Pyrethrum is a natural insecticide derived from the flowers of the Pyrethrum Daisy Tanacetum cinerariifolium (syn. Chrysanthemum cinerariifolium). It works by paralysing the insect’s nervous system, which eventually results in the death of the target pest.
Being of natural origin, it’s no surprise that pyrethrum is not a single ingredient at all, but a4dctually a mixture of six different pyrethrins (pyrethrin 1, pyrethrin 2, cinerin 1, cinerin 2, jasmolin 1, and jasmolin 2).
In this review, we will have a look at the best way to use pyrethrum to control pests and the ways to minimise its environmental impact.
Toxicity to Non Target Species
Just because an insecticide is natural and organic doesn’t mean it’s 100% safe all of the time! Pyrethrum is a direct contact insecticide, it has to be sprayed directly on the pest insect to be effective, and it is also a broad spectrum insecticide, which means it will non-selectively kill any insect you spray it on, including ‘good bugs’ such as predatory and beneficial insects.
It is important to note that bees are highly sensitive to pyrethrins, either by ingestion or contact. Doses in the micrograms (millionths of a gram) will kill bees! [1] Additionally, pyrethrum is toxic to fish and aquatic invertebrates. Pyrethrum is described as ‘probably nontoxic’ to earthworms by the Penn State Extension, College of Agricultural Sciences, with the qualifier ‘not enough data to categorise’ [2].
Environmental Persistence
As far as insecticides go, pyrethrum has a very low persistence, as it is unstable in light and air, and has a half-life (where 50% breakdown occurs) of less than 24 hours. It breaks down mainly by exposure to UV light (sunlight) even when it’s on the soil surface or in water. In soil, the pyrethrins (which make up pyrethrum) have a half-life of around 10 days under aerobic soil conditions and around 86 days in anaerobic conditions [3]. Remember, pyrethrum is one of the shortest acting insecticides, manufacturers always state a ‘best case scenario’ under ideal conditions.
NOTE: Under certain conditions many pesticides can persist far longer than claimed. It’s different for every pesticide, but factors such as soil acidity or alkalinity, presence or absence of organic matter, aerobic or anaerobic conditions, clay or sandy soils, light or dark, wet or dry soils can affect how long a pesticide persists in the environment.
Soil Mobility
Pyrethrum has very low soil mobility. It is not very water soluble and binds so tightly to soil it is considered immobile, so there is little to no risk of run-off and contamination of groundwater and waterways. Once pyrethrum is bound to the soil, it is broken down by soil microbes.
Bioaccumulation
Pyrethrum is so easily broken down that it does not build up in the food chain, and therefore it does not bioaccumulate – that is, the levels of pesticide do not build up in plants, animals and other living things.
Appropriate Use
Most pesticides can’t be used without affecting non-target species, including beneficial insects. Pesticide use is really a matter of calculated risk, and assessing the risk requires is a knowledge of the products being used, an awareness of their environmental impact, careful judgement and discretion as well as responsible use by the gardener. All pesticides can be used more safely and effectively with less impact on the environment when they are used appropriately as intended.
- Pyrethrum is a direct contact insecticide that has to be sprayed directly on the pest insect to be effective, the most appropriate way to use it is when pests are active, when you can see them!
- Pyrethrum is toxic to bees, so it is best to spray at times when the bees are unlikely to be around. Apply only during late evening, night, or early morning. [4]
- Pyrethrum is toxic to fish and aquatic invertebrates, so care needs to be exercised when spraying pyrethrum to avoid wind drift near waterways, ponds and water gardens. Where pyrethrum is absorbed by the soil, the risk of runoff into waterways is low.
Pyrethrins vs. Pyrethroids – Why pyrethrum is better than pyrethroids
Pyrethrins are natural plant-based insecticides that make up the mixture known as pyrethrum, while pyrethroids are synthetic chemical insecticides with a chemical structures similar to their natural counterparts and which work in a similar way.
While natural pyrethrum is quite safe, the synthetic pyrethroids are usually much more toxic and persistent, because they’re designed to be. Pyrethroids are modified to increase their stability in sunlight.
It is therefore important not to mistake insecticide formulations containing synthetic pyrethroids as natural pyrethrum, because if we consider a situation where it is appropriate to use natural pyrethrum, it may be totally inappropriate to use a synthetic pyrethroid.
The synthetic pyrethroids are highly toxic to aquatic organisms and US Agencies undertaking water quality monitoring efforts in California have discovered a new risk posed by these pesticides, especially the newer second-generation pyrethroids, which are more toxic, more resistant to breakdown under sunlight and persist longer in the environment.. “Findings further indicate that the unique physical, chemical, and toxicological properties of the pyrethroid class of chemicals contribute to their propensity to accumulate in sediment at toxic levels.” [5] This is significant because the aquatic sediment is the home of many aquatic invertebrates, such as crustaceans (crayfish and shrimp), molluscs (clams and mussels), insect larvae, worms and snails.
Synthetic pyrethroids include the following active ingredients – check your labels!
First generation pyrethroids:
Bioallethrin, D-Allethrin, Imiprothrin, Phenothrin, Prallethrin, Resmethrin, Tetramethrin
Second generation pyrethroids:
Tau-Fluvalinate, Tralomethrin, (S)-Cypermethrin, Beta-Cyfluthrin, Bifenthrin, Cyfluthrin, Cypermethrin, Deltamethrin, Esfenvalerate, Fenpropathrin, Gamma-Cyhalothrin, Lambda-Cyhalothrin, Permethrin
Clearly, natural pyrethrum is a safer pesticide than the synthetic pyrethroids.
All Natural Pyrethrum?
Gardeners looking for a natural low environmental impact pesticide will very often reach for pyrethrum. Sometimes, due to mistaken identity, even when reading the labels, pyrethroid insecticides are often confused with pyrethrum, but they definitely aren’t the same thing at all! Synthetic pyrethroid insecticides are definitely not natural and have a much higher environmental impact.
Another factor to consider is that of synthetic additives in natural products. Both natural pyrethrum and synthetic pyrethroids have a knock-down effect on pests, but just because a pest is knocked down doesn’t mean it has been killed! Insects have natural enzymes in their bodies which can rapidly detoxify pesticides, allowing some of them to recover.
To delay the enzyme action and ensure that the dose of pesticide is lethal, most natural pyrethrum formulations and some synthetic pyrethroid products also contain synergists, such as piperonyl butoxide and MGK-264 to enhance the effects of the pesticide.
So, even if you think you’re choosing a natural pesticide with pyrethrum, check the label and keep in mind that in many cases it will also contain a synthetic synergist.
Pyrethrum is one of the few insecticides allowed for use in Certified Organic Production of crops in the USA, Europe, Australia and New Zealand. The synergists piperonyl butoxide (PBO) and MGK-264, however, are not certified to be used on organic crops, so growers who want to meet organic certification standards need to be aware of this. [6]{7}
NOTE: It’s important to remember that the use of botanical and other natural pesticides is neither the first, nor the only option for controlling pests, but it can form an important part of an Integrated Pest Management (IPM) approach to pest control.
[1] Oral LD50 22uG/bee, contact LD50 130-290uG/bee. Environmental Fate of Pyrethrins, Amrith S. Gunasekara, Environmental Monitoring Branch, Department of Pesticide Regulation Sacramento, CA, November 2004, (Revised 2005)
[2] Earthworms Fact Sheet UC182, Penn State Extension, College of Agricultural Sciences, The Pennsylvania State University
[3] National Pesticide Information Center – General Facts Sheet, Pyrethrins (NPIC is a cooperative agreement between Oregon State University and the U.S. Environmental Protection Agency)
[4] University of California Division of Agriculture and Natural Resources Statewide Integrated Pest Management Program, Pesticide Information – Pyrethrin (http://ipm.ucanr.edu/TOOLS/PNAI/pnaishow.php?id=62)
[5] California Department of Pesticide Regulation, Reevaluation of Certain Pesticide Products Containing Pyrethroids, California Notice 2006-13
[6] Colorado University CSU/Denver County Extension Master Gardener 2010 – Some Pesticides Permitted in Organic Gardening
[7] Oregon State University – What is Organic? (http://extension.oregonstate.edu/lane/sites/default/files/WhatisO.pdf)