Vegetable Notes 2025 Vol. 37:25

Ways to prevent the evolution and proliferation of herbicide-resistant weeds on your farm

Herbicide-resistant weeds are a subset of a weed species that have a genetic variation that causes them to no longer by killed by a normal rate of an herbicide^1,2. The important aspects of that definition are:

• The trait is genetic
• The weed species used to be killed by the herbicide
• Now a subset of the population of that weed species, with the genetic trait, isn’t killed

Herbicide resistance occurs the same way any other resistance issue develops, such as insecticide or antibiotic resistance³: through genetic variation and selection pressure (Fig. 1).

There is always natural genetic variation within a species, and sometimes that variation just happens to result in a small change that protects an individual from a management tool. Resistance can develop for any management tool; for example, by changing the color of their seedlings, weeds can even evolve resistance to hand-weeding⁴. Herbicide resistance mechanisms have been categorized into two types of genetic changes. In target-site resistance there is a change to the binding site of an herbicide, which makes it so that the herbicide can no longer work⁵. In non-target site resistance, a change prevents an herbicide from getting to its site of action, for example by growing a thicker cuticle, or changing the function of transporter molecules^6,7.

After there is a genetic variation that protects an individual from a management tool, then there needs to be unidirectional selection pressure. Repeated use of the same tool creates unidirectional selection pressure and the genetic variation that protects an individual can proliferate and become prevalent in a field. The stronger the selection pressure—meaning the more effectively it kills the susceptible individuals of a species—the quicker resistant individuals of that species become dominant. Herbicides can do an excellent job killing susceptible weed species, so unfortunately, that means they exert a really strong selection pressure. In the United States, we rely heavily on herbicides for our weed management, since they are both effective and relatively affordable to use⁸. Because we rely so heavily on herbicides, and herbicides exert a strong selection pressure, there are a lot of herbicide-resistant weed populations in the U.S^8,9.

This results in real-world consequences. Once there are large populations of weeds resistant to herbicides that a farmer had relied on, they may struggle to achieve adequate weed control and the cost of weed management increases dramatically¹⁰. You should not hold out hope that new herbicides will become available that can replace the ones that are no longer effective; since 1980 there have only been 3 new herbicide active ingredients brought to market⁹. Instead, herbicides should be treated like a finite resource¹¹. 

Luckily, there are actions you can take on your own farm to slow the evolution of herbicide resistance, regardless of whether there are herbicide-resistant weeds at neighboring farms³. Below are strategies specific to vegetable crops that have been proven to help¹:

1. Scout the weeds on your farm regularly.
   If you know what weeds are where, by creating a weed map (Fig. 2), you can track how well different management tools control your weeds, identify individuals that have escaped management, and understand whether populations are growing or shrinking. This information will help you spot the signs of resistance development quickly. Try to scout after an herbicide has been applied and the weeds are dead, but before seeds of any escaped weeds would be produced.
    
2. Understand the biology of the weeds on your farm.
   It’s great to know which weeds are where, but then you should try to learn how those different weed species behave. Understanding emergence patterns, lifecycle, and the primary form of reproduction are most important. For example, if you know a species doesn’t produce much viable seed, you might be less worried about herbicide resistance development compared to a species that produces hundreds of thousands of seeds from one individual.
    
3. Be especially aware of weed species that are prone to evolving herbicide resistance.
   There are several common weed species that make up the majority of herbicide-resistant populations because they can easily evolve resistance¹¹. Be especially mindful of these species (Fig. 3) and focus on making sure to prevent seed-set by managing escapes. These species are:

• Amaranthus spp. (pigweeds)
• Conyza canadensis (horseweed)
• Ambrosia artemisiifolia (common ragweed)
• Setaria spp. (foxtails)
• Chenopodium album (common lambsquarters)
• Bassia scoparia (kochia)
• Ambrosia trifida (giant ragweed)

 

4. Ensure that your field is weed-free when you plant.
   This is helpful in two ways. If you plant into a field with established weeds, the weeds will have a competitive advantage over the crop. The rest of the season you will be playing catch-up with weed control, rather than focusing on growing competitive crops, and your risk of weed escapes is higher. You also have more weed-control options before planting, including more non-herbicide options, so take advantage of this stage to control all existing weeds.
    
5. Manage the movement of weed propagules.
   Be careful not to inadvertently bring seeds from herbicide-resistant weeds to your farm. Weed seeds can easily get lodged in harvest equipment and be deposited into new fields. Sometimes, these escapes have survived until harvest because they are herbicide-resistant. Weed seeds are also often brought in through contaminated organic amendments like straw, woodchips, or compost, or even through contaminated crop seed when the weed seed and crop seed are similar. Be sure you trust the source of any resource you bring to your farm.
    
6. Use appropriate cultural weed management tools to reduce reliance on herbicides.
   Cultural weed management tools will improve the competitive ability of your crop and reduce your reliance on herbicides. Some common cultural tools for vegetable production include using effective crop rotations, growing competitive cultivars, fertilizing the crop not the weeds, decreasing planting spacing, timing weed management activities when the weeds are most competitive with the crop, planting cover crops, and using fallow periods when appropriate.
    
7. Use appropriate physical and mechanical weed management tools to reduce reliance on herbicides.
   Using a variety of weed control tools will reduce your need to use herbicides. It will also help control weeds that do happen to have genetic traits that confer herbicide resistance by incorporating another, non-herbicide, weed management event. Most vegetable production systems incorporate some sort of mulch, whether plastic, organic, or living, as well as tillage, which both removes germinated weeds and redistributes seeds within the soil profile, decreasing the density of any herbicide-resistant seed near the soil surface. There are many other physical and mechanical tools that are less ubiquitous in vegetable production, such as flame weeders, electrical weeders, and laser weeders.
    

8. Hand-weed escapes as necessary.
   Weed escapes may have survived an herbicide application due to herbicide resistance. You definitely do not want an escape that potentially has herbicide resistance to set seed. Hand-weeding escapes may seem annoying, but it’s ultimately much easier than managing herbicide-resistant seeds in the soil seedbank.
    

9. Use the suggested rate of an herbicide at the correct timing.
   Applying a sub-lethal dose of an herbicide to a weed will increase its chances of evolving resistance, especially non-target site resistance. If you apply too little herbicide, or wait until the weeds are too big, then you are applying sub-lethal does. You also can’t apply too much herbicide or get weeds too early, because then you risk damage to your crop or not catching the majority of weeds. You really need to take the Goldilocks approach—apply the correct amount of herbicide at the correct time.
    
10. Rotate herbicide modes of action.
    If you change the modes of action being used on a weed community from year to year, then the selection pressure is no longer unidirectional. If some individuals do begin to develop resistance, using a different mode of action that still targets that weed species the next year will successfully kill those individuals. If there aren’t enough herbicides labelled for use in a crop to feasibly rotate modes of action, rotate the type of crop grown in that field instead. To make it easier to know if you are rotating through different modes of action, each herbicide is assigned a group number which corresponds to a mode of action. If you are using herbicides with different group numbers, you are using different modes of action. If all your herbicides have the same group number, they all have the same mode of action. These numbers are assigned by the Global Herbicide Resistance Action Committee. They re-assigned several herbicides in 2020, so be sure you’re using the most up-to-date numbers.
     
11. Apply multiple modes of action together.
    The chance of a weed having natural genetic variation to two or more different modes of action at the same time is much smaller than just one mode of action. Applying at least two modes of action together can reduce the risk of herbicide resistance development. However, this is only true if the weed species community is not already resistant to one of the herbicides in the mix. If a weed is already resistant to one of the modes of action, then the chances of evolving resistance to both modes of action is the same as evolving resistance to just one mode of action. Use this strategy when you are sure that no weeds in your field are already resistant to an herbicide, otherwise you are selecting for a weed that is resistant to two modes of action and life will be even harder. 
     
12. Be especially careful with herbicides that weeds easily become resistant to.
    Similar to just a few weed species being responsible for the majority of herbicide-resistant species, a few modes of action are especially easy to become resistant to¹¹. In vegetables, be careful using herbicides with these modes of action and be sure to effectively rotate through them.

• Acetolactate synthase inhibitors (group 2)
• 5-enolpyruvl-shikimate-3-phosphate synthase inhibitor (group 9)
• Acetyl CoA carboxylase inhibitors (group 1)
• Plant growth regulators (group 4)
• Ureas (group 7)

Works Cited

1. Boyd, N. S. et al. Occurrence and management of herbicide resistance in annual vegetable production systems in North America. Weed Sci. 70, 515–528 (2022).
2. Norsworthy, J. K. et al. Reducing the Risks of Herbicide Resistance: Best Management Practices and Recommendations. Weed Sci. 60, 31–62 (2012).
3. Evans, J. A. et al. Managing the evolution of herbicide resistance. Pest Manag. Sci. 72, 74–80 (2016).
4. Ye, C.-Y. et al. Genomic evidence of human selection on Vavilovian mimicry. Nat. Ecol. Evol. 3, 1474–1482 (2019).
5. Gaines, T. A. et al. Mechanisms of evolved herbicide resistance. J. Biol. Chem. 295, 10307–10330 (2020).
6. Délye, C., Gardin, J. a C., Boucansaud, K., Chauvel, B. & Petit, C. Non-target-site-based resistance should be the centre of attention for herbicide resistance research: Alopecurus myosuroides as an illustration. Weed Res. 51, 433–437 (2011).
7. Mucheri, T., Rugare, J. T. & Bajwa, A. A. Mechanistic understanding and sustainable management of non-target site herbicide resistance in modern day agriculture. Adv. Weed Sci. 42, e020240056 (2024).
8. Heap, I. Global perspective of herbicide-resistant weeds. Pest Manag. Sci. 70, 1306–1315 (2014).
9. Heap, I. International Herbicide-Resistant Weed Database. https://www.weedscience.org/Home.aspx. Impact of weeds on Australian grain production.
10. Scott, B. A., Vangessel, M. J. & White-Hansen, S. Herbicide-Resistant Weeds in the United States and Their Impact on Extension. Weed Technol. 23, 599–603 (2009).

Calantha: A New Tool for Colorado Potato Beetle Management

Insecticide Resistance in CPB

Colorado potato beetles (CPB) are among the most difficult pests to manage on both organic and conventional vegetable farms in Massachusetts, in large part because of their widespread resistance to many common insecticides. CPB are unusually well-adapted to detoxifying chemicals they are exposed to, and they have now developed resistance to more than 50 pesticides from nearly every IRAC class. Populations can be isolated on different farms and may remain locally susceptible to some chemical control options, but using one class of insecticides against a population just a handful of times in succession can rapidly lead to resistance. 

This is why resistance management is crucial for controlling this pest in the long term. Rotating between different classes of insecticides for consecutive sprays helps slow down this process, and the more options that are available, the more time there can be between uses of the same class of insecticides. Every new option helps stave off resistance to every other insecticide option, leading to more sustainable IPM. 

One new tool for conventional growers is Calantha, the newest insecticide registered in Massachusetts for use against CPB. It is a foliar-applied formulation that uses a completely new mode of action making use of a natural biological process called RNA interference or “RNAi” (Group 35). This is an area of rapid innovation and more of these products are likely to come on the market in the coming years, so we will take some space to explain the biology below, then share our own research results using Calantha to control CPB in 2025.

How RNAi Works

RNA plays a key role in how cells create proteins. Messenger RNA (mRNA) carries instructions from the genome to the cell's ribosomes, where proteins are assembled. Double-stranded RNA (dsRNA) is another type of RNA which is used by viruses for a range of purposes. Many organisms, including some plants, animals, and fungi, have a natural defensive response against viruses that is triggered when dsRNA is detected. This response is called RNA interference (RNAi). When cells detect dsRNA, they chop it into small RNA fragments. These fragments guide the cell to find and destroy viral mRNA strands with matching sequences, obstructing the virus from creating proteins and replicating.

Calantha uses this same natural process to control CPB. Its active ingredient, ledprona, is a specially designed dsRNA molecule. When eaten by CPB, the dsRNA is absorbed by gut cells, triggering the RNAi response. However, instead of the resulting RNA fragments matching viral mRNA, this dsRNA breaks into fragments that match a set of CPB’s own mRNA which normally carries instructions for a piece of protein machinery that is essential for breaking down damaged proteins. This results in the CPB cell silencing its own genes instead of those of a virus. Without this machinery, damaged proteins accumulate inside the beetle's cells, disrupting essential cellular functions and ultimately causing the beetle to die.

Why is this important? 

Because Calantha targets such a crucial gene for CPB’s regular cellular functioning, it may be more difficult, though not impossible, for CPB to develop resistance to Calantha. Also, because Calantha is designed to target one highly specific piece of genetic code that is only found in CPB, it can selectively control CPB without harming other organisms, including natural enemies and pollinators. Prior to registering the product, the EPA conducted an extensive risk assessment of its effects on a range of non-target organisms including other insects, fish, birds, and mammals, and only observed adverse effects in a few closely related beetle species with similar target gene sequences. However, even among beetles that are somewhat susceptible to the insecticide, there is low risk due to several other factors, described below:

1. Double-stranded RNA breaks down rapidly in the environment (within 3 days), so there is a narrow window in time and space where non-targets may be exposed to these molecules.
2. CPB are constantly feeding on the treated crop, so they receive a much higher dose of the insecticide than any non-targets would.
3. Even if exposed to high enough doses of the dsRNA, few other organisms are as susceptible to this mode of action as CPB; most organisms, including humans, do not take up ingested dsRNA into their cells as readily as CPB or have such a strong systemic RNAi response to dsRNA. 

All this combined means there is an extremely low likelihood of non-target effects, making it safer for human and environmental health relative to other insecticides. The EPA also evaluated these and other factors impacting the safety of the product, releasing a decision document detailing its more than four-year review of Calantha’s safety and efficacy data and information submitted by GreenLight Bio, including its response to submissions received during an extensive public comment period.

Research Trials

This summer, we evaluated the efficacy of Calantha in preventing damage to foliage and maintaining yield in potato. On May 14 we planted 4-ounce seed pieces of Yukon Gold potatoes at 12-inch in-row spacing and 5-feet between rows. Plots were 8-ft long with 2-ft planted buffers between plots within the bed. Plots were arranged in a randomized complete block design with 4 replications of each treatment, and buffer beds were planted on both sides of the trial beds to aid with CPB distribution. After planting, Lorox (linuron) was applied at a rate of 1.5 lbs/A for pre-emergent broad-leaf weed control; subsequent weed control was done by hand. 

Treatments included two rates of Calantha compared to a conventional standard (Admire Pro, foliar 1 week after emergence), an organic standard (Entrust SC), and an untreated control (see table 1). We monitored damage to the foliage weekly and harvested the trial at the end of the first generation of CPB. The data collected met the assumptions for normality and so were analyzed using a general linear model (PROC GLM) and means were compared using Tukey’s honestly significant difference test (α = 0.05) using SAS (SAS v.9.4, SAS Institute, Cary, NC). 

CPB pressure in the trial was extremely high, with untreated plots being completely defoliated within 6 weeks. Plots treated with the conventional standard had similarly high levels of foliar damage and low yields, and performed statistically as poorly as the untreated control. This control failure indicates that the population of CPB at our research farm is resistant to imidacloprid, despite it not being used there for at least a decade. The two rates of Calantha performed equally well, meaning a grower could achieve just as good control with 2 applications as they would with 3, saving some time and money, and they performed as well as the organic standard, Entrust SC, which provided statistically significant reductions in damage and increases in total yield and tuber size. 

Our results highlight Calantha as an effective new chemical control option for conventional CPB management. However, like with any other insecticide, resistance to RNAi-based biopesticides is likely to emerge if resistance management strategies are not used. As stated on the Calantha label, this product should target hatching eggs and small larvae, and can only be used on 1 of the 2 generations of CPB that we see each year in MA. Calantha is a liquid formulation (SL) with no special storage requirements, is compatible with many other tank-mix partners, and should integrate well into potato IPM programs.

References

Chen, Y. H., Cohen, Z. P., Bueno, E. M., Christensen, B. M., & Schoville, S. D. (2023). Rapid evolution of insecticide resistance in the Colorado potato beetle, Leptinotarsa decemlineata. Current Opinion in Insect Science, 55, 101000. https://doi.org/10.1016/j.cois.2022.101000

Dimase, M., Bradford, B. Z., Weissner, M., Buzza, A., Manley, B., Alyokhin, A., Groves, R. L., & Nault, B. A. (2025). Optimizing application timing and frequency of a novel dsRNAi-based insecticide for Colorado potato beetle management. Pest Management Science. https://doi.org/10.1002/ps.70294

Rodrigues, T. B., Mishra, S. K., Sridharan, K., Barnes, E. R., Alyokhin, A., Tuttle, R., Kokulapalan, W., Garby, D., Skizim, N. J., Tang, Y., Manley, B., Aulisa, L., Flannagan, R. D., Cobb, C., & Narva, K. E. (2021). First Sprayable Double-Stranded RNA-Based Biopesticide Product Targets Proteasome Subunit Beta Type-5 in Colorado Potato Beetle (Leptinotarsa decemlineata). Frontiers in Plant Science, 12. https://doi.org/10.3389/fpls.2021.728652

US EPA, O. (2023, December 26). EPA Registers Novel Pesticide Technology for Potato Crops [Announcements and Schedules]. https://www.epa.gov/pesticides/epa-registers-novel-pesticide-technology-potato-crops

Zhu, F., Xu, J., Palli, R., Ferguson, J., & Palli, S. R. (2011). Ingested RNA interference for managing the populations of the Colorado potato beetle, Leptinotarsa decemlineata. Pest Management Science, 67(2), 175–182. https://doi.org/10.1002/ps.2048