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Aphids

There are about 4000 species world wide, and 500 species in Britain, but only about 250 species worldwide are serious plant pests.

Aphid digestion

I think this is the really interesting thing about aphids. They feed on the phloem sap of plants. This liquid is very sugar-rich, (think of maple syrup), has a high water content, but is low in nitrogen. So the bug must eat large quantities to get sufficient nitrogen.

The gut is modified so that the excess water and sugar can quickly pass from foregut to hindgut then rectum, bypassing the midgut (see right which shows the sap feeding hemipteran digestive tract). The midgut is where the nitrogen and the amino acids are absorbed. This means that the excreted liquid is very sweet, and it is sometimes called honeydew. It tastes quite nice, but soon goes mouldy as you find out if you park your car uder an aphid-infested tree.

Some aphids can excrete as many as seven droplets of this sugar-rich liquid an hour - that can be as much as 133% of the insect's weight! And some hemipterans consume more than 100 times their body weight per day.

 Hemipteran (aphid) digestive tract showing the passage of sap through the aphid gut

Ants farming aphids

Sometimes the honeydew is in quantities large enough to be used by man. In the Old Testament the manna given to the Israelites was probably anal excretions of Trabutina mannipara, which feeds on the tamarisk. The Arabs still collect it today, and call it "man". The Australian aborigines also collect honeydew.

Ants farm aphids

The photograph on the left shows ants guarding aphids whie the aphids suck sap from a rose bush. The aphids get too much sugar from the sap and excrete it (this liquid is called honeydew) and the ants lap it up, drinking some themselves, and taking the rest home for their nestmates and grubs.

The tip of every leaf was guarded by at least one ant, and other ants patrolled over the aphids, and up and down the stems. Any other insect, or even a camera strap would be nipped and squirted with formic acid. So the aphids are free to suck in peace and the ant gets a sugary reward. And the rose bush? Well it was quite a big bush and a short time later it had many fragrant blooms.

When there are no ants around to collect the honeydew the aphids flick or squirt the droplets away.

On the right is a drawing showing the position of the stylets as it enters the plant cell wall to suck sap.

Aphid parthenogenisis

Aphids are also prodigious breeders. Females can give birth without mating (parthenogenesis). During summer the young are born live, and one female can give birth to numerous females in a day. Theoretically a single female aphid could produce over 3 million descendants in a single year.

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Some aphids have wings and others are wingless, see the drawings below.

the stylets of a hemiptera (bug)

greenfly, aphid

wingless aphid

 Aphid winged

Rhopalosiphoninus latysiphon - the bulb and potato aphid

Rhopalosiphoninus latysiphon - the bulb and potato aphid

On the left is Rhopalosiphoninus latysiphon - the bulb and potato aphid. As its name implies it is commonly found underground - note how its antennae are flexed backwards to ease movement in the soil.

White flies

Whiteflies belong to the Aleyrodidae family. They look like minute white moths from 1 - 3 mm long. Their wings are covered in a waxy white powder, and their antennae have 7 segments.

They suck from the undersides of leaves, and exude a large amount of honeydew. And often it is this sticky honeydew that is the first sign of whitefly that the gardener notices when there is a whitefly infestation. The Cabbage Whitefly, Aleyrodes brassicae, attacks cabbages and related plants. The Greenhouse Whitefly, Trialeurodates vaporariorum, attacks cucumbers, tomatoes and many other plants.

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