How to Control Aphids

How to control Aphids? Flawlessly, every year, from April to June this question dominates my work, encompassing the concerns of professional growers, landscapers and home gardeners. Aphids are so common, and there are so many species affecting so many plants. Home gardeners often make the mistake of not associating their winged stage with the typical non-winged stage, and not recognizing that aphids can be different sizes, colours and shapes and some are specific to only one or two kinds of plants. Aphids are complicated; they are sticky and damaging, unsightly and overwhelming; and here is how to control them:

Water!

Seriously. Take a stream of water from a hose and with a little pressure, wash them from the leaves. This sounds too simple, and you’ll be asking “won’t they survive and climb right back up?” Trust me: If you are looking up how to control aphids because you have an aphid problem at the moment, stop reading and go spray them off with water. I list this first because it’s something you can do now that will help and not prevent other control methods from working. Do it now. Then read further.

Winged aphid live-births some clones. But a hoverfly egg demonstrates how quickly predators find them.

Spraying them off with water does several things: First, aphids continually feed by sucking plant phloem (sap) through straw-like mouth pieces. They seldom mate or lay eggs, but just produce live birth clones. When the feeding is good, they reproduce rapidly. Spraying them off with water interrupts their feeding and slows their reproduction. Second, many of their mouth-pieces will break off in the leaf tissue, so while they may climb back up, they will still starve. Third, as this water may also injure or disrupt aphid predators, you are not applying a soap or any chemical and home remedy that will kill off predators or stress your plants. Furthermore, when you find aphids, I can guarantee you aren’t the first to notice them. Many will already be parasitized, predators will be present or their eggs and larva will be, so knocking the aphids off the plant does not prevent the parasitoids from emerging, or prevent the predators from feeding on them and establishing in your yard. Essentially, spraying with water is buying you and your plant time. Your plant will not suffer damage as quickly and the natural predators (who will eat all the aphids eventually….even if you do nothing) will still come and do their job.

Release Predatory Midges

Aphidoletes aphidimyza is the top biological control for aphids. Not ladybugs! (Never buy ladybugs) In almost all commercial greenhouses, nurseries, public and private gardens Aphidoletes has been the primary aphid control for several decades. They are inexpensive, easy to use, and they do all the work for you.

Aphidoletes arrive as pupae. Within a day the adults (which look like mosquitos) emerge and mate within the container. You simply open up the container in your yard or greenhouse (best to do so at dusk). You don’t even need to release them close to the aphids. The adults find aphids by the smell of the honeydew. So, as long has you haven’t sprayed the plant with a soap or smelly compound which masks their smell, the Aphidoletes will find the honeydew, feed on it and lay hundreds of eggs in the aphid colony. Each egg hatches into an aphid-destroying larva capable of killing hundreds of aphids in their short 7 day larval stage. At that point they drop to the ground to pupate in the soil and they start the process over again.

Three Aphidoletes fight over the last aphid in an aphid graveyard.

If you like the idea of releasing beneficial insects to control aphids, then Aphidoletes is best.

There are other beneficial insect options too:

Release Brown Lacewing Adults.

The Brown Lacewing adult is an aphid predator. It finds and eats aphids. It also lays eggs and its larva are incredible aphid predators. Similar to Aphidoletes, you just open a container and they fly-out, find aphids and go to work. This is a little less satisfying than Aphidoletes since they are hard to find after release and control takes longer. They are also the only cold-weather bio-control for aphids. They are excellent for cold-weather fox-glove aphid.

Release the American Hoverfly

Eupeodes americanus is an unmatched aphid predator. Each worm-like larva can eat as many as 2200 aphids in their 7 day larval stage. However, like Aphidoletes, these are packaged as pupa. The adults you get are not aphid predators, but fantastic pollinators. They need pollen and nectar for 3-5 days before they lay the eggs that will result in aphid control. Best applications are still being researched, but these are great if you have lots of flowers or kids that like bugs! Otherwise, shop for Hoverfly Eggs to apply directly to aphid colonies.

That hoverfly larva at the edge of the leaf can eat 2200 aphids in only 7 days. (Don’t spray your plants!)

Release Aphidius (or wait for them)

Some Aphidius species are available commercially. They are tiny parasitoid wasp that parasitize aphids. You have likely seen them, since they often show up naturally. They are the little bronze/brown, rounded aphid bodies that you see in aphid colonies or where they were. In the right conditions they can spread easily as each adult can parasitize many aphids. The only draw back is they are specific to specific types of aphids, and their mummies remain on the plants after the aphids are gone. This is not ideal for ornamentals or edibles, but might work for you. However, a gut bacteria found in aphids, known commonly as Hamiltonella, prevents aphids from being parasitized. And while this was found in only a few specific areas, it has since been discovered in aphid colonies all over the world.

Cabbage aphid on mustard. This aphid only affects brassicas. The brownish balls are parasitized aphids.

Use a bio-pesticide

These are becoming highly popular. Essentially you spray the leaves with a parasitic fungi. When aphids (and other soft bodied invertebrates) come in contact with it…and conditions are right…the fungi infects and kills the aphids. The draw backs to this is they don’t specifically target aphids, and you’ll be killing many other insects you might want around. It can also be expensive, and the more these are used the more resistance is created. Some common names are Beauvaria or Metarhizium.

Do Nothing

I know, right!? Aphids serve one purpose: to feed other animals. Birds love them, bugs love them, you’ll even find other small animals feeding on them or their honeydew. Although unsightly, aphids almost never kill a plant (except young seedlings). A bit of tolerance for aphids usually results in you finding aphids, seeing their population grow and then seeing them disappear slowly or overnight as predators eliminated them naturally. Aphids also come earlier in the year than predators so that their populations can increase in order to feed everything – this is the natural cycle of aphids. Whenever you can leave them, do. For example, the native ladybugs (not the ones you buy) have several life cycles during the growing season. They eventually have large enough numbers to cause massive reductions in aphid populations. But early in the season, there are few ladybugs, and lots of aphids. This is normal.

Native ladybugs always show up, even if it’s later than you’d like.

Aphids also are sometimes specific to plants. For example, the hop aphid can only infect hops. The lupin aphid only affects lupins. The oat aphid affects only grains. So, my master-growing-tip: Plant these. Put them places where you won’t care how damaged they get and watch them attract predators. My hop gets the hop aphid, and then ladybug eggs show up, then their larva, then the adults, then more eggs and suddenly my yard is inundated with thousands of free aphid predators that will move to aphids that ARE a concern on other plants, like the green peach aphid, pea aphid, or that black aphid that seems to affect all plants in your garden. And don’t necessarily pull that one Brussel Sprout covered in cabbage aphid. If the other plants are clean be confident that the aphids are attracting predators. That Brussel becomes your one sacrificial one. (If you don’t have a sacrificial one, just plant a couple extra next year.)

Stop Fertilizing

Aphids feed on the phloem in plant tissue (the sap). The more nutrients that plant is taking up, the more it is feeding aphids. In a garden growing in natural, lean soil, aphid (and other sucking insects, like spider mites and whitefly) do poorly. But in rich soil amendments, like manure, or liquid, quick fertilizers, you are feeding insects as well as your plants. One of the first steps in aphid management in a greenhouse is to reduce the nitrogen component of the fertilizer: the speed that aphids reproduce is reduced immediately. In a yard, consider this next year: grow plants lean. They might be smaller, but they won’t be less attractive. When you use a quick fertilizer then stop providing it is when leaves turn yellow. That is not the case when they grow lean from the beginning. See more on this and all the goods on fertilizer here.

Talking about Aphids is part of my job. I do it endlessly, and while everyone’s concerns about aphids are high and their individual situations complicated, aphid control is always so simple. I assess a situation and suggest either spraying dense populations off the plant with water and doing nothing else, or releasing Aphidoletes, the predatory midge. In a protected structure like a greenhouse you must use Aphidoletes because other predators will have a hard time finding their way inside. Outside I usually suggest doing nothing, but when people have a high value plant (commercially or aesthetically) or a specific need, like hosting a patio dinner under a tree dripping honeydew from aphids, or a yard with a history of high aphid pressure, then I use Aphidoletes outdoors. Fortunately in most of Europe and North America, one, single release of Aphidoletes (maybe costing you $30) can be all you need until that population disperses 5 years later.

Like this picture, I seldom get a shot of only aphids at my organic farm. Here there are aphids and winged aphids (black) but hoverfly larva and eggs as well as the swollen pupa of aphids parasitized by natural aphidius. Even if only the larva on the bottom right were there, it would eat every aphid over the next 5 days.

But this only sounds simple, because true problems with aphids are from situations made complicated by our own interventions. A few seconds of internet searching and you’ll find a “home remedy” for aphids. It’ll be a combination of household ingredients turned into a spray. Yes, this will kill aphids (usually) but it is no different than using a harsh chemical: the aphid fix will be temporary, but the affect to your plant and natural predators will be long-lived. The “active ingredient” in a chemical aphid spray is usually effective for the first few years after it is created and then there is aphid resistance. However, the chemicals work longer because of the other ingredients – primarily water and a spreader (used to produce fine droplets) and sticker (used to help those droplets stick to plant material long-enough to work). But spreaders and stickers are generally just soaps, detergents, acids and bases. They all kill soft-bodied insects and damage your plants. When you grab a bit of dish soap, blend an onion or a hot pepper and spray your plants you are causing your plant to react permanently to a chemical attack. It washes off the natural waxes on the leaves protecting the plant from UV or …get this….insects. Once it washes off, the plant is more easily attacked by aphids and the smell of both what you sprayed and the plants natural chemical reaction to the spray will ward-off natural predators. So you better make a big batch of that home remedy because it’ll be the only thing that works for you for the rest of the season, and your plant will suffer.

So keep it simple: Look at the aphids closely. You’ll probably never see an aphid egg, so anything that looks like an egg mass or a little grain or rice is an egg from something else. Chances are any other thing you see near the aphids is a predator. If that’s the case (you see aphids and anything else), leave them be. The predators eventually win. If you can’t wait, spray them off with simple water. If you have a protected area or a low threshold for aphids, buy and apply Aphidoletes from the closest supplier that doesn’t store them cold or for long periods. Your plants will thank you.

Black aphid on poppy. Hoverfly larva feeding, with other hoverfly eggs and small larva.

Read more: Why I put aphids in my garden.

Read more: Grow Hard

Read more: Hardening off and leaning out.

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