Aphids: How to Identify Them and Control Them by IRAC Group (Without Making It Worse)

Aphids are small, soft-bodied insects that suck sap from plants. They live on almost every crop in the world: cereals, vegetables, cotton, oilseeds, potatoes, fruit trees and flowers.

A few aphids do little harm.

The trouble is how quickly a few become thousands, and the viruses they carry from plant to plant.

In this video: how to recognise an aphid in seconds, how to tell it from whiteflies, mealybugs and thrips, why the numbers explode, the three kinds of damage, how to scout, which natural enemies do the work for free, what to do before you spray, and how to choose an insecticide by its IRAC group, so that the aphids on your farm do not become resistant.

Aphids are one to four millimetres long and pear-shaped, and they come in green, yellow, pink, grey or black.

Look at the rear end.

Most aphids carry two small tubes pointing backwards, called cornicles.

Think of them as two tailpipes. No other common crop pest has them, so if you see the tailpipes, it is an aphid.

Most of a colony has no wings.

When the plant gets crowded or starts to decline,

winged aphids appear and fly off to new plants and new fields.

These winged forms are the ones that carry viruses over long distances.

Look on the undersides of young leaves, on growing tips, on stems and in flower heads.

Several clues give a colony away.

White cast skins, which are often mistaken for whiteflies.

Shiny, sticky leaves.

Black sooty mould. And ants running up and down the stem.

Ants farm aphids for their sweet honeydew, and chase away the insects that would eat them.

Three pests are often confused with aphids.

Whiteflies are tiny insects with white wings that fly up in a cloud when you shake the plant.

Mealybugs are covered in white wax and move slowly, and they have no tailpipes.

Thrips are slender and fast, and they leave silvery scars rather than sticky leaves. The control choices differ, so get the identification right first. A hand lens that magnifies ten times is the most useful tool you can carry.

For much of the year, an aphid colony is all female.

Each female gives birth to live young without mating, and each daughter is born already carrying the beginnings of the next generation.

In warm weather a newborn aphid can be giving birth itself within about a week, so a colony can double in just a few days.

This is why timing matters so much with aphids. A colony that is easy to manage on Monday can cover the plant by the following week. It is also why resistance to insecticides builds so quickly.

There are many generations in a season, and each one is a chance for the survivors to multiply.

Aphids harm crops in three ways. First, direct feeding.

They suck sap from the phloem, so leaves curl and pucker,

shoots are stunted, and heavy infestations weaken the plant and cut yield.

Second, honeydew. Aphids pass the excess sugar out as a sticky liquid. It coats the leaves below, and black sooty mould grows on it, shading the leaf and spoiling produce such as cotton lint and fruit.

Third, and often the most costly, viruses.

Aphids spread many plant viruses, such as barley yellow dwarf in cereals,

potato virus Y and potato leafroll in potatoes,

and cucumber mosaic in many vegetables.

Some viruses, like potato virus Y and cucumber mosaic, are picked up and passed on in a few seconds of probing.

No insecticide acts that fast, so for these viruses the answer is clean seed, barriers, mineral oil sprays in some crops, and removing infected plants early.

Others, like potato leafroll and barley yellow dwarf, need the aphid to feed for longer, so controlling aphids early does reduce their spread.

Scout at least once a week while the crop is young or flowering, and more often in warm weather. Walk a set pattern, such as a W across the field, and check a fixed number of plants, for example twenty plants in each of five places.

Record the share of plants with aphids, the number per leaf or tiller, and whether you also see natural enemies.

Yellow sticky cards and yellow water traps catch winged aphids as they arrive, and give you early warning. Then compare your counts with the action threshold for your crop and region. Thresholds differ widely.

Seed potatoes, which must stay free of virus, tolerate almost no aphids, while a grain crop late in the season can carry many without loss. If the threshold is not reached, do not spray.

Aphids have a long list of enemies, and on many farms these enemies keep aphids below damaging levels without any help.

Lady beetles, both the adults and their alligator-shaped larvae,

can each eat dozens of aphids a day.

Hoverfly larvae, which look like small green or brown slugs, and lacewing larvae are just as hungry.

Tiny parasitic wasps lay an egg inside an aphid. The aphid swells into a papery, tan or black mummy, and a new wasp cuts its way out. In damp weather, fungal diseases can wipe out whole colonies.

When you scout, count these too. If you find many mummies and lady beetle larvae,

the aphid population is often about to crash on its own. A broad-spectrum spray at that moment kills the enemies as well, and the aphids, which recover faster, can come back worse than before.

This is called resurgence.

Several steps lower aphid pressure before you ever need a spray.

Avoid excess nitrogen, because soft, lush growth is exactly what aphids like. Remove weeds and volunteer plants that carry aphids and viruses from one season to the next. In vegetable nurseries, fine insect netting keeps aphids off seedlings, and reflective mulches confuse incoming winged aphids. Where barley yellow dwarf is a risk, sowing date matters.

In some regions, cereals sown a little later in autumn escape the main flight of winged aphids.

In small plots, a strong jet of water, an insecticidal soap or a horticultural oil will knock colonies back.

Soaps and oils act by contact, so they must hit the aphids, which are mostly on the undersides of leaves.

If your counts pass the threshold, choose an insecticide by its IRAC group, the mode-of-action number on the label.

Products with the same group number act on the same target in the insect, so switching between them is no rotation at all.

Several groups are used against aphids.

Group four includes the neonicotinoids, group four A, often used as seed treatments, and related chemistries such as the sulfoximines, four C, and the butenolides, four D. Because they act on the same receptor, treat all of group four as one family when you rotate.

Group nine acts on the insect's feeding and balance organs, and stops aphids feeding within hours.

Group twenty-three is systemic and moves both up and down the plant, so it reaches aphids hidden in curled leaves.

Group twenty-nine also stops feeding.

The older groups, the organophosphates and carbamates in group one and the pyrethroids in group three A, are mostly broad-spectrum and hard on natural enemies, and the green peach aphid resists them in many regions.

Registrations differ from country to country, and some of these chemistries are restricted in some regions.

Only use products registered for your crop, at the label rate, and respect the pre-harvest interval.

Aphids are among the fastest insects to evolve resistance.

The green peach aphid alone has populations resistant to more than seventy active ingredients, across several IRAC groups, including some neonicotinoids. Four habits slow resistance down.

Spray only when the threshold is reached. Do not use the same group twice in a row on the same generation of aphids; switch group for the next application window. If the seed was treated with a group, do not follow it with a foliar spray of the same group. And get good coverage at the full label rate, because aphids that survive a weak or patchy spray are the start of the next resistant population.

To recap.

Look for the two tailpipes to confirm it is an aphid.

Scout every week, count aphids and their enemies, and use your local threshold.

Remember that some viruses spread too fast for any spray. Protect lady beetles, hoverflies, lacewings and parasitic wasps. Cut excess nitrogen, weeds and volunteer plants.

And when you do spray, choose by IRAC group, and rotate groups, not brands.

For more on aphids, and on every other crop pest, visit laisha.ai. Thank you for watching.