration with Dr. Williams of USDA/Univ Illinois, 53 conventionally managed snap bean fields in the Willamette Valley were surveyed by the lab of Dr. Peachey for weeds present at time of harvest. Pigweeds (Amaranthus spp.), lambsquarters, and nightshades (Solanum nigrum L. and S. sarrachoides L.) were the dominate weeds present that impacted yield and harvest. Shepherds’ purse was common, but weeds of this species were usually small and caused little interference (Figure 1). The most common herbicides used were bentazon at 75%, EPTC at 67% (EPTAM, Group 15), and imazamox at 58% (Raptor, Group 2) (Figure 2). It will be interesting to follow up on a similar survey in the coming years and determine if any changes in practices have affected the dominant weed species.

Figure 1. Top 13 weeds present in 53 conventionally managed snap bean fields in 2019 and 2020. Percentages shown in the chart (based on average density across all sites) are relative to the 13 weeds presented, not the entire suite of weeds found in fields. Amaranthus species included A. retroflexus, A. powellii and hybrids.

Nearly all herbicides available for snap beans are soil residual herbicides because of limited postemergence options (Figures 2 and 3). Historical knowledge of the weeds present in the field becomes integral so that the appropriate herbicides and timings are used for best control. Herbicides have become a major tool in the area and the practitioners should have good knowledge of the available options because of the herbicide soil persistence concerns which can limit crop rotation options.

In snap beans, there is potential to integrate various tactics like cultivation, flaming, electricity, and hand weeding with herbicides; however, these tactics must be demonstrated to be cost effective and readily available. New technologies like smart cultivators, precision sprayers, and other new technology are gaining interest among producers and can create more management tools to integrate into production systems that may eventually allow reductions in herbicide use and hopefully the cost and complexity of managing weeds.

Agroecosystems are complex, and developing an IPM plan can be very challenging, especially when considering the many interactions among other pests like insects and diseases. However, IPM should be viewed as a continuum where we slowly aim for the highest level by learning and adapting to scientifically based knowledge (Table 1). I am looking forward to working with practitioners to develop IPM for weeds in snap beans and in many other Oregon vegetable and specialty seed crops. The diversity of crops and engaging practitioners provides many opportunities for research and extension in the region.

References

Benbrook, C.M., Growth III, E., Halloran, J.M., Hansen, M.K., and Marquardt S. (1996). Pest Management at the Crossroads. Yonkers, NY: Consumers Union.

Flint, M.L. (2012). IPM in Practice, Principles and Methods of Integrated Pest Management. 2^nd ed. Oakland, CA: University of California, Agriculture and Natural Resources Publication 3418.

Riboldi, L., Peachey, R., Hulting, A., Brunharo, C. (2020). Integrating UPLC-qTOFMS and UPLC-MS/MS to characterize resistance to bentazon in Chenopodium album L. populations from Oregon. Pg 461 in Proceedings of the Western Society of Weed Science. Maui, HI: Western Society of Weed Science.

Swanton, C.J., Mahoney, K.J., Chandler, K., and Gulden, R.H. (2008). Integrated weed management: knowledge-based weed management systems. Weed Science, 56(1), 168-172.