Natural sprays

There are several botanical and naturally derived substances which can be sprayed in the organic garden to control pests and disease. Widely used before the arrival of chemical products, several of the natural alternatives have become less fashionable and may be difficult to obtain.

I first appreciated the benefits of natural garden sprays several years ago when I helped a close friend whose vegetable gardens became infested with a particularly virulent species of cutworm. Around the peak of spring, she attempted to control them with a well known chemical treatment. Despite repeated applications, the number of cutworm began to increase rather than diminish. By the approach of autumn, her summer vegetables were all but devastated and my friend became reluctant to attempt a winter crop.

When she requested assistance, my initial plan was to work steadily on the soil. By increasing the quantity of organic materials, I hoped to improve the soil’s nutrient distribution and simultaneously encourage the larval predators which might eventually control the cutworm problem. One of my local contacts suggested a heavy dose of natural insecticide. He gave me the address of an elderly gentleman with his own pyrethrum recipe. Now I considered myself sufficiently knowledgeable to dismiss pyrethrum for treating cutworm. A significant dose might control caterpillars and leafhoppers, but would probably be ineffective against serious cutworm infestations, I predicted. Natural pyrethrum breaks down to nothing within a few days. “Cutworm”, I asked doubtfully, “you’re convinced this will control cutworm?” The old gentleman touched his nose. “Yeah”, he said. “It’ll get ‘em”.

And it did! Within several days the cutworm were history. After that result I returned with a few questions. The pyrethrum supplier referred me to a publication on natural garden sprays. It was currently out of print, so with his permission I reproduced a couple of sections for my own reference. There was an insightful chapter on safety precautions required for the preparation and application of natural garden remedies. I was concerned by the toxicity of several plant derived insecticides, particularly the nicotinic and anabasinic compounds.

Nicotine-based insecticides are extremely poisonous to humans, domestic livestock, pets and native animals. Thankfully, I’ve managed to avoid these particular compounds, although many of my friends have recommended them to me. I prefer relatively safe natural compounds such as rotenoids and pyrethroids. These can still occasionally be purchased from local garden centres and organic suppliers.

It appears that toxicity of natural rotenoid compounds was initially exploited by indigenous South American tribes who used various plant extracts to paralyse fish. Organic gardeners may be familiar with the insecticide products based on these compounds. Derris powder is the most popular extract from plants of the genus rotenone. The product is still prepared by mixing the finely ground plant roots with a clay base. When sprayed appropriately, derris is effective as a general insecticide. Unlike the majority of fast acting insecticides, rotenone does not function as a nerve toxin. It is believed to inhibit oxygen transport and basic energy mechanisms within cells. It is extremely toxic to fish and amphibians, so care is required when spraying near rivers, creeks or ponds.

Organic pyrethrum

Organic pyrethrum is the botanical extract from pyrethrum flowers (chrysanthemum cinerariaefolium). This is distinct from synthetic pyrethroids which are chemical approximations of the naturally occurring compounds.

When formulated as liquid or aerosol, natural pyrethrum compounds prove rapidly effective against a wide range of insects. The knock down effect is particularly evident within enclosed environments such as greenhouses, propagation sheds, and indoor gardens. In higher doses, it can be effective in orchards, and other outdoor settings. Like derris, organic pyrethrum will break down completely after several days exposure to sunlight.

On the positive side, they are considered as one of the safest insecticide compounds with a low toxicity to humans and other warm blooded mammals, although some skin irritation and allergic reactions have been associated with repeated handling and use. Unwashed traces of pyrethrum compounds are usually destroyed by exposure to heat, sunlight, and other atmospheric influences. Negatively, pyrethrum is toxic to fish, amphibians and indiscriminate applications can obviously harm beneficial insect organisms such as pollinators and natural predators.

Since my encounter with the cutworm, I’ve received numerous enquiries about the availability of natural pyrethrum compounds. There is a general consensus that it’s becoming more difficult to obtain. Commercial crops of pyrethrum flowers are grown in Africa, Japan, and Hawaii, although the volume of production has declined significantly over recent decades. Within most organic gardening communities, there are individuals and groups who continue to raise pyrethrum chrysanthemums for the purposes of insect management. My advice is to contact the nearest organic society. They can usually provide the best indication of local availability.

Microscopic pests

The majority of nuisance insects which attack our plants from one season to the next are tiny, almost microscopic in size. Once they have identified their plant hosts, many of these pests will generate large colonies within a surprisingly short period of time. In addition to encouraging their natural predators, minute insects can be controlled with natural substances which remain effective for at least several days.

Various light oils are used to control scale, mites, thrip, flies, and many of the softer larvae which attack foliage. They work by interfering with the insects normal mechanisms of respiration, feeding, and temperature regulation. To eliminate the risk of leaf damage, these products can be sprayed when the shrubs and trees enter their dormant winter phase. Insecticidal soaps are also made from botanical oils combined with the salts from natural stearols and other fatty acids. When applied in solution to foliage, they are an effective treatment against soft-bodied thrip, mites, aphids, and flies. During times of infestation, they need to be applied several times each week although some sensitive plants can develop leaf burn with repeated applications. A weaker solution should eliminate leaf burn or assist by reducing the severity of the problem.

No magic bullet

Since committing myself to an organic approach, I’ve attempted to understand each living garden as a complex and integrated environment. Given this, I try to avoid the imbalances which can accumulate whenever insect populations are rapidly affected by toxic compounds. In short, I remain suspicious of the magic bullet approach, even when this involves natural compounds such as pyrethrum. Once again, I admit struggling to appreciate how an old-fashioned pyrethrum solution was so devastatingly effective against my friend’s cutworm problem while the contemporary chemical treatments had simply failed. The reasons were quite obvious, yet I needed time to reflect upon them.

With few exceptions, synthetic chemical products establish a toxicity which lingers in the soil for an extended period. Under these circumstances insect populations are provided opportunities to adapt, and develop their own resistance against the chemical agent. Most of the local cutworm population were probably killed through initial contact with the synthetic chemicals, although a small percentage may have survived then experienced a significantly reduced exposure over an extended period of time. It is these low level exposures which promote adaptation and, ultimately, resistance. By contrast, the residual toxicity detectable at several days following the pyrethrum treatment would have been negligible. Any surviving insects would be denied the benefits of adaptation. For this reason alone, it is often recommended that pyrethrum be used as a high potency single strike application rather than as a modest dosage regularly applied.

To date, my friend has raised many vegetable crops without the impediment of the serious cutworm infestation. We remain thankful for the effectiveness of the pyrethrum treatment although both of us believe that the decisive remedy was founded upon our integrated and longer term strategies of soil improvement and the encouragement of natural insect predators.