Plant Doctor Archive

Using pyrethrum

I would like to know of plants containing pyrethrum please so I can get rid of bugs naturally.

Pyrethrum (Tanacetum cinerariifolium) is the name of an aromatic plant of the daisy family that typically sports feathery foliage and white daisy flowers with yellow centres. The closely related T. coccineum is also known as pyrethrum but has less impact as a natural insecticide than its cousin, T. cinerariifolium.

The natural insecticidal properties contained in pyrethrum are called pyrethrins, and it is these components that are extracted and used commercially in products such as garden sprays, flea bombs, fly sprays and the likes. The flower heads are either dried and powdered or the oils are extracted from within the flower.

Pyrethrum sprays are excellent for use against sap sucking insects, such as aphids, woolly aphids, spider mites, thrips, scale, whitefly, and other pests including caterpillars, earwigs, leaf miners, beetles and slaters.

You can make your own spray or dust recipe by drying the flower heads (picked shortly after blooming) on sheets of newspaper in direct sunlight — the quicker the drying process the higher the pyrethrin percentage. Once dried, finely grind the flowers using a mortar and pestle, then sprinkle the powder over your plants.

To make a spray, mix half a cup of coarsely ground flower heads (dried) with 1 litre of boiling water, leave to cool (about three hours), strain, then add a teaspoon of pure soap. The soap allows for better leaf coverage and increases efficiency by a factor of four. Apply the spray immediately following preparation.

Note, however, that pyrethrins are also toxic to bees, so make sure you spray early morning or in the evening when bees are inactive.

The effect of pyrethrum is immediate. It works by paralysing the insect or bug and killing it, but if the solution is not strong enough it will simply stun the insect, allowing it to recover again in a few hours time. If this is the case, treatment will need to be repeated with a stronger dose.

Pyrethrum breaks down in sunlight, therefore it gives only a short-term protection of about 24-48 hours. It will also need to be reapplied after rain.

Feverfew (T. parthenium) is also a good insect repellent and contact insecticide that contains pyrethrins and can be used in place of true pyrethrum (T. cinerariifolium). A feverfew spray can be made as above (using 2 cups of ground dried flowers), and a dust recipe can also be made as for pyrethrum. Some say that feverfew is the perfect deterrent for bugs, keeping pests away from all plants nearby.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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