Organic sprays decoded

Ever wondered how organic insecticides, fungicides and herbicides actually work? Here is an introduction to botanical sprays, their active ingredients and modes of action.

Neem

Neem oil, extracted from neem tree seeds, is an incredibly effective insecticide. It provides some smothering properties like other oil sprays, but its power lies in a compound called azadirachtin: a compound that acts as an insect growth regulator and anti-feedant. Ingested by insects it distorts growth and switches off appetite, causing death. It also impacts fertility of insects and eggs. It works on a broad range of chewing and sap sucking pests.

Two catches: azadirachtin breaks down, so buy neem products with an expiry date and only mix up what you’ll use that day. It will not store once diluted. Secondly, neem sprays aren’t approved for use on edible plants in Australia, though they are overseas.

Pyrethrum

Extracted from a chrysanthemum species, pyrethrins attack insects’ nervous systems, causing over stimulation to the point of paralysis and death. This is a broad-spectrum insecticide, affecting pest insects and beneficials as well.

Pyrethrins are susceptible to UV light so spray at the end of the day.

Note: Despite being plant-derived most pyrethrum sprays are not organic. Nearly all contain a synthetic booster, piperonyl butoxide, which is banned under organic standards, you’ll find it listed in the ‘Active Constituents’ section on the front label.

Cyclotides

One of the newest organic insecticides is made from the butterfly pea plant (Clitoria ternatea). It contains proteins called cyclotides that kill small, soft-bodied insects on contact. On larger insects it acts as an anti-feedant, causing starvation and it’s also shown to reduce egg-laying on sprayed plants. Despite the broad action, cyclotides have been shown to be safe for bees.

Bacillus thuringiensis subsp. kurstaki

This soil dwelling bacteria, commonly called BT, is an effective, organic caterpillar killer. Once ingested it paralyses the gut so caterpillars stop feeding and starve. It’s highly targeted and only toxic to caterpillars. Check the expiry date before purchasing and spray late in the day as BT degrades quickly in sunlight.

Note: Spinosad, another bacteria-derived insecticide, kills a broader range of insects but home gardener products aren’t organic.

Pick up Issue 169 of ABC Organic Gardener in stores now to read Steve Falcioni’s full guide to organic sprays in which he looks at oils, soaps, fungicides and herbicides. You can also subscribe at subscribe at mymagazines.com.au to get every issue delivered straight to your door.