Everything you need to know about aphids for growing plants indoors and out, commercially or as a hobbyist.
Everyone has dealt with aphids at one point or another….or all the freaking time! They are ubiquitous, pervasive and the world’s number one economic pest in agriculture.
There is a wide range of damage caused by aphids, or as a result of aphids. Most commonly, an aphid infestation (or even just a few aphids) reduces the saleability of ornamental crops, or can be a cosmetic concern in your garden. The sticky and shiny honeydew they produce further reduces saleability but also attracts other insects, and can lead to secondary infections from molds and mildews.
But the damage goes further. Many aphid species will attack new, succulent plant growth and stunt or deform it with both the physical damage of their feeding, and toxicity in their bite. Their ability to increase in population quickly can lead to young, tender plants succumbing to the infection and dying. And, perhaps the greatest risk, worldwide, from aphids is their ability to vector plant diseases and viruses.
In commercial settings, particularly indoors and in protective greenhouses, aphids must be closely controlled. Outdoors, monoculture crops are at great risk, but most gardens seldom need interventions of aphid control, as nature quickly takes over. (Despite most gardeners’ quickness to do so.)
And that’s a point I want to clarify first: aphids are a primary food source for a seemingly unlimited number of other living creatures: wasps, beetles, fungi, birds, reptiles, mites….you name it! Aphids – in a natural setting – are meant to produce food by being the food. They start early in the spring, attack young, fast-growing plant tissue, and multiply fast enough to feed the many predators that rely upon their presence throughout the year.
In an organic garden and with the right amount of tolerance you can watch aphids come and go throughout the season on a variety of plants. Some damage will be dealt, but it’ll never become a “problem.” And before you jump to solutions, know that there is no circumstance where you should be buying ladybugs that have been collected from the wild. Read more. Or watch the video here.
But here we are talking about Aphids because it is the pest most people are driven to panic over. With a little information you may find that aphids are easily dealt with and nothing to panic over. Here’s what you need to know:
- Types of aphids
- Aphid Life Cycle
- What they like and don’t like.
- Common mistakes that make aphids worse.
- Aphid predators
- When there are enough aphid predators to relax.
- How to prevent aphids in the first place.
Types of Aphids:
We are starting here very simply because most people find this fascinating. There are thousands of types of aphids! Every once in a while I run into a comment like “That’s an aphid!? I thought aphids were black.” And I have to explain the many colours of aphids, which also runs into conversations about the size of aphids and then (to the surprise of many) that there are wooly aphids and also root aphids in the soil. Check out the variety:
You may recognize many aphids by name: “Green Peach Aphid,” “Foxglove Aphid,” “Black Cherry Oat Aphid,” “Pea Aphid,” ect. Typically we know these aphid species because they are of a great commercial significance, and the reason for that is they have a wide host range. Meaning: there are several species or varieties of plants that they will happily infect and damage.
However, too often gardeners will see aphids on something like a lupin (for example) and panic, remove the plant or spray it with a chemical or home remedy. But, had they known that MOST aphid species can only infect one species of plant they wouldn’t need to panic. For example, if in a garden full of a variety of plants, you notice a lupin is covered in a teal/grey aphid, you can be sure that is the Lupin Aphid, and will not spread to other plants that are not lupins. So you could leave that plant alone, since the predators that will arrive to eat the Lupin aphid will spread into your yard to eat other species of aphids.
What complicates this, is that many plant specific aphids have a different over-wintering plant from their primary host plant. For example, the Hop Aphid infects Hops (both the brewing type and the ornamental type), and stinging nettle and (sometimes) cannabis since they are all closely related. However, the Hop Aphid needs to overwinter on Prunus trees. (Plum family). Here are some more plant-specific aphids that either have a single host, or a small group of hosts and an overwintering host (not, wide-spread species that can infect many plants). Their name usually gives away the specific plant they infect.
Size Matters too! Size is important for ID. If you’re used to seeing large aphids, like the Melon Aphid, then you may overlook the tiny Hop Aphid, or think it’s not an aphid. Furthermore, aphid size can restrict some predators, and is (along with species) essential for parasitoids. Many aphid parasitoids (wasps) will be host specific and may only parasitize one species of aphid. Even if they try to parasitize another species, often they cannot because they are specially designed for their specific host aphid. Sometimes they are simply uninterested in a different species, and sometimes it is the size that makes the parasitoids too small to do damage, or too big to expect a viable offspring to be produced within the smaller aphid.
Further complicating aphid identification is that sometimes they have wings. These “Alate” aphids can be males, but more often they are females that have reached a population or environmental trigger and formed wings to spread-out.
Male aphids are rare. Most aphids reproduce by live-birthing exact clones of themselves. When a new female aphid is born, it already has thousands of clones of itself developing inside. This is why aphid populations can “explode” seemingly overnight.
As mentioned above, sometimes female clones are produced with wings and sometimes winged males are produced. There is sexual reproduction when males are produced, but this is extremely rare in most aphid species. So, when someone says they see aphid eggs, question them. In all my years studying and working with them, I’ve only seen aphid eggs in the wild, once. (Instead, it’s almost always smaller aphids or predator eggs.)
Typically, aphid infestations start when winged aphids fly into a yard or greenhouse. They immediately produce clones and begin feeding. Aphids feed by piercing plant tissue with a mouth piece and allowing the plant’s veinous hydraulic system to pump nutrients into the aphids’ mouths. They are feeding on phloem (think of it as sap), which is the nutrients the plant is pumping to and from its above ground tissue and its roots. Aphids almost continuously feed, which allows them to reproduce almost constantly, as well.
So, other than the cloning live birth of a male in order to start a sequence of sexual reproduction, you can think of the aphid lifecycle as simply this:
Winged female finds your plant.
Produces non-winged female clones that feed and reproduce further.
Aphid population reaches a tipping point and new female clones are winged.
Winged clones leave to infect more plants.
Seasonally, they tend to be at their worst in Spring and early summer. By Autumn they are usually well matched by natural predators and moving towards overwintering sites. In the cold months, some aphids like Foxglove aphids may still be problematic, especially if their predators are gone for the season. Then in Spring, as soon as there is green plant tissue and a sun high-enough in the sky to induce rapid plant growth, aphids will be happily feeding and preparing for another year.
What They Like and Don’t Like
Other than the obvious, (certain plants,) aphids don’t require many specific conditions. Again, there are so many species that simply giving a temperature range doesn’t give them the respect they need. But here are some major generalizations that are useful for growers:
Like – High nitrogen: If you want to grow aphids (which I do) you simply grow a plant normally with lean soil, then fertilize the heck out of it so the aphids can reproduce faster. Fertilizers rich in nitrogen are best because it is the element required for rapid, soft, green growth. That growth is tender, and easily pierced by aphid mouthpieces, especially those of the small, new clones. (Read more about fertilizers’ impacts). You may have heard the theory that Brix (sugar content) is what aphids are after, but at very high levels of Brix the aphids can’t feed. This has been disproven with peer-reviewed research. Plants can produce compounds aphids don’t like (nicotine, caffeine…etc) and these may be produced at different ratios to sugar at different times, but, very simply, the more sugar the faster they reproduce.
Like – Soft plant tissue: As above, nitrogen can cause this, but so can a positive change in weather. After rain, especially in the spring, when the sun comes out and roots are wet, plants are often poised for rapid growth. This does the same as high nitrogen and aphid populations can expand quickly. Similarly, when a plant flowers based on day length, there is a chemical switch, and often the stalks shoot up. This can cause a similar explosion in aphids.
Like – Weakened plants: Again, nitrogen weakens the plant by producing succulent green growth without the necessary potassium and phosphorus to firm up the stems and outer leaf membranes. But fertilizers aren’t the only culprit. General plant stress: drought, infection, too much or too little sun..etc also contribute. Also, washing or spraying leaves with any soap, acid, base, some oils, or other home concoctions can wash away the thin waxy membrane on leave surfaces. Days later the plant becomes highly susceptible to UV, infection and pests. (Think of it like stripping your skin down to a fragile, thin layer.)
Like – Absence of predators: New research currently being conducted is showing aphids react to a smell put-off by some beetle predators. Earlier research (Lucas, et al) from Université du Quebec á Montreal found that large predators with feet are detectable by the aphids via their footfalls. An approaching predator can induce defensive actions like dropping from the plant, kicking at predators, or simply walking away. An “absence of predators” might be obvious to you, but it’s necessary to point out that indoor plants and those in greenhouse often have aphid populations in the absence of predators.
Like – Ants: Ants farm aphids for the honeydew they produce. They literally pick up and move aphids to new plants or new leaves and actively fight off aphid predators to maintain the aphid colony. Ants are even known to carry aphids underground for the winter in order to establish them as a food supply in the next spring. Ants must be controlled in commercial settings before aphid predators are applied. A recent study found that ants will choose highly refined sugars over aphid honeydew. So the common and effective ant trap is a mixture of table sugar and borax. Famous entomologist for Agriculture Canada, Dr. David Gillespie suggests this recipe:
1/2 Cup sugar
1 1/2 Tbsp Borax
1 1/2 Cups warm water.
Soak cotton balls in the mixture and place near the ants. They’ll feed on the sugar and borax and take it back to the hive to feed the queen. This will poison all of them.
DO NOT over-do the concentration of Borax – this can cause all kinds of plant injuries.
Dislike – Strong blasts of water: heavy water flow, like from a hose can cause mouthpieces to break off. The aphids eventually starve to death.
Dislike – Vermiform Predators: They obviously dislike all predators, but vermiform (meaning wormlike) predators do not elicit defence actions from aphids. Instead they sit there feeding and get eaten. So the most damaging predators tend to be the larva of hoverflies or aphidoletes.
Dislike – Movement: Foxglove aphids are classic for this: Wave your hand above the plants and they drop to the ground. Cabbage aphids, when approached will begin to do the “wave”, where in sequence they invert their bottom halves, perhaps to look more like one big organism. Regardless of the individual reactions, appearing to be a threat from above can disrupt their feeding. Some growers pass a spray boom over the crop just to create a moving shadow that interrupts foxglove aphids and slows their feeding and damage.
Common Mistakes that Make Aphids Worse:
All home remedies! Stop doing this insane thing! You’re not doing anyone any good by recreating a chemical pesticide with stuff off your shelf – and that is exactly what you’re doing. Commercial pesticides are typically an active ingredient (which seldom works, thanks to overuse and built-up resistance) and a spreader and a sticker to form small droplets to cover more area and stick to plant surfaces long enough to work. Those are soaps and oils. So even your blended spicy pepper (oil) and a bit of dish soap is essentially a powerful pesticide. And before you say “Good!” consider this:
These pesticides work because they kill insects by contact. Some leave behind a smell or residue that keeps some pests away for a few hours or days. But all of these do irreparable damage to your plants. Commercial growers that leave-behind sprays and use bio-controls often find that plant health is so much improved (and plants are quicker to market or fetch a higher price) that it not only makes biocontrol more effective, but less costly. You may not see the plant damage when using soaps and oils, but it thins the leaves, removes the wax coating and makes the plant very susceptible and attractive to pests and infection.
All that is really needed is pure water. It washes off the honeydew, knocks some aphids off and doesn’t disproportionately kill predators, which are likely already present.
Washing them off: I mention it above because sometimes it is warranted. To the untrained eye the unsightly mess of aphids, shedded exoskeletons stuck to honeydew…etc is an eyesore needing to be eliminated. However, I rarely get called to an “aphid infestation” that isn’t already populated by natural predators. Sometimes we have to add more predators, and sometimes we don’t. Washing aphids off is a last resort to save a bloom you need to sell. Otherwise, let nature do it’s work and augment with commercial bio-control agents (aphid predators).
Conducting a Push: If you’re in the cannabis industry your plants are always in a “push.” For the rest of us, we sometimes conduct a push when weather has kept a crop behind where it should be. If hanging baskets destined for Mothers’ Day sales looks like they won’t be ready, we pump up the nitrogen and “push” the plants. The most seasoned growers order more beneficial insects to be present when they conduct a push because the influx of nitrogen will make any sucking insect present multiply. Aphids are one such sucking insect. One hundred aphids, with their mouthpieces tapped into your plants can easily become hundreds of thousands of aphids when you return after the weekend hoping to see bigger and better plants. Because cannabis is grown for accelerated growth and a quick turn-around, they always have too much nitrogen in them and so the demand for pest control is much greater. Conversely, ornamental landscape nurseries may never use beneficial insects. Some are intentionally grown in very poor soil so the plants are grown “lean” which strengthens their resistance to pests and infections and strengthens stems and roots. These growers typically don’t need any chemicals or bio-controls.
Buying Ladybugs: Commercially available bio-control agents are produced and collected from laboratory settings. This makes them safe, well studied and legal. When you buy ladybugs you are buying a bag of wild-collected ladybugs from a very specific part of North America. They are collected in high density while they overwinter. When you buy them, they are still in hibernation mode and HAVE to travel back to that spot. The ones you buy are not the ones that establish in your yard. Among the reasons this is problematic is because wild collected ladybugs bring with them ladybug parasites. These parasites are often invasive and spread to native ladybug population and wipe them out. Do not buy ladybugs! or watch the video here:
Panicking: Often when someone sees black aphids on their nasturtiums they pull the plant and compost it. They have been down that road before and don’t want the unsightly, struggling plant to ruin their garden. However, if you try to grow aphids in your yard, you will fail. Aphids always result in aphid predators. Leaving aphid populations to support predator populations is a difficult place for many growers to get to, but it results in major successes. For example, nasturtiums can be planted out of sight. Ornamental kales can be planted behind bigger plants and in acidic soil to weaken them and attract a cabbage aphid – seldom one that affects other ornamental plants. Stinging Nettle can be grown for early (March/April) infestations of the hop aphid, to attract suitable aphid predators ahead of commercial hops emerging from the ground (May). My point is: an aphid colony is a “banker plant” for you to establish aphid predators whether you can wait for native ones, or buy commercial ones. Speaking of predators, here they are:
Knowing aphid predators is essential for feeling calm in the garden, and making the right decisions for interventions. Here is a non-exhaustive list of aphid predators that are native (to my area in the Pacific Northwest – BC) or commercial available predators for all of North America (and in most cases, Europe).
Native – Aphidius species and other parasitoids: There are thousands of aphid parasitizing species. Most are tiny, black and plentiful. Some are specific to an aphid species, but others are more general and can parasitize multiple aphid species.
Native – Ladybug species. There are too many to list. But the one you can buy is not a native one. Don’t buy them. Generally they show up early and generally, they are generalists – meaning they eat all sorts of things. It isn’t until aphid populations become extreme that you’ll see ladybug populations increase. Like other predators, ladybugs are cannibalistic. If eggs are laid in small aphid populations, the ladybug larvae will starve and eat one another. Later in the year, larvae feed and pupate before emerging as adults – sometimes all on the same plant.
Native – Hoverfly species. These are similar to Ladybugs since they are native, big, and cannibalistic. Only when aphids are largely available does one see many hoverfly larvae. But, unlike ladybug adults and larvae, hoverfly larvae crawl along undetected by aphids and can consume up to 10 time more aphids than a ladybug.
Native – Lacewing species. Brown lacewings and green lacewings are both native. Both are largely nocturnal or shy and are seldom seen. The eggs of green lacewing are often the first sign. They are singularly laid eggs attached to a long thread. Brown lacewings lay eggs singularly as well, but without the string. Both try to spread out their young so they don’t eat each other. When they don’t eat each other, they are awesome aphid predators. Brown lacewings which are also predators as adults, also continue to hunt aphids throughout the winter in some milder areas.
Native – Birds. The picture I took of the winged green pea aphid (cover photo) was taken only a moment before a hummingbird picked off the adult. I had never seen it, and honestly never considered birds as a major predator. Now I watch all sorts of birds feed on aphid colonies.
Native – Spiders. A couple from Manitoba, in their retirement, began researching a wild aphid in a park nearby. It was hard to find and track aphid populations because when they’d return the next day the aphids were massively reduced. They set up cameras all day and all night and found that most wild predation of aphids was done at night by larvae like lacewings and hoverflies but by all sorts of other predators including, what appeared to be the main predator – spiders. Once again, aphids are food for anything.
There are likely so many more. But we tend to know the “big” ones best. Typically it’s because they are easiest to see. But also, it is because they eat so many aphids (to support their offspring and long lifecycles) that we notice them in high numbers towards the end of summer when aphid populations are in their decline. However, often the best predators are ones that sneak in and maybe only eat a few, but reproduce as fast or faster than aphids. When we find those, the usually become commercialized as new aphid predators.
Commercial – Anystis predatory mite. This “Crazee Mite” is fast, and a generalist. It can wipe out aphid populations or just contribute to predation while feeding on most other pests as well.
Commercial – Aphidoletes aphidimyza predatory midge. This is the industry standard. They reproduce faster than aphids (a single adult can lay hundreds of eggs in a couple nights) and each larva can kill up to 200 aphids. They attack almost all species of aphids and outdoors a single release can last all season. For more on Aphidoletes, read this: or watch this video.
Commercial – Eupeodes americanus predatory hoverfly. This has only recently been commercialized. It is known as the #1 aphid predator. Each larva can consume up to 2200 aphids in its short, one week life span. However, as an adult it only feeds on pollen and nectar, and adults must be fed with both for 3-5 days before eggs can be laid amongst aphids. In a flowering crop, hoverflies can do damage to aphids! Read more here:
Commercial – Aphidius species and other parasitoids. Ervi and Colemani are two species popular for commercial reasons. Many growers have tremendous success controlling aphid populations with Aphidius. However, ornamental growers, or growers of leafy green produce cannot use them because the “aphid mummy” carcasses they leave behind remain with the plant. Also, other predators, like lacewings, hoverflies and aphidoletes lick up the honeydew as well as eat aphids, which cleans the plants. Even when aphidius work, they seldom help with plant saleability. Furthermore parasite resistance through the gut bacteria hamiltonella and hyper parasites have wiped out commercial Aphidius populations.
Commercial – Micromus variegatus predatory brown lacewing. Green lacewings are also available, but like the hoverfly, the adult must feed elsewhere. Brown Lacewing Micromus variegatus is a predator for its adult and larval stages and has a longer lifecycle. It is also effective from 40C down to 4C. Most growers use only a small amount of these for year-long aphid predation and the added benefit that they are generalists and will eat mealybugs, small caterpillars, soft scale and other soft pests. Read more:
How To buy Beneficial Insects:
Sometimes this is a stumbling block for people. Sure, you want bugs, but how the heck do you get them? Asking at a garden shop often limits you to refrigerated products like wild-collected ladybugs or old nematodes, but you can ask for specific bugs and see what happens. Or you can check out this list of distributors for Applied Bio-Nomics – North America’s largest independent commercial insectary. There you can find someone close to you and place an order.
Please note, there are many insectaries and some may better suit your needs than others. However, be sure to evaluate their efficacy. No matter what, they should ship fresh product that is not packed with ice or refrigerated along the way. Temperatures below 8C can reduce efficacy of beneficial insects by over 90%. So while they may be cheeper you’ll need as much as 10X more to get the same results.
When There Are Enough Predators To Relax:
Outdoors, if you see anything that isn’t an aphid amongst them (other than an ant), it is likely a predator. One predator means the aphids have been discovered and you likely need to do nothing. For example seeing an orange aphidoletes larva means it has been there feeding for several days. Because the adults cycle outdoors all year you’d probably find many aphidoletes eggs or early instar larva under or on the aphids. But, it would require a magnifying glass to see them. Likewise, a parasitized “mummy” aphid means that it was parasitized days earlier. Therefore, in the same colony, it is more than likely that several more are already parasitized but showing no sign. When the adult wasp emerges it will continue to parasitize. So a single mummy is usually all that is needed.
There are special circumstances, however. Sometimes a plant is so overwhelmed and stressed that aphids take over. Predators are there, but never clean it up. This could be an underlying problem with the plant, but it could also mean that the plant has become unsuitable for predators to lay their eggs. If they don’t think the plant will survive, or the leaf is dying and will become dry, they will leave that space. In cases like this, no amount of predators will help. Your best bet would be to wash every leaf with water.
Furthermore, plants like cherries that get the black aphid early in the year will have a tough time fighting off the infestation without damaging young growing tips. Establish Aphidoletes early on these aphid populations. If you see several aphidoletes larva per group of aphids by the next week you will be in a good place.
Indoors you have to buy predators. Likely you’ve watched the aphid colony grow, so you’ll need to assess the performance of the beneficial insects to determine if they are keeping up. Within a day or two you should see the beneficials where the aphids are. If things haven’t improved the following week, then it’s time to get more. In practicality, there are people like myself that need to curtail indoor commercial application rates according to a variety of factors (wind speed, plant, day length, temperature, other invertebrates present…etc). So if you’re at home, just buy slightly more than you’d think, or at least buy them twice, two weeks apart to ensure they are taken care of. If you’re a commercial grower, contact an IPM specialist of beneficial insect producer/seller.
How to Prevent Aphids:
If you’re read this far, you’ve probably got it mostly figured out. The missing piece is early applications of commercially available beneficial insects. But before we get there, let’s recap for outdoor home growers: Leave aphids alone as long as you can to let predators establish. If you are at your tolerance threshold then wash them off with pure water and buy commercially produced aphid predators to release in your yard. Next season, establish plants hidden amongst your yard that you know will get aphids, particularly aphids that won’t spread to other plants. Allow these to be a battery of aphid control for your garden.
However, when aphids outdoors are a problem, there is an easy solution. Release 250 aphidoletes when aphid populations are present. (250 per large tree or per half acre). Outside, they will lay eggs and the larvae will feed, drop to the soil to pupate and new adults will emerge two weeks later at a nearly 20X growth rate. If you do it early enough in the season (but no earlier than mid April) you will only have to do this once per year (sometimes once every few years).
It’s always different commercially. Growing plants to attract aphids away from your crop is always good practice, but you run the risk of attacking predators to those plants instead of your crop. And while establishing predators on that plant is considered a “banker plant,” you still want predators to find individual aphids within your crop, not just on the banker plant. If you wait until your crop has a preferable population of aphids, then it may be too late. Always, when books are examined at the end of a season, buying small amounts of beneficials that starve to death because they are eating every single aphid ends up being more profitable than banker plant systems or otherwise trying to establish predators in your crop. Essentially: don’t feed you predators, let the actual pests do that.
It’s always suggested you keep a journal of pests and predators. What you’ll find is the same week every year (within one or two) is when aphids arrive. Two weeks before that time, buy and apply aphidoletes at a rate of 250 per acre(min) or per hectare (max) every two weeks. Yes, they will find single aphids that have entered your crop and yes, many will starve to death from having too few aphids. Luckily aphidoletes are cheap and aphid damage is expensive. When aphids start to arrive in higher numbers, simply adjust the release schedule to weekly, or increase release rate up to 1,000 for the same space. In special circumstances 3,000 can be used.
Also release Micromus variegatus once per month or once per two months. As adults they need less food and survive much longer in your crop. Their larva will devour aphids and pupate down near the roots, emerging a few weeks later as new adults.
In crops with lots of pollen and nectar, the release of adult hoverflies can be used, but is seldom recommended if the two predators above are used.
There you have it! A total crash-course in aphids, and how to control them. I’d love to know if you have any questions and if you found this helpful. Feel free to contact me! And good luck with your aphid control.
There is still lots more to read and learn. Check out these other topics: