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Aphids

4 December 2021
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Aphis nerii or oleander aphids

Aphids are very much a part of the complex natural jigsaw that is your garden. We need to understand the part they play in a 'butterfly habitat'.

In an earlier blog post I stressed how everything in Nature is connected. When you have swan plants (or milkweed), oleander aphids (Aphis nerii) will soon find your plants. Aphids, bright yellow, pear-shaped insects, only a few mm long, with their soft bodies, will suck the juices from your plants, causing them to wither and fail. They particularly like the tender, young, new shoots or plant tips of your plants.

Just when you're imagining how many monarch caterpillars will be able to dine on your plants, the aphids arrive and start to play havoc with your 'crop'. By piercing the cell wall of leaves and stems to suck the sap they weaken the plant, leaving a wound that can allow disease to get in.

Normally, with a healthy swan plant, the aphids will do no more damage than when we graze our knee. A healthy plant can withstand aphids, but weaker plants go downhill quickly.

No chemicals, please

Because you want your swan plants to be free of chemicals you cannot reach for a commercial product. You have to help Nature using natural means.

Do nothing!

The first step is to do nothing. That's right, nothing. Just observe, be vigilant and patient.

Are your plants well-watered but not standing in wet ground? Run your finger over the soil – if it comes away dusty the plant needs watering. (Muddy? Probably not.) Are they well fertilised, mulched? Do they have plenty of room? If they're in a pot, check that the roots aren't bursting out the bottom. If you see a tangle of tiny roots the plants should be in a larger pot.

Don't grow all your swan plants in the same place, year after year. Each plant species removes certain nutrients from the soil. By growing different plants on the same piece of land it improves soil health, optimises nutrients in the soil and combats pest and weed pressure. This will lead to healthier swan plants, more resistant to disease. (This is called 'crop rotation'.)

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While you're waiting...

While you're waiting patiently to see what is going on, there are some actions you can take without interfering too much. High-pressure hosing the aphids will remove them from the plants. So will squishing them with your fingers – wear gloves if you don't want your fingers stained orange. Or you can also cut the piece of plant that is infected with aphids and put it in a plastic bag and straight into your waste system (or you could wash the stem and leaves you've cut off, and feed it to monarch caterpillars.) But remember, if you see signs of other life with your aphids, you need to stop immediately and let Nature take its course.

Encourage beneficial insects

Encourage beneficial insects – the kind that will get rid of the 'bad guys'. Some of these work from the outside (predators) while others work from within (parasites).

Most ladybird species will eat aphids. After the first aphids arrive in your garden you might notice ladybirds arrive. Aphid larvae look like tiny lobsters. Both the adults and the larvae eat aphids – ladybird larvae can eat up to 400 aphids in 2-3 weeks. And a female ladybird will lay more than a thousand eggs in her lifetime. (Yes, they might also eat a few butterfly eggs as well.)

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Adult and nymph ladybirds

Ants

Watch out, also, for ants. Ants encourage aphids, because aphids secrete a sugary fluid called honeydew which ants use as food. Ants also eat insect eggs. Honeydew has a negative effect on the plant, inhibiting photosynthesis, which can affect growth. Honeydew also provides a good breeding ground for fungi, especially the fungus called sooty mould.

sooty-mould

sooty mould

See what I mean: everything's connected!

Some people use a product called ant sand to control ants, or if your plant stands alone you can wrap sticky tape around the base to prevent ants from climbing up.

Aphidius colemani

My favourite control for aphids is a parasitic wasp called Aphidius colemani. (Yes, it is a wasp, and yes it's a 'good guy'. There are probably over 2,000 wasps in NZ and only a few are pests.) Female A. colemani lay their eggs inside aphids, and when the egg hatches the wasp larva eats the aphid from the inside. Eventually more wasps emerge and of course when they mate, they lay eggs in other aphids. Soon your aphid 'problem' will be no more!

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A. colemani ovipositing egg inside aphid

You need to watch out for aphid 'mummies' – these are the aphids in which the wasps are growing. You don't want to remove them! That's when you should squishing or high-pressure hosing and leave Nature to do the job for you.

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Mummified aphids on rosebud

A. colemani are probably already present in your garden, if your garden is a friendly place for them. Don't use chemical controls. Tiny flowers like alyssum, coriander, dill and parsley will attract them. They are perfect for A. colemani and other tiny beneficial insects.

If you don't think you have any – remember these are tiny, wee wasps – you can buy them from a supplier such as Bioforce, an IPM (integrated pest management) specialist. Mummified aphids arrive by courier and you leave them in the garden near the infected plants.

Check out our video HERE.

Remember...

If you see aphids on your swan plants, it’s very important to look out for the natural controls already at work. If you use any manufactured ‘control’ then you will be throwing everything out of balance.

Aphids don’t bother butterflies. Butterflies have no sense of aesthetics. And the wasps won't hurt anything else in the garden. Practise patience, wait and be vigilant, and learn about what’s going on in your garden.

Thanks to Bioforce, Nicholas Martin and The NZ Institute for Plant and Food Research Ltd for photos.

Want to know more? Interesting video here!

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