Sycamore Aphids

Recently there has been a lively exchange of comments in the Guardian on a piece by George Monbiot that extols the virtues of native trees because they support insect biodiversity (birch hosts no less that 266 different invertebrates, for example) and deplores the tendency of local authorities to plant non-native trees that are of relatively little value to our native fauna.

One of the most frequently cited examples of a non-native tree with a very limited insect fauna is sycamore, which hosts a paltry 15 species. It's a long-established non-native species with a  documented history here that dates back to the 16th. century, but since then only a small number of our insects have adapted to it. But what it lacks in biodiversity, it certainly makes up for in the sheer quantity of one particular insect that lives on its leaves - the sycamore aphid.

Over the summer these little insects multiply in vast numbers under its leaves - and they produce a lot of honeydew, so woe betide anyone who parks a car under the tree; it will be as sticky as a toffee apple if it's left there for long. 

The positive aspect of this sticky secretion is that the mildew that grows on the sap that the aphids excrete is an important food source for the orange ladybird Halyzia sedecimguttata, whose distribution is linked to that of sycamore.

All these aphids are certainly of real value to some of our native birds - I've often watched blackcaps and blue tits picking them off the leaves. 

One of the most striking features of the sycamore aphid is the way in which the individuals space themselves evenly under the leaf. the spacing is such that they are just close enough together to touch each other with their long antennae, so if an individual in one part of the leaf is attacked the alarm spreads from aphid to aphid in a wave of antennae-waving across the whole leaf.

The annual sycamore aphid population explosion has just about ground to a halt now and soon they'll fly to sites on twigs and buds where they'll lay overwintering eggs. They hatch in spring just as the buds burst, with perfect timing to plug their stylets into the fresh young foliage as it emerges - and that's the time when you'll often see blue tits and great tits picking the aphids off the bud scales.

The Insect that Turns Cars into Toffees

Take a close look at sycamore Acer pseudoplatanus buds at this time of year, just as they are beginning to swell, and you'll probably find some of these tiny insects huddled in groups around the edges of the bud scales. They're sycamore aphids Drepanosiphum platanoidis and they will have recently hatched from minute eggs that have survived the winter on the surface of the bark, just below the bud. It's often claimed that hard winters tend to reduce pest numbers, but that doesn't seem to be the case with this insect - buds on sycamores that I looked at today had a large pupulation of these aphids.

As soon as the tender new leaves begin to burst out of the bud scales the aphids will crawl onto them and begin to feed and breed - very rapidly. At this stage of their life cycle the aphids clone themselves, giving birth to live young that already have the next genetation developing inside them when they are born.

This is what the aphids look like at the moment, magnified about forty times. They cluster together with their antennae touching those of neigbouring insects, so any threats are transmitted through the colony very rapidly.

And this is what they do if they are threatened- all stick their tails up in the air. I'm guessing that they must be emitting something that might repel insect predators....

Once the sycamore leaf expands and begins to photosynthesise a winged generation of aphids develops. Later in the season these can migrate from tree to tree. Notice how most of them are lined up along the leaf veins. Their stylets, like hypodermic syringes, are inserted in the phloem inside the veins that transports the sugars, made by photosynthesis, to other parts of the plant. The sugary solution is rich in energy but very poor in other essential nutrients that the insects need for growth and reproduction, so they need to consume vast quantities of it, most of which is emitted from the tail end of the insect as sticky honeydew - and if you park your car under a sycamore on a hot summer's day when it's raining honeydew it will have the tactile qualities of a toffee when you return.