I have attended the annual symposiums of the California Invasive Plant Council (Cal-IPC) for 5 years. I have always learned something new and the most recent symposium in November 2022 was no exception. This year there was a lot of important information about herbicides that are widely used to eradicate non-native plants.
Several presentations reviewed the California laws that regulate pesticide use in California. (Slides for one of those presentations are available HERE.) The laws are designed to reduce risks of exposure to both applicators and the public.
The presentations emphasized the importance of legally mandated personal protective equipment (PPE) for applicators. The minimum PPE required by California law is protective eyewear and chemically resistant gloves:
Source: 2022 Cal-IPC Symposium
The toxicity of pesticides is rated by federal law as “Caution,” “Warning,” or “Danger,” with “Danger” indicating the most toxic and “Caution” the least toxic. These ratings are defined as signal words. Signal words of “Warning” or “Danger” require the applicator to also wear protective coveralls, in addition to protective eyewear and gloves.
Other types of PPE may be required by the product label, shown in this picture:
Source: 2022 Cal-IPC Symposium
Comparing the toxicity of organic and synthetic herbicides
Signal words can be used to compare the acute toxicity of different products. For example, the signal word on glyphosate products is “Caution,” indicating that it is considered less acutely toxic than other herbicides with higher toxicity ratings of “Warning” or “Danger.” Signal words are not a measure of long-term health damage of pesticides, such as cancer or kidney damage. Epidemiological studies of long-term health effects of pesticides are hotly disputed and are usually dismissed by the manufacturers of pesticides.
When glyphosate products were rated as a “probable human carcinogen” by the World Health Organization and tens of thousands of product liability lawsuits were filed by users of glyphosate products with cancer, there was a public backlash against the use of glyphosate partly because it is the most widely used herbicide on the market. Glyphosate is found in most of our food and in the urine of most people. The health damage done by glyphosate is the result of 40 years of widespread use by agriculture. Glyphosate’s “Caution” signal word does not reflect the long-term effects of its use.
Consequently, glyphosate has been banned in many places all over the world. Los Angeles County has banned glyphosate. Locally, it is no longer used by East Bay Regional Park District (EBRPD) in developed park areas such as picnic areas, parking lots, and playgrounds. Although EBRPD made an exception for “invasive” plants outside developed areas, they have significantly reduced their use of products containing glyphosate. They are using more “organic” herbicides. Marin County banned the use of glyphosate. They are using exclusively organic herbicides.
What is the difference between synthetic and organic pesticides? In general, organic products are derived strictly from sources in nature with little or no chemical alteration. Synthetic pesticides are products that are produced from chemical alteration.
Are organic pesticides less toxic than synthetic pesticides? The general public tends to assume that organic pesticides are less toxic than synthetic pesticides, such as glyphosate. Based on the signal words the EPA assigns to pesticides to evaluate toxicity, organic pesticides are not necessarily less toxic than some synthetic pesticides. Remember the signal words are “Danger” (the most toxic), “Warning,” and “Caution” (the least toxic.)
Several presentations at the Cal-IPC conference compared the toxicity of organic and synthetic pesticides, using signal words as a proxy for toxicity. This is a slide from one of the presentations:
I also compared the signal words of the organic products used by Marin County and East Bay Regional Park District. Although they are using some organic products not evaluated by the presentation at the Cal-IPC Symposium, many of the organic products they are using have a “Warning” signal word, which means the EPA considers them more toxic than glyphosate.
Clearly organic herbicides are not necessarily less toxic than synthetic herbicides and many organic herbicides are more toxic than glyphosate.
Comparing the efficacy of organic and synthetic herbicides
Are organic herbicides as effective as synthetic herbicides? One of the presentations made at the Cal-IPC Symposium reported the results of a field study comparing the effectiveness of three organic herbicides with three synthetic herbicides, all with “Caution” signal words:
Here’s a description of the field trial:
Here are the results of the field trial (one organic herbicide was removed from the field trial when glyphosate was reported as an undisclosed ingredient in the product):
WeedZap and Fireworxx are the organic herbicides used in the field trial. The organic herbicides used in the field trial were found to be less effective than synthetic herbicides considered equally toxic.
This finding was corroborated by a publication of the UC Nursery and Floriculture Alliance, entitled “Organic Herbicides –Do they Work?” The short answer to that question is, not very well:
- “Organic herbicides kill weeds that have emerged but have no residual activity on those emerging subsequently. Further, while these herbicides can burn back the tops of perennial weeds, perennial weeds recover quickly.”
- “These organic products are effective in controlling weeds when the weeds are small but are less effective on older plants.” The organic herbicides were significantly less effective when weeds were more than 12 days old.
- “…broadleaf weeds were easier to control [with organic herbicides] than grassy weeds.”
Comparing the cost of organic and synthetic herbicides
The field study comparing organic and synthetic herbicides also compared the costs of these different product types:
In other words, organic herbicides are considerably more expensive than synthetic herbicides.
The publication of the UC Nursery and Floriculture Alliance agrees: organic herbicides “are expensive and may not be affordable…Moreover, because these materials lack residual activity, repeat applications will be needed to control perennial weeds or new flushes of weed seedlings.”
Clearly, organic herbicides are not a substitute for synthetic herbicides because they are not less toxic, not as effective, and are very expensive. Cal-IPC considers that assessment of organic herbicides a justification for continued use of synthetic herbicides. I consider it an argument for declaring a truce in the war on “invasive” species. We have waged that war for over 30 years. We have not won that war. In fact, we lose ground every year. We have done more damage to the environment with our chemicals than the “invasive” species did. We have reached a dead end.
Herbicides and Climate Change
The most valuable lesson I learned at the Cal-IPC Symposium was that climate change is making herbicides less effective. Higher temperatures and higher levels of CO₂ are reducing the effectiveness of herbicides. This revelation was mentioned only briefly in a presentation by Regional Invasive Species and Climate Change (RISCC) Management Networks. A search of the scientific literature substantiated that revelation:
- ”Increased temperatures and elevated CO2 levels reduce the sensitivity of Conyza canadensis and Chenopodium album to glyphosate:” “Therefore, overreliance on glyphosate for weed control under changing climatic conditions may result in more weed control failures.”
- “High CO₂ and Temperature Increase Resistance to Cyhalofob-Butyl in Multiple-Resistant Echinochloa colona:” “…herbicide efficacy was reduced on Multiple-Resistant plants grown under high CO₂ or high temperature about 50% compared to Multiple-Resistant plants at ambient conditions.”
- ”Herbicide effectiveness in controlling invasive plants under elevated CO2: Sufficient evidence to rethink weeds management:” “We found that responses of the weed species to herbicide under elevated CO2 were species-specific… However, the C3 [cool season] grasses tended to be the most sensitive to herbicide application followed by the herbs and C4 [warm/hot season] grasses while shrubs and vines demonstrated the highest resistance. Our results highlight the need for broader testing to determine the species most likely to exhibit increased tolerance to herbicide in the future in order to improve management options beforehand and thus offset a future liability.”
These studies are just a small selection of the studies that respond to a search for “impact of heat and CO₂ levels on herbicide efficacy.” They all point to yet another reason why the chemical crusade on introduced plants is a dead end.
Climate change is a reality and it is here to stay. Climate change has changed the ranges of where native plants can survive and it has made it impossible to destroy the non-native plants that are capable of surviving in the changed climate. Switching from one poison to another will not overcome the forces of evolution, which dictate that vegetation changes when the climate changes.