Pyrethrum Insecticides: Are they Safe?

Pyrethrum is one of the best known botanical insecticides, effective against a wide variety of insect pests and generally considered safe to use. Is it really safe? To answer any question about pyrethrum it must first be explained that what is referred to as “pyrethrum” can be many different products. There is pyrethrum, the raw flowers; pyrethrins,the extracts from the flowers; and pyrethroids, synthetic pyrethrum. In addition many other insecticides and enhancers are often added to formulations which are called “pyrethrum”.

            True pyrethrum is the powdered flowers of the African Daisy, Chrysanthemum cineraraefolium.This product has been used as an insecticide for almost 2,000 years, and is generally considered a least-toxic pesticide, permitted with restrictions by the California Certified Organic Farmers. However even though it has relatively low mammalian toxicity, it is a poison and can cause sensitive people to have strong allergic reactions.

            Pyrethrins refer to insecticidal compounds found in the pyrethrum flowers. They are extracted from the flowers and used in a variety of formulations. Whereas pyrethrins are low in mammalian toxicity and break down in 12-24 hours outdoors, they are most frequently combined with other products to improve their toxicity and the length of time they are effective (their residual activity). This is why we need to read the labels carefully and/or request a label reprint from the company doing spraying for us. Some added products are called synergists because they enhance the effects of the chemicals they are added to. One synergist most commonly found in pyrethrins is PBO (piperonyl butoxide). PBO deactivates enzymes that break down toxic substances, which is why it makes pyrethrins more effective against insects and more dangerous to humans. Originally it was considered an “inert” ingredient; now it is registered as a pesticide. Both diatomaceous earth and silica gel are often added to pyrethrins to strengthen the pesticidal effect, but these are not poisons, acting by dehydration. As a dust, they should not be breathed as they can damage the lungs. Pyrethrins are also often combined with other botanical insecticides like rotenone and ryannia, both of which are much stronger toxins. However even though pyrethrins are poisons and are often combined with PBO and other products, their effect is short-lived, usually a few hours to a few days; and they are safer to humans and more environmentally friendly than organophosphate insecticides. Cats can be very susceptible to pyrethrin poisoning and flea formulations containing pyrethrins should be used exactly as the directions say. Products containing pyrethrins are Revenge, Pursue, Drione and Diacide.

            Pyrethroids are not botanical insecticides; they are synthetic compounds that are more toxic and more long-lasting than pyrethrins, some persisting for up to ten days or more. The question of their toxicity is unfortunately complicated because many different pyrethroids have been formulated: for example, resmethrin has very low mammalian toxicity; cypermethrin is moderately toxic and long lasting; and permethrin can range from low to moderate toxicity depending on which solvent is used as a carrier, and it is very long lasting and highly toxic to fish. Permethrin is marketed as Ambush, Ectiban and Pounce etc.

            Pyrethrum, pyrethrins and even pyrethroids are far safer than most chemical pesticides, however it must be remembered that they are poisons and should be used with caution. Not only are cats, fish and birds vulnerable, but killing beneficial insects can cause pest outbreaks; and overuse can and has led to the development of insects that are resistant to these environmentally preferable insecticides. It seems logical that the safest form to use pyrethrum is as the powdered flowers or as a pyrethrin without PBO. Since it is only effective for one day, it must be sprayed at exactly the right time. Also other non-toxic controls should be considered like cultural methods, horticultural oil, neem, insecticidal soaps and the old strong blast of plain water. Before doing anything, try to determine if the insect damage is minor cosmetic harm or if it is major and seriously stressful to the plant. Whitney Cranshaw’s  book Pests of the West is a good guide to determine if serious damage is likely, and to understand the life-cycle of the pest which is important to know for accurate timing of controls. A gardener who is willing to give more attention and more study to a pest problem can get as good or better control with less toxicity.

(much of this information came from the Bio-Integral Resource Center in Berkeley, Calif.; and from the Office of Ag.-Entomology at the U of Illinois, Urbana.)