Sunday, April 17, 2011
You may be familiar with the word 'Pyrethrum' from the packaging of many ready-to-use organic insect sprays. However it was once one of the most popular insecticides available until the introduction of modern synthetic insecticides.
This insecticidal chemical is derived from the dried, powdered flowers of the pyrethrum daisy, Tanacetum cinerarifolium, and has been used as early as 1880 as a treatment to control mosquitoes. The active ingredients 'Pyrethrins' are mainly concentrated in the seeds of the flower head, and work by way of a contact insecticide. This means that the insect only has to be touched by the active ingredient to be affected.
Pyrethrins have a quick knockdown effect on insects, working in some ways like a nerve toxin. With the right dosage insects can be paralyzed in mid flight, but if the dose is too low they will just be knocked out and fly off later on once they've recovered. On food crops pyrethrins can be applied up to one day before harvest because they are quickly destroyed by light and heat. This means that they are not persistent in the environment and this it why pyrethrins have their 'organic' label. Be careful though as Pyrethrins will kill ladybirds, aquatic insects and the preditors that eat them although they do not appear to be harmful to bees.
HOW TO MAKE YOUR PYRETHRUM INSECTICIDE: Pyrethrum daisies are easy to grow in the English garden and are readily available at most good plant retailers. That way - if you have pyrethrum in the garden - you will have the main ingredient conveniently close by when you are ready to make your spray. The importance of this becomes clear when you realise how quickly the active ingredient within the pyrethrum flower will degrade..
The concentration of pyrethrums is at its peak when the flowers are in full bloom, this is recognised as the time when the first row of florets on the central disk opens - up until the time that all the florets are open. Pick the flowers in full bloom and then hang them in a dark sheltered spot to dry.
Traditionally, in Japan, the flowers were harvested with their stems intact, and hung upside down in water for between 24 to 48 hours before drying. The reason for this process is that it can increases the pyrethrin levels. Once dry, crush the flowers into a powder using a mortar and pestle or a blender. The finer the powder is the more effective it will be against insects, but it will deteriorate more rapidly.
To apply as an insecticidal dust, simply apply the dried and crushed flowers on to the leaves of plants that require its protection.
To use as a spray, soak ten grams of pyrethrum powder into three litres of warm water for three hours, after this it is ready to be sprayed. It is possible to use fresh flowers instead of dried but you will need to use up to four times the amount of planr material to get the same concentration of active ingredient.
The efficiency of pyrethrum can be greatly improved with the addition of other products such as sesame seed oil or washing up liquid. These can be added at a dose of one teaspoon per litre of solution and can increase the effectiveness of your spray up to four times the norm.
As mentioned before Pyrethrum breaks down quickly after application giving no more than 48 hours of protection ( 12 hours is generally nearer the mark) depending on the concentration of the mixture sprayed. One of the ways that this degradation can be slowed down is to add anti-oxidants such as tannic acid, a chemical found in the bark of several tree species. Even so it will be necessary to reapply after rain.
You may need to experiment with the amount of water your powder is being added to as the concentration of pyrethrins in the dried flowers will be an unknown variable. If your spray does not seem to kill insects, try using use less water next time you make your spray.
Posted by Andrew King at 5:25 PM