Many formulations of natural pyrethrum insecticide (derived from the flowers of the Pyrethrum Daisy Tanacetum cinerariifolium) also contain piperonyl butoxide. Gardeners often wonder what this chemical is, what it does, whether it’s environmentally friendly, and if its approved for use in organic gardens.
Piperonyl butoxide (PBO) is a synthetic organic (carbon-containing) compound that plays a crucial role in enhancing the efficacy of many insecticides, particularly those derived from natural sources such as pyrethrum. This article explores the chemical nature of PBO, its mode of action, and its significance in the formulation of natural insecticides.
Chemical Composition and Properties
Piperonyl butoxide is a semi-synthetic derivative of safrole, a natural phenylpropene compound that is extracted from sassafras oil. It is an oily pale yellow to light brown liquid with a mild odor and a faint bitter taste. Its chemical structure is characterized by a methylenedioxyphenyl (MDP) moiety connected to a butoxyethyl side chain. The molecular formula of PBO is C19H30O5.
Why Is Piperonyl Butoxide (PBO) Added to Natural Insecticides?
The main function of PBO in insecticide formulations is to act as a synergist. It does not possess any insecticidal properties of its own, but increases the potency of other insecticides such as carbamates, pyrethrins, pyrethroids, (synthetic pyrethrins), and rotenone. Since it is enhances the effectiveness of pesticides, it is also referred to as a potentiator.
Although natural pyrethrins are potent insecticides, their impact is often short-lived because they are rapidly broken down by enzymes called cytochrome P450 monooxygenases (P450s) within the insect’s body. The enzymes that play a critical role in the metabolism and detoxification of many substances, including pesticides.
PBO enhances the effectiveness of pesticides such as pyrethrins by inhibiting P450 enzymes, which prevents the insecticide from breaking down. This allows the insecticide to stay active in the insect’s system for much longer, thus increasing its effectiveness. PBO may also impact esterases, another set of detoxifying enzymes insects utilise to degrade pesticides within their bodies.
This extended activity also means that smaller amounts of pyrethrins can be used to achieve effective pest control. Additionally, PBO helps prevent insects from developing resistance to pyrethrins by disrupting their metabolic pathways, further bolstering the efficacy of the insecticide.
Toxicity and Environmental Impact of Piperonyl Butoxide
Piperonyl butoxide (PBO) is an important synergist in insecticidal formulations but, like all chemical compounds, it is not without concerns, and has specific environmental and health impacts. The toxicity of PBO to various organisms, its persistence in the environment, mobility in soil, and potential for bioaccumulation are critical factors in assessing its overall environmental footprint. Here’s a detailed overview of these aspects:
Toxicity of PBO to Various Organisms
- Humans: PBO is considered to be of moderate toxicity to humans. Acute exposure can cause skin and eye irritation, and inhalation may lead to respiratory discomfort. Chronic exposure concerns primarily relate to potential liver effects.
- Aquatic Organisms: PBO is highly toxic to aquatic organisms. It has been shown to have acute lethal concentrations (LC50) ranging from 0.008 mg/L to 0.136 mg/L for various species of fish and aquatic invertebrates. For instance, the LC50 for rainbow trout is around 0.015 mg/L over 96 hours.
- Bees and Other Beneficial Insects: PBO is of low toxicity to bees. However, when combined with pyrethrins (which are toxic to bees), the mixture can be more harmful due to enhanced potency.
- Birds: It is generally of low toxicity to birds; for example, the acute oral LD50 for bobwhite quail is greater than 2150 mg/kg.
Piperonyl Butoxide (PBO) is classified by the U.S. Environmental Protection Agency (EPA) as a Group C, possible human carcinogen. This classification means that there is some evidence from animal studies to suggest that PBO could potentially cause cancer in humans, but the evidence is not conclusive. Group C is used when there is limited animal evidence of carcinogenic effects and insufficient or no data from epidemiological studies in humans. Curiously, the International Agency for Research on Cancer (IARC) evaluated PBO and reported that it is “not classifiable as to its carcinogenicity to humans.”
Persistence of PBO in the Environment
Piperonyl butoxide (PBO) persistence in the environment can be assessed through its half-life in various media such as air, soil, and water. The half-life of a chemical refers to the time required for it to reduce to half of its initial quantity,, and is a significant indicator of how long it remains active and potentially impactful in the environment.
Below are details on the half-life of PBO in different conditions, along with explanations on how environmental factors influence these figures.
1. Half-Life in Air
PBO in the atmosphere is primarily subject to photodegradation (breakdown due to sunlight). The half-life of PBO in air is relatively short, typically ranging from a few hours to one day, depending on the intensity of sunlight and the presence of other atmospheric conditions that might accelerate degradation, such as ozone and other reactive gases. Photodegradation leads to the breakdown of PBO into smaller, generally less harmful compounds.
2. Half-Life in Soil
The persistence of PBO in soil can vary widely based on several factors:
- Aerobic Conditions: Under aerobic conditions (presence of oxygen), PBO tends to degrade faster. Its half-life in soil under aerobic conditions typically ranges from 10 to 14 days. This is due to the action of soil microorganisms that use oxygen in their metabolic processes to break down organic compounds like PBO.
- Anaerobic Conditions: In anaerobic conditions (lack of oxygen), such as waterlogged soils, the half-life of PBO can be significantly longer. Under these conditions, the degradation processes are slower, and PBO can persist for several weeks to months.
- Soil Type: The type of soil also affects the persistence of PBO. Soils with high organic matter tend to have shorter PBO half-lives due to higher microbial activity, whereas sandy soils with low organic content might see longer persistence due to reduced microbial degradation and increased leaching potential.
3. Half-Life in Water
In aquatic environments, the half-life of PBO can vary:
- Temperature and Light Conditions: Warmer temperatures and higher levels of UV light can reduce the half-life of PBO in water, typically ranging from a few days to a couple of weeks.
- Oxygen Levels: Similar to soil, the presence of oxygen can enhance the breakdown of PBO. In well-oxygenated waters, PBO degrades more quickly than in oxygen-poor (anaerobic) conditions.
4. General Environmental Conditions Impacting the Half-Life of PBO
- Temperature: Higher temperatures generally increase the rate of microbial activity and chemical reactions, thereby decreasing the persistence of PBO.
- pH Levels: The acidity or alkalinity of the environment can also affect the stability of PBO, with extreme pH conditions potentially enhancing degradation.
- Presence of Other Chemicals: The presence of various organic pollutants and heavy metals found in the environment can inhibit the breakdown of PBO, increasing its persistence, leading to longer residual times and potentially greater environmental impact. For instance, certain types of pollutants might react with PBO, altering its degradation rate.
- Organic pollutants like hydrocarbons and chlorinated compounds can compete with PBO for the same metabolic pathways in microorganisms, potentially slowing down its degradation.
- The presence of heavy metals such as copper and zinc can inhibit the microbial activity that is crucial for the biodegradation of organic compounds like PBO. These metals can interfere with enzyme systems in microorganisms, reducing their efficiency in breaking down PBO.
Understanding these dynamics is crucial for assessing the environmental impact of PBO and for designing strategies to mitigate its persistence in sensitive habitats.
Soil Mobility of PBO
PBO has moderate to high soil mobility depending on the soil’s organic matter content and the compound’s water solubility. It has a relatively low water solubility (about 2 mg/L at 20°C), which suggests it can bind to soil particles, particularly in organic-rich soils.
In contrast, PBO’s mobility may increase in sandy soils or those with low organic content, leading to concerns about the potential for groundwater contamination. Given its moderate to high mobility in less organic soils, PBO can leach into groundwater or runoff into surface waters, causing wider environmental dispersion and possible contamination of water sources. This not only poses risks to aquatic life but also to other wildlife and potentially human health if these water sources are used for drinking water or irrigation.
Bioaccumulation Potential of PBO
PBO has a low potential for bioaccumulation in aquatic and terrestrial organisms. The bioconcentration factor (BCF) in fish is relatively low, suggesting that it does not accumulate significantly in aquatic biota. Studies indicate that BCF values are generally less than 500, which is considered low in terms of environmental persistence.
Despite having a low bioaccumulation factor (BCF) in individual organisms, PBO’s moderate persistence in the environment and potential for chronic exposure raise concerns about its cumulative effects in food chains. Its presence in water systems and soil can lead to repeated exposure for aquatic and terrestrial organisms, possibly leading to unforeseen long-term ecological effects.
Environmental Implications
While PBO is an effective insecticide synergist, it does pose certain environmental risks, particularly to aquatic life due to its toxicity and moderate soil mobility. However, its low persistence in soil and low bioaccumulation potential do mitigate some of these issues.
To minimize environmental risks from Piperonyl butoxide (PBO), it’s important to use precise application methods to prevent wind drift when spraying, and to asses the nature of the site where it is used, to minimise off-target impacts, especially in aquatic environments.
Can PBO-Containing Pyrethrum Insecticides Be Certified Organic?
Organic certifications generally do not approve the use of Piperonyl Butoxide (PBO) in products that are labeled as organic. PBO is a synthetic chemical, and most organic standards strictly limit or prohibit the use of synthetic substances.
To be certified as organic, natural pyrethrum insecticides should be free of synthetic additives such as PBO. They are permitted to contain only those ingredients that meet organic criteria. Consequently, insecticides with PBO are generally ineligible for organic certification by most established standards, which prioritise natural and eco-friendly ingredients.
References
- Cross, A.; Bond, C.; Buhl, K.; Jenkins, J. 2017 Piperonyl Butoxide (PBO) General Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. npic.orst.edu/factsheets/pbogen.html.
- National Center for Biotechnology Information. PubChem Compound Summary for CID 5794, Piperonyl butoxide. https://pubchem.ncbi.nlm.nih.gov/compound/Piperonyl-butoxide.