Safe Pest Control - Step 5 and 6

Best-practice safe pest-control for food-gardeners involves six steps (in this order):

1. Make your food garden as healthy as possible 
2. Don’t please your pests 
3. Keep them out 
4. Catch and remove 
5. Is further action needed? 
6. Use a low-impact pesticide 

Step 5 and 6 are discussed below.

Step 1 and 2 were discussed here.
Step 3 and 4 were discussed here.

Margaret W is an entomologist and member of the Food Garden Group. I would like to thank her for proof-reading drafts of this blog post, suggesting changes and additions and making sure that this blog post is accurate and relevant to Tasmanian food gardeners.


Step 5 - Is further action needed?

Okay, so Safe Pest Control Step 1 – 4 have not solved your pest problem.

You may now decide to spray the pest to get rid of it, but there is a problem: many pesticides, even the ones with low toxicity to humans, kill many more species of insect than just the pest you want to get rid off.  You may kill the beneficial insects that right now are busy attacking the pest.

In her inspiring book Beyond Organics Helen Cushing mentions an aphid infestation in an apricot tree:

With lots of sweet young growth, the aphids arrived to sip on the gently flowing sap, and the new growth started to deform.  However, I did not interfere, though I could have gone in with pyrethrum or simply with a jet of water from the hose to remove the aphids.  I knew from experience that both these measures bring only temporary respite and must be repeated again and again.  I decided to wait and observe nature taking its course.  Soon, there were a couple of ladybirds.  Next time I checked, I counted eight ladybirds just on one small twig.  Now, both aphids and ladybirds have gone.  All I have done is watch and admire and learn.  That is low-maintenance, ecological gardening.  It requires and understanding and a patience that leads to minimum intervention, rather than a reactive approach.

Ask yourself ‘might the natural enemies of this pest already be busy removing it? Should I wait to see whether they succeed?'

Your answer may be ‘I don’t see any signs that other insects are removing my pest for me.’

If that is the case, ask yourself one more question ‘will I get very little produce or a very damaged crop if I don’t kill this pest?’

Proceed to Step 6 if your answer is affirmative.  The better you know the pest you are dealing with, the better you can answer this last question.

In spite of my best efforts I had whitefly in my beans. Whiteflies (see photo below) are tiny insects that suck sap from plants. They often fly away in small swarms when you touch the plants. Whitefly in large numbers can debilitate plants by their feeding and the sooty mould that grows on their excrement reduces quality of produce and interferes with photosynthesis.
In my case, however, the actual beans were plentiful and untouched. I would waste my time and effort if I tried to get rid of these insects. I did not go to Step 6 to kill this pest on this crop.

Very seldom will you see just one Whitefly

If you can live with the damage done by a garden pest to your produce, consider not taking any further action.


Step 6 - Use a low-impact pesticide


First a few pesticide terms

Some pesticides are contact pesticides. This means that insects die, if they are directly touched by the pesticide when it is applied, or if they touch it later, or ingest it by eating leaves.

Some pesticides are systemic pesticides. These pesticides can be absorbed by a plant and then move around its tissues in its sap. Many sap-feeding insects (eg. scales, aphids) take in lots of plant sap during feeding. A pesticide that is in the sap may be ingested and kill it.

Some pesticides are narrow-spectrum, because they kill just one type of insect.

Most pesticides unfortunately are broad-spectrum. This means they kill a wide range of insects, including often beneficial insects, such as pollinators (bees etc.), and predators (ladybirds etc.) that help you to get rid of your garden pest.


A sensible approach when using pesticides:

1. Select and use the least toxic pesticide that does the job effectively.
2. Always read the directions before use. The labels on pesticide containers are actually legal documents. A lot of effort has gone into getting the right message across about how to use it safely and how to get the best result.  Don't ignore this advice.
3. When applying a pesticide always wear protective gear, ie. gloves, a long-sleeved shirt and long trousers, solid shoes and a simple nose-mask.  Again read the label.
4. For all pesticides (‘contact’ or ‘systemic’) always cover the insects themselves, the tops and bottoms of leaves, and stems.
5. Store the pesticide according to the label (or if home-made, in a cool shady place) and keep out of reach of children.

Commonly used pesticides – alphabetical summary

The table below shows pesticides that are commonly used by food gardeners in Tasmania, Australia. It shows generic pesticide names, not brand or trade names. All the pesticides mentioned below are contact pesticides, except Neem, which is systemic.

The table may well be incomplete. Please email details to foodgardengroup@gmail.com if an environmentally friendly type of pesticide that you use successfully, is not mentioned. That way you can help improve this blog post.

Most of the pesticides mentioned below are insecticides, ie. they kill insects. Most are benign. Some are not.  I included a few more-harmful ones to make you aware of their toxicity.

Here is what each column in the table means

Kills what – column in the table above
For each pesticide this column shows the range of insects it kills.

Active for how long – column in the table above
This column shows how long the pesticide remains active. It may help you determine when to spray once more if it was not completely successful the first time.
An entry like 7 days unless rain means ‘whichever comes first’.

Withholding period – column in the table above
This is the minimum number of days you need to wait before harvesting vegetables or fruits after spraying the pesticide. The period mentioned is based on data available in February 2016. Future new research may prove the period inaccurate.
When the withholding period is over, you can pick the crop, but always wash it before eating.
More info about withholding periods set for Australia can be found here.

Certified organic farms – column in the table above
Australian Certified Organic (see http://aco.net.au) is Australia’s largest organisation for certification of organic produce. This column in the table indicates whether ACO’s Australian Certified Organic Standard 2013 (see http://austorganic.com/australian-certified-organic-standard2/) approves the use of this pesticide or not.

More info about each pesticide mentioned in the table

Bacillus thuringiensis
Commonly known in Australia under the label Dipel, Bacillus thuringiensis (Bt) is a biological insecticide that is ingested by caterpillars when they eat leaves. It is a stomach poison. The caterpillar stops eating soon after and is dead within two or three days.
Bt is a great narrow-spectrum insecticide because it kills caterpillars, and nothing else.
Water washes Bt off leaves, so after rain or overhead irrigation it needs to be re-applied.

Derris
Derris is a climbing Southeast Asian plant. Its roots contain rotenone, a strong pesticide and fish poison. This is an example of an organic pesticide that is very toxic to insects and some mammals.
It is moderately toxic to humans (for more info on rotenone’s toxicity see for instance here).
Australian certified organic farms are not allowed to use it. Canada and the United States are phasing out the use of rotenone for everything except fish poisoning.

Garlic Spray
Garlic spray is a simple spray that is effective for slugs and snails, aphids, caterpillars, and other soft-bodied insects. Strong garlic can kill them outright. Garlic spray also has anti-fungal and anti-bacterial properties. The smell may mask the smell of the insect’s preferred host plants, so it can also protect plants that way.
Garlic spray is effective for between 24 and 72 hours, but this varies depending on temperature, moisture and amount of direct sunlight.
In spray packs with fine-spray nozzles, it is best to use a commercially produced garlic spray because it is finely filtered. However, it is not hard to make your own.
Here is a recipe:

1. Chop or crush three or four entire bulbs (approximately 100g).
2. Mix with one tablespoon of vegetable oil and leave to soak overnight.
3. Strain and add one litre of water.
4. Add one teaspoon of liquid soap.
5. Steep for at least 24 hours.
6. Filter and dilute at 15 mls per 1 litre of water.

There are variations of this type of spray that use warm water instead of oil and/or add onions and/or hot chillies (chilli spray). See for instance ‘All-round insecticide’ here.

Heat
Heat is of course not a pesticide in the traditional sense of the word, but I mention it here because it is a completely non-toxic way of killing pests that is worth consideration if you are not in a hurry.
If you have a persistent pest in a particular part of your garden or a garden bed, applying heat through solarisation can be the way to go.
Solarisation is the simple environmentally-friendly method of using the sun to kill garden pests. In summer (in Tasmania temperatures may be too low in other seasons) cover the area with black plastic and seal down the edges.
Make sure the black plastic does not blow off, so it covers the area at all times.
Six to eight weeks later (4 – 6 weeks in warm conditions) a wide range of soil-inhabiting pests and disease organisms will have died in the heat generated under the plastic.
For more information see for example Soil Solarization for Gardens and Landscapes and Gardens Alive.

Horticultural oil / White oil
Horticultural oil, also known as White Oil, is a broad-spectrum pesticide that kills soft-bodied insects by blocking their breathing pores, causing suffocation and rapid death. Because it kills insects by suffocating them, insects can not build resistance to it, therefore horticultural oils will never cease to be effective.
Horticultural oil is used to remove scale, aphids, moths, whiteflies, leaf miners, mealy bugs and thrips.

Scale with their friends the ants

Horticultural oil also kills lacewing larvae that eat aphids, caterpillars and scale, so like any other broad-spectrum insecticide use it as a last resort. Horticultural oil does not kill bees and ladybirds.

Horticultural/white oil can be easily made at very little cost (see below).  Advantage of buying a ready-made product is that the liquid is thinner because it is made with petroleum oil, not vegetable oil.  Thin oil is preferable if the air temperature at the time of spraying is over 30 degrees. There are many variations of horticultural oil on the market, many with their own special ingredients and claimed advantages. Natrasoap and Eco-oil are two examples. If the temperature at the time of spraying is under 30 degrees there is, in my opinion, little reason to buy a ready-made product.

Certified organic farms in Australia are allowed to use horticultural oil that contains fatty acids naturally derived from animal or vegetable oils.

How to make you own horticultural oil/white oil:

1. Use a wash-up liquid that is environmentally friendly.
2. Combine in a jar with a lid 1 cup of thin vegetable oil + ¼ cup wash-up liquid.
3. Put the lid on and shake well.
4. Mark the jar ‘horticultural oil’ + expiry date, which is the day you make it + 3 months.
5. Store somewhere cool.
6. The vegetable oil might go off after a while, so discard this concentrated mix after 3 months.

Don't be concerned if your home-made product is not actually white.  It will only be white if the wash-up liquid you use is white. The photo below shows a bottle of concentrated 'while oil' I made.

My bottle of 'white oil'

How to use horticultural oil/white oil:

1. Don’t use it on hairy or soft leaved plants like lettuce, because it is likely to burn the leaves. 
2. Don't use home-made horticultural/white oil in temperatures over 30 degrees Celsius because it may burn leaves.
3. Preferrably spray early in the morning when temperatures are lower.
4. In a spray bottle mix 1 dessert spoon of the concentrated mix with ½ litre of warm (not hot) water (or mix according to the advice on the bottle if you are using a commercial product). 
5. Shake well: in my case the content of the spray bottle was now white - hence the name, but the colour of the mix is irrelevant.
6. Focus on areas on plants and in the air where there are whiteflies. Thorough coverage of insects at the time of spraying is essential.  Spraying of plants that have no whiteflies on them will only help if a whitefly lands on the plant while it is wet.
7. Do this once a week for at least three weeks.

Insecticidal soap
Insecticidal soap is a mixture of water and soap, whereas horticultural oil is a mixture of oil and soap.
It is claimed that horticultural oil is more effective than insecticidal soap, but the latter is easier to make.
Soap acts on contact and must be applied directly to the insect to be effective. Thorough coverage of pests is essential. Spray once a week for at least three weeks.
Insecticidal soaps are effective against soft-bodied insects such as aphids, some scales, whiteflies, mealybugs and thrips. Hard-bodied insects, such as bees and wasps, are not harmed. For more info about insecticidal soap see for instance here.
Don’t use it on hairy or soft leaved plants like lettuce, because it is likely to burn the leaves.

Here is a recipe for insecticidal soap:

1. Use a soap that is environmentally friendly.
2. Combine, in a hand sprayer, 1 tablespoon of soap flakes to 500 ml of water.
3. Put the spray top on and shake well until the soap is completely dissolved.
4. Spray this mixture without diluting it.

Here is my own even easier recipe: pour a bit of dish-washing liquid in a hand sprayer, add water, put the spray top on and shake. Spray this.
When I have an aphid problem that can only be solved by spraying, I keep a spray bottle near the plants that have the problem as a reminder to spray again after a few days, if needed.
Certified organic farms in Australia are allowed to use insecticidal soap that contains fatty acids naturally derived from animal or vegetable oils.

Milk Spray
If Powdery Mildew is present, milk spray will kill it, but interestingly, only when there is sunlight, so don't spray milk spray at dusk or after dark. Give both the leaves and stems a good even coverage.

Milk spray is very simple to make:

1. Combine water and full cream milk in the ratio 9:1
2. Shake well
3. Apply early in the morning

You can find more info about milk spray here.

Neem
Neem oil is a vegetable oil made from the fruits and seeds of the Indian Neem tree. Neem is a systemic pesticide, ie. it is absorbed by plants and kills insects when they ingest the plant’s sap. It is effective against a wide variety of leaf-eating and sap-sucking pests, such as aphids, scale, whiteflies and caterpillars.
Neem is not harmful to mammals, birds, earthworms and beneficial insects such as bees and ladybirds.
It is moderately toxic to fish and other aquatic animals. It has some toxicity for humans.
For more info on Neem see for instance Neem Oil General Fact Sheet.

Pyrethrins
Pyrethrins are pesticides made from pyrethrum, which is a natural insecticide that comes from the white chrysanthenum daisy. The Chinese crushed chrysanthenum plants and used the powder as an insecticide as early as 1000BC.
Pyrethrins are potent insecticides that kill a wide variety of insects, including beneficial ones, very quickly. For this reason, and because of their relatively low toxicity to most mammals including humans, pyrethrins remain popular. Pyrethrins are highly toxic to fish and cats.
Pyrethrins break down quickly upon exposure to light and air, often within a few hours.
Over time some insects have become resistant to pyrethrins. In answer to this, synthetic substances such as piperonyl butoxide (PBO) are now often added to natural pyrethrins.
Pyrethrins that do not contain PBO are allowed on certified Australian organic farms.
Do not confuse pyrethrins with pyrethroids (see below). Please check labels before purchasing.

Pyrethroids
Pyrethrum has always been expensive, and this is why after WWII pyrethroids were developed.
Pyrethroids are manufactured chemicals, very similar in structure to pyrethrins, that were chemically designed to be more toxic with longer breakdown times. They are also a lot cheaper.
In the 1960s a number of new “second-generation” pyrethroids were patented. These are many times more toxic to a broad spectrum of insects than natural pyrethrum.
In air, many, but not all, pyrethroids are broken down within a few days by sunlight. In areas with limited sunlight, such as grain silos, pyrethroids can persist for months.
Pyrethroids bind strongly to soil and organic matter and break down much slower there, but are eventually degraded by micro-organisms. They generally do not move from soil to groundwater.
For more info on the toxicity of pyrethroids see for instance here and here.
Australian certified organic farms are not allowed to us Pyrethroids.


I do my best to solve pest problems in my garden using Safe Pest Control Step 1 - 4.  On rare occasions I use horticultural oil, insecticidal soap, Dipel or milk spray.

But that is just what I do.  I hope that this series of blog post on pest control have provided some helpful info, so you can follow your own pest control approach.

May the pests not be with you!