Understanding aphids

We all know an aphid when we see one… Except you almost never see just one. At some stage in almost every tree crop season, they arrive like tourists after the COVID borders are opened.

No magnifying glass is required; you recognise them as a crowd encrusting the growing tips and, as you take a breath to say a swearword, you know your potentially perfect crop is jeopardised.

Identification

They get to about 4mm, their jointed antennae extend forward more than half the length of the body, they have piercing and sucking mouthparts and at the rear, two cornicles, like a pair of guns pointing upwards/backwards. When adults have wings, they are both membranous, forewing much larger than the hindwing.

Life cycle

How come this population eruption happens so quickly? Tick the usual factors: warmth, humidity, succulent new shoots to literally get stuck into – and reproduction without sex to save time. Sex is sort of optional but is generally required to lay eggs which are the usual overwintering stage in temperate zones of Australia… Less so in the tropics.

There are many aphid species. Green peach aphids have been listed as pests on hundreds of plants; most aphid species have varied taste preferences beyond the common name they carry – such as rose aphid, cabbage aphid, black bean aphid and other you-name-the-plant aphids.

The brief message from me to you is this: No matter which aphid arrives on your crop, it has probably come from surrounding plants/weeds, (but sometimes overwintering in the bark of your trees) and, you should do something about it no matter what it’s called.

Overwintering eggs on nearby weeds/pastures hatch in early spring when those plants get new growth earlier than the trees (especially deciduous trees). These hatching nymphs get to be adults in about two weeks. The first adults are usually wingless and mostly females which produce living young.

The aphid population is soon overcrowding the weed plant. The ‘mood’ changes and adults develop wings, look lasciviously at each other… And any nearby trees that are shooting/flowering and offer the bonus of amino acids coursing through the phloem. (Aphids prefer amino acid-laced sap).

Once airborne, they can be carried by the breeze, find your tree crop and trouble begins. Migration between host plants is more likely to be measured in metres rather than big kilometres.

Your mission is to be monitoring the nearby weed growth a couple of times a week, getting your neighbour to do likewise so you can make mayhem on weeds and aphids before they transfer their attention to your trees.

Once aphids arrive, they go back to reproduction by parthenogenesis (no sex) by wingless, mostly female adults. Up to 10–20 generations a year is possible, more in the tropics where winter is not an issue. Each time aphids on a tree become overcrowded, they start producing winged adults which will fly to nearby trees until most trees are under attack.

As the summer progresses and the aphid-friendly shoots are fewer, tougher and unappetising, the aphids you haven’t killed develop wings and leave for a new fast-growing crop… Maybe your vegetables or nearby weeds.

Damage

With their mouthparts inserted into the sapstream they hardly need to suck. The plant phloem (bark) is loaded with sugars after photosynthesis in the leaves and the aphids can’t handle all those carbs. What is called honeydew is excreted out through those cornicles. Ants just love it. Bees like it too.

It’s almost like a gangland protection racket; the aphid excretes payment so the ants don’t eat them… May even protect them from various predatory insects. The sugary ‘spillage’ ferments and darkens into what is called sooty mould and, if it settles on your fruit, the market won’t buy it – or pays you less.

Viruses – very tiny and easily transmissible – are also transmitted by aphids directly into the sapstream.

Then there’s the physical damage to your fruit. A little insertion mark is not such a big deal on leaves but marked fruit is marked down. Then there’s the attrition rate when aphids partying on buds and pea-sized fruit. Too many suckers and the bud or fruit falls off, means less yield.

Control

Many info sources have a tendency to demean the value of insecticides. They advise use of beneficial insects and weed management and infer that because of the short lifecycle and many generations in a year, aphids tend to build up resistance due to the same insecticide being used, leaving only the more resistant ones to breed.

That may have been the situation when the options were either organophosphates or synthetic pyrethroids. Even then, thinking growers could alternate their spray applications to cope with insect resistance. Now we have more insecticidal groups available with different actions on the pathways insects use to de-toxify them.

There are some new systemic insecticides; reading and following their labels needs concentration, but hey! In the ‘Directions For Use’ table you can readily find if your tree crop is listed, see if aphids are included, then check the application rate/timing and the withholding period.

In principle, systemics are usually applied early, after leaves are open and able to absorb the chemical. It then circulates and kills insects feeding on the plant. A bonus. You can simultaneously kill scales, mealybugs and larvae which have hatched from eggs laid into the plant, as well as aphids, various bugs, leaf-eating beetles, caterpillars, etc.

You can often apply a follow-up ‘dose’ as long as you don’t get too close to harvest and infringe the withholding period.

A rampant pest population is almost always beyond the capability of beneficials to bring under control – simply because they cannot match their population growth rate. This means the beneficials should be introduced soon after they have pests to feed on.

There’s also the choice to use a contact insecticide such as pyrethrum spray with a 1-day withholding period that kills immediately without harmful residues.

Beneficials can be introduced soon after because you really won’t kill every last pest if the spray doesn’t directly contact them; these survivors will provide some sustenance for the beneficials and the beneficials should then be able to maintain control.

Written by Ion Staunton - entomologist at Pestech.com.au – maker of PyBo Natural Pyrethrum Insecticidal Concentrate. Send questions by text +61 407 308867. Email: ion12@me.com

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