Aphids are sap-sucking pest insects that every gardener deals with at some point. Unfortunately, these are one of the most destructive pests. Having some ‘tools’ in your pest management toolkit for getting rid of aphids quickly is essential. So, let’s take a look at who they are, why they are such a problem plus how to prevent aphids and minimise the damage with practical science-backed strategies to get rid of aphids naturally. I’ve lost plenty of plants to aphids so I’m sharing what really works and why.
Aphids – Who are They?
Aphids (Hemiptera: Aphididae) are tiny insects that clone themselves and feed on plant juices. They literally suck the nutrient-rich sap and life out of your plants. There are over 5000 species in various colours from green to pink and black!
In small numbers, they’re not always a problem on mature plants. Especially if you have natural predators in your garden like hoverflies, parasitic wasps or ladybirds. If these hungry omnivores are residents in your garden, they will often seek out and enjoy aphid dinner, taking care of minor numbers.
Why are Aphids Such a Problem?
However, aphids generally are a BIG problem. They may be tiny but can do a LOT of damage in just a few days. Colonies with thousands of aphids can build up VERY quickly. How’s that possible? To be blunt, their highly successful reproductive rate happens because female aphids don’t waste time or energy on looking for a male partner, courtship, sex, or laying and incubating eggs! Sorry boys, not needed here as aphids are primarily asexual. In most species, males are rare or absent! Males don’t get much attention until cold weather when they’re occasionally required to fertilise eggs as a backup plan! Even then, mother aphids control their population by laying eggs that morph into males when necessary!
Aphid colonies are started by a stem mother who flies to a new food source location. She carries live babies and gives birth on arrival to start the new colony.
Aphids are female cloning experts that take a shortcut. Mother aphids give birth to multiple live female nymphs rather than eggs. Not only are all these daughters born hungry, but also ready within days to produce their own look-alike families! There’s no time buffer for you to miss their arrival before the exponential population explosion gets out of hand. A gardener’s nightmare, right? Take a look at this video to see how quickly this scenario unfolds.
How Do You Know if You Have an Aphid Problem?
If you notice tiny dots on your plants, curled leaves or discolouration, check carefully. Look on the undersides of leaves, stems, on flowers and new lush bud or tip growth. Aphids are really good at hiding. Especially the tiny nymphs. They are masters of camouflage, often blending in with the same colour of the plant they are on. A magnifying glass may be helpful to locate them.
How Do Aphids Feed, Move and Multiply So Fast?
Aphids have two main goals in life. Eat and reproduce! They feed on plant sap to convert nutrients into aphid biomass as fast as possible. This enables them to duplicate themselves sooner. They devote their short lives and energy to eating, motherhood and finding the next host plant. So essentially these wingless, sedentary insects are continually pregnant and giving birth. They don’t waste energy on exercise! They only move to locate more sap on the host plant or hide from natural enemies – and maybe you.
Mature adult aphid with wings – Image source
If one plant gets too crowded with aphid clones, they walk to the next. So the cycle starts again.
Winged aphids have a different role. Adult aphids fly to locate new host plants and establish new colonies. They take off with embryos inside and give birth when they find their next plant victim. Aphids just keep coming back with new versions of themselves. If you eliminate most of them, it only takes one single aphid to repeat the nightmare. Sounds like an alien horror movie, right? They’re definitely a pest to take seriously.
What Damage Do Aphids Cause?
When aphids feed, they disturb the balance of the plant’s growth hormones. Leaves wilt, wither, yellow, and dry out. Aphids prefer new shoots and buds to older leaves. Buds may not open at all or produce distorted flowers. Aphids also transmit plant viruses.
After feeding, they secrete honeydew (a sugary substance). This food source often attracts ants who act as bodyguards for a sweet reward. It’s a win-win relationship.
Ant receives a sweet honeydew reward from an aphid for providing security services – Image Source
The ants keep potential predators like ladybirds away and get paid with free food. Sticky honeydew covers the leaf creating the perfect growing environment for black sooty mould to develop. This, in turn, slows and stops photosynthesis so the plant can’t produce energy to grow. It’s a domino effect. Forewarned is forearmed!
So, one small dot on your plant can have serious consequences! A single aphid can lead to other diseases and plant death. If ants are present as guards, you have to remove them too. That’s why you need to act fast to prevent and control aphids.
Host Plants and Aphid Species
If you grow any vegetables, fruit or citrus trees, roses, perennials or annuals, it’s likely you’re going to encounter at least one species of aphid sap suckers! Most aphid species look for hosts of a particular plant genus, but others are generalist feeders. They’re not fussy about which plants they eat whereas others target specific plant family groups. Onion aphids, for example, are specialist aphids that feed on host plants (HP) in the onion family. Have you ever noticed those black bugs on your leeks, onions, chives, spring onions and garlic? They’re common in spring.
Severe infestation of onion aphids Neotoxoptera formosana on shallots – Image source
Neotoxoptera formosana is one of these species. It’s a global pest insect that sucks nutrients from the onion (Allium) family including onions (Allium cepa); shallots (Allium ascalonicum); spring or green onions (Allium fistulosum); garlic (Allium sativum); garlic chives (Allium tuberosum); chives (Allium schoenoprasum); leeks (Allium porrum) and Chinese onions (Allium chinense).
Research studies (Pickett et al., 1992; Pickett & Glinwood, 2008) show onion aphids can detect their preferred host plants in the Allium family by scent using olfactory cues. Aphids use their sense of smell to identify their host species by the unique aromatic airborne plant volatile compounds (volatiles) released by the leaves. They move directly towards the odour source, kind of like using GPS. Or pet dogs that magically appear in the kitchen when they smell their favourite food!
Two sulphur-containing compounds that are characteristically found in Allium plants are diallyl disulphide and dipropyl trisulphide (Hori, 2007). Since both these compounds are relatively uncommon among other plant taxa (Webster, 2012), aphids can accurately target the onion family. Amazing huh? That’s why you’ll see specific aphid species on thousands of different plants. Read on to find out how to use this to your advantage.
Natural Pest & Disease Management
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How to Prevent and Get Rid of Aphids Naturally
So what can we do to prevent and minimise aphid damage? I use a few key strategies and principles because you need more than one tool in your toolkit. From my experiences with black onion aphids on members of the onion family over the years, I’ve adapted my gardening practices.
7 Preventative Pest Management Strategies for Aphid Control
How do you avoid an aphid infestation on your plants? These are some science-backed suggestions for you to consider.
1. Be observant. It’s much easier to tackle a dozen aphids when they first arrive than a major infestation problem in plague numbers. Check your plants daily if possible. As the weather warms up in spring, you can expect aphids to become more active with all the new growth to feed on. Early detection and intervention will help. If you see a few, don’t wait a week to do something about it! Act quickly.
2. Maintain plant health and watering. Weak nutrient-deficient plants are a magnet for pest insects. Keep up the soil moisture, nutrients and mulch. Strong, healthy plants have a better chance of resisting attack. Studies have found plants with adequate bioavailable phosphorus and potassium have higher resistance to aphid populations. Water in dry times so plants can access nutrients in the soil. Drought or heat-stressed plants release chemical cues that insects pick up on. If your plants are showing signs of wilting or leaf discolouration, check nutrient availability. Foliar spraying with seaweed can also help strengthen cell walls and encourage earthworm activity.
Foliar feed plants with liquid seaweed to strengthen and protect against pest attack
3. Avoid excess nitrogen. An imbalance of too much nitrogen can create a flush of new sappy growth. The allium family is particularly vulnerable. Whenever I’ve applied nitrogen-rich organic fertiliser pellets and watered them in, within 24 hours, aphids appear. Like magic. Every time. Scientific studies confirm excess nitrogen fertilisers attract aphids and other sap-sucking pest insects like whiteflies. You can’t go wrong with slow-release compost and worm castings. Nature’s food with a balance of nutrients.
4. Practice biodiversity. Aphids use scent cues. They are attracted to the volatile compounds your host plants release. So, avoid planting large numbers of the one plant family all in one spot. It may look pretty to have a row of onions or broccoli and make crop rotation in garden beds easier. However, this just spotlights your crops making it easy for aphids to find them. Instead, spread them around the garden. Interplant alliums as beneficial companions near other plants like beets, brassicas, carrots, cucumber, dill, lettuce, potatoes, roses, spinach, strawberries and tomatoes. The scents and diverse leaf shapes of other vegetables and herbs can also help make it more difficult for the aphids to find them. Win-win!
“Solid blocks of the same plant variety, though easy to seed and harvest, act as an ‘all you can eat’ sign to insect pests and diseases. Harmful bugs will stuff themselves on this unbroken field of abundant food as they make unimpeded hops from plant to plant and breed to plague proportions.” – Toby Hemenway, Author – Gaia’s Garden
5. Plant flowering species that attract aphid predators. Aphid natural enemies include omnivorous hoverflies, ladybirds, lacewings and their respective larvae, plus predatory wasps. These beneficial insects also dine out on nectar from many flowers as a supplementary food source. They eat meat and veg! So, try growing herbs and flowers like oregano, dill, buckwheat, sweet alyssum, nasturtiums and bachelor buttons in your garden prior to spring when aphids generally are most problematic. You’ll have your friendly predatory ‘armoured guards’ ready to take care of these unwelcome aphid guests on arrival. Early intervention can keep the pest-to-predator ratio in balance. Insect-eating birds that feed on aphids include wrens, silvereyes, willy wagtails, finches, honeyeaters, pardalotes and some sparrows. Make your garden bird-friendly with habitat and water!
A Silvereye feeding on aphids on a Red Russian kale plant
6. Interplant with strongly scented herbs like rosemary and pennyroyal. The volatile organic compounds (VOCs) in these non-host plants have been found to help mask the odour of alliums and repel aphids of various plant species. (Hori, 1996). Keep in mind though, you need to maintain your herb health as well. The production of these volatile compounds that help repel aphids is often dependent on moisture availability. Research studies (3) show rosemary, for example, decreased the release of VOCs after a four-day water deficit. Pruning can also help increase the release of VOCs. So you can encourage plants to provide a protective role at critical times in spring.
7. Cover your crops. Obviously, if you use exclusion insect netting or crop protection bags, aphids will find it hard to access their host plants.
Exclusion Insect Netting for Crop Protection
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Companion Planting – How to Confuse and Lose Aphids
This is a complex area of pest insect and plant interactions. Research studies vary widely in their findings. However, some of the science-backed findings regarding companion planting are interesting and give us the confidence to use plants as part of our integrated pest management (IPM) toolkit. Evidence-based research shows many companion plants have been found to work in several beneficial ways.
- By Repellency. By releasing strongly scented volatile compounds some companion plants make it difficult for aphids to find their host plants. For example, studies (1, 5) show companion planting either chives (Allium schoenoprasum) or Rosemary (Rosmarinus officinalis) near capsicums (Capsicum annuum) helps repel aphid species Myzus persicae. How? The chives and rosemary produce repellent chemicals that cause aphids to move away from the host plant and reduce the attractiveness of the capsicums. The chives also help mask odours. Aphids basically get confused by the different chemical signals and can’t locate their target!
- Food and Accommodation. Other companion plants entice predatory insects that are natural aphid enemies by providing food and shelter. Many studies suggest companion plants can provide a valuable source of pollen and floral nectar. A bit like a bed-and-breakfast for beneficial insects. Grow it and they will come!
- Disruption to Finding Host Plant and Acceptance Behaviour. Aphids choose their host plant using numerous cues including visual stimuli. i.e. what a plant looks like! The ‘appropriate/inappropriate landings theory’ (5) suggests by interplanting host plants with other green-leafed non-host plants provides alternative options to land on, which can lead to lower pest density. Some plants respond to aphid attacks by activating defensive genes that produce physical barriers and/or chemical toxic compounds. (7)
- Interfering with pest egg-laying behaviours. Indirect plant defences such as producing oviposition-induced plant volatiles (OIPVs) that are used to attract natural enemies (parasitoids) able to kill the eggs or hatching larvae.
- As a Trap Crop. You can use some plant species, particularly herbs, as decoys. Trap crops attract pest insects including some aphids to feed on them as an alternative food source. Treat them as a sacrificial plant, drawing aphids away from your more valuable crop. For example, nasturtiums.
Potential interactions between companion plants, aphids, natural enemies, and host plants. Interactions and mechanisms described are based on the available literature. CP: companion plant, HP: host plant, VOC: volatile organic compound. Image source
Does Companion Planting Really Work to Repel Aphids?
There’s a significant volume of research in this area (5) across a wide range of plants but let’s just look at a couple. One study (2) explains the ‘push and pull’ effect. “Companion plants may work simultaneously, influencing both top-down (i.e., reinforcing host plant defence) and bottom-up (i.e., enhancing biological control) mechanisms. In several cases, companion plants have been shown to simultaneously have a repellent effect against pests and an attractive effect on natural enemies.”
Another study (1) explains how strongly scented plants work. “Aromatic plants have also been used as companion plants to enhance the abundance of natural enemies. For example, in a pear orchard ecosystem, Beizhou et al. found that using aromatic plants such as summer savory (S. hortensis), ageratum (A. houstonianum), and basil (O. basilicum) between pear tree rows enhanced the activity of the C. septempunctata and reduced the incidence of Aphis citricola. Results showed that intercropping with aromatic plants contributed to the earlier appearance of natural enemies and the shorter occurrence duration of pests, mainly aphids.”
8 Ways to Get Rid of Aphids from Plants Naturally
These are simple natural strategies to get remove aphids off infested plants.
1. Squash by hand. If just a few aphids turn up, you can try running your fingers over the leaves to crush them. Check each day as you will always miss a few!
2. Prune the plant. If you only have minimal numbers contained to a leaf or stem that won’t impact the overall growth of the plant, you can simply trim this off and bin it.
3. Hose off. Sometimes a short sharp spray of water from the hose over 2-3 days helps dislodge a small number of young aphids. This may buy you time to institute other strategies. Why does this work? Adult aphids are slow but after being knocked off a plant by wind or water, they walk and climb back up on the same plant or another host nearby. Make sure not to spread aphids to other plants in your garden! Hosing can be effective because the nymphs are weaker and smaller. They may run out of energy before they can reach a food source. However, if adult aphids are blown off, some nymphs may hitch a ride on their back to reach the food factory again! Other babies will starve and die before they make it. So at least some temporary reduction in numbers can be achieved.
4. Suffocate them. It sounds gruesome but if aphid populations increase rapidly and the predatory insect ‘cavalry’ hasn’t arrived, you need to take more drastic action. I use an organic horticultural oil like EcoOil or EcoNeem. Another option is a soap-based spray to suffocate the aphids.
Soap Spray – use to foliar spray plants with aphids and other sap sucking insects as a natural pest management strategy
Soap Spray
Mix 10ml pure castile liquid soap* with 1 litre of water in a spray bottle. Shake well to mix. Spray directly onto the insects. Some will be small and hiding! There are readily available insecticidal soaps suitable for organic gardens like NatraSoap and other brands. (* Plant-based without essential oils or fragrances). Note: Liquid dishwashing soap products that are formulated to remove harsh grease and oils generally contain synthetic detergents and other potentially phytotoxic chemicals. They may cause leaf burn and damage to a plant’s waxy cuticle.
Once the aphids are dead, wipe them off with paper towels or hose them. If you leave the dead bodies on the leaves, they will have difficulty photosynthesizing and you won’t know if more arrive. Insecticidal soap spray is a useful remedy for treating many pests that have a soft exoskeleton including aphids, scale, mealybugs, spider mites and white flies. Soap spray works by coating these soft-bodied insects’ external protective barrier. This causes the membrane to dry out. It works by direct physical contact and doesn’t poison insects. It is best tested first on foliage and sprayed late in the day. Avoid applying on a hot sunny day, to drought-stressed plants or sensitive species. e.g. succulents, ferns, plants with hairy leaves, fuchsia, bleeding hearts, lilies, cucumber and sweetpeas. To minimise the potential for leaf injury, hose off plants within a couple of hours after spraying.
5. Sticky yellow traps. These are not my preferred solution as there will be collateral damage with other insects stuck to the trap as well. The idea is to hang the sticky trap in amongst the foliage of the affected plant to help attract flying aphids. Importantly, check to see if they are effective in your situation and capturing the aphid clones rather than beneficial bugs. It’s a ‘win some, lose some’ option to use carefully.
6. Sprinkle with diatomaceous earth (DE). Aphids moult or shed their exoskeleton multiple times as they mature. Food-grade DE is a fine powder made from the fossilized cell walls of aquatic microorganisms (diatoms). Their skeletons are made of silica. Despite its smooth feel, food-grade DE acts like shards of glass when in contact with the exoskeleton of insects. It slices and dices them, so their bodies dry out and die. It must be dry and needs direct contact to work. However, if other beneficial insects are present, these minute sharp skeletal remains will also shred and desiccate your garden helpers. For this reason, I use this method very judiciously. I sprinkle it with a tea strainer to contain the powder just where I want it. Be sure to use PPE and not breathe it in when using it.
Food Grade Diatomaceous Earth
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7. Drop and Drown. Use an artist or pastry brush to broom aphids off your plant into a container of eco-friendly biodegradable liquid soap in water underneath. The soap suffocates them and they drown. Winged aphids will fly away but you may be able to remove the majority this way. It’s surprisingly effective. You can also use this method to remove any ants that may be farming the aphids for honeydew.
8. Wash and Replant. This is a more drastic option but I’ve done it! Dig up affected plants, wash them thoroughly in a big tub of soapy water until the aphids drown. Replant in a new location. However, it works and has saved quite a few of my allium plants, especially precious garlic, onions and leeks.
Final thoughts. Aphids, like most insects, are part of a healthy biodiverse ecosystem. They are an important food source for other predatory insects. In balance, they are not a problem. If you end up with a population explosion of aphid clones, I hope these tips and strategies will give you some options to try.
Related Articles
- Why are Ants in my Plants, Pots and Soil?
- Organic Aphid Control with Hoverflies
- How to Control Garden Pest Insects Naturally
- Imitate Nature for Higher Yields & Less Pests
- Organic Fruit Fly Control Strategies
- Coping with Caterpillars – Part 1
- Coping with Caterpillars – Part 2
References
1. Companion Plants for Aphid Pest Management
2. Effects of intercropping with aromatic plants on the diversity and structure of an arthropod community in a pear orchard
3. Water deficit stress induces different monoterpene and sesquiterpene emission changes in mediterranean species
4. Defense of pyrethrum flowers: repelling herbivores and recruiting carnivores by producing aphid alarm pheromone
5. Intercropping Rosemary (Rosmarinus officinalis) with Sweet Pepper (Capsicum annum) Reduces Major Pest Population Densities without Impacting Natural Enemy Populations
6. http://www.aphidsonworldsplants.info/
7. Aphid-plant interactions: a review
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