Leaf rolling

Rosy Leaf-curling Apple Aphid (Dysaphis devecta agg.)

These eye-catching leaves with their bright red rolled edges caught my eye when I walked past our apple tree the other day. They're evidence of the presence of the Rosy Leaf-curling Apple Aphid (Dysaphis devecta agg.), a minor pest which attacks the leaves of various apple species. There are actually three closely related aphid species which make up the Dysaphis devecta group. All look similar, but one lives only on apple trees while the other two alternate between apple and Cow Parsley or Rough Chervil. The aphids feed on the tiny veins in the leaves, and the longitudinal red galls are formed in response to substances found in the aphid's saliva. Fortunately, the aphids don't do much harm to the tree itself. Here in our garden, we have plenty of natural predators to help keep them under control, including Eurasian Blue Tits, various ladybirds, some species of hoverfly larvae, and several types of parasitic wasp. It's all part of the circle of life!

Mike's picture of the aphids themselves

Mugwort suckers

Mugwort Aphid (Macrosiphoniella artemisiae)

One of the fun things about this project has been learning just how little I knew about a lot of things before we started. For instance, I had no idea that there were so many species of aphid in the world – almost 600 in the UK alone! Many are extraordinarily hard to identify, though knowing the plant on which they're found can help. But some can be figured out, particularly with the help of awesome websites like the one developed by Influential Points. I found these aphids on a Mugwort plant (Artemesia vulgaris) the other day. They are, predictably enough, Mugwort Aphids (Macrosiphoniella artemisiae). Greenish-grey and covered with a waxy powder, they have black legs, antennae, siphunculi (the two little spikes on their backs) and cauda (the tail-like spike on their back end). Their front femurs (the part of their leg closest to their bodies) are brown; I had to look with a magnifying lens to see that! Their bright red eyes were certainly a surprise. Like all aphids, these are pretty small, measuring only 2-3.5 mm (less than 1/8 of an inch). They're found on a variety of Artemesia species, though we've only found them on Mugwort (so far, anyway) in our garden. Because Mugwort blooms from July through September, that's reputedly the time when the aphid's numbers peak. They're typically found right near the top of the plant, among the flowers. Like all aphids, they excrete "honeydew", but that honeydew doesn't contain melezitose, a sugar that is particularly attractive to ants, so the species isn't ant-tended. The ladybirds will be happy to find them though.

Artemesia aphids

Absinthe Aphids (Macrosiphoniella absinthii) on Shrubby Wormwood (Artemesia arborescens)

When I looked out the office window the other afternoon, I noticed a dark smudge at the top of one of the Artemesia plants in the herb garden. Intrigued, I ventured out for a closer look and found these colourful aphids (photographed by Mike). A sniff around the internet led me to the amazing Aphids ID website run by the folks at Influential Points. This site packs a serious punch. It has detailed ID information for hundreds of species, with some great pictures as well. Sifting through the plant list (you can search for potential aphids IDs by checking the plants on which they're feeding) quickly led us to our list's latest addition — the Absinthe Aphid (Macrosiphoniella absinthii). Its one of only 25 species of aphid that Carl Linneaus himself named, in the 10th edition of his seminal work, Systema Naturae. As is suggested by its name, it's typically found on Absinthe plants. We don't have Absinthe in the garden, but we do have a close relative — Tree Wormwood (Artemesia arborescens). And that's where these little creatures have made themselves at home. They feed only on the upper parts of various Artemesia species. All of the insects you see in the picture are female. Like other aphids, this one typically reproduces parthenogenically. For most of the year, females give birth to only to other females, which are clones of their mother. And those newly-born females are already pregnant with clones of their own. In the autumn, some males will also be produced, and these will then reproduce sexually with some of the females. This allows for a bit of gene mixing (provided, of course, that they're not just mating with their sisters).

These little aphids are unexpectedly colourful; many aphids solidly green or brown or grey. The bold pattern may help to make the aphids less obvious to passing predators by helping to break up their outline. Alternatively, it may aposematic, indicating to potential bird predators that the aphids are toxic; the compound thujone in Artemesia plants render the aphids inedible to birds. The two little spikes sticking up from the aphids' back ends are called siphunculi (or cornicles); they're tubes that exude a defensive fluid if the aphid is threatened. The pointy "tail" is known as a cauda; the aphid uses it to flick away its excrement (also known as honeydew because of its high sugar content). Unlike some aphid species, this one is not guarded by ants. This species naturally occurs from northern and central Europe, north Africa and the Mediterranean region east to Siberia and Iran, and has been accidentally introduced to the US, Canada, and Argentina. Our record appears to be the first for East Anglia — how exciting! Of course, it could also just mean that most people aren't looking...