Currant blister aphids

The the new leaves have recently unfurled on a redcurrant I bought over the winter. And the leaves have bright red blisters.

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Discoloured blistering on a young redcurrant leaf

These blisters look like a text book image for currant blister aphids. Sure enough, when I looked at the underside of the leaf, there was a big fat aphid with a brood of babies…

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Redcurrant leaf with blistering and aphids

These bugs must have come in as eggs on the plant. According to the RHS, the aphids need to move to hedge woundwort (Stachys sylvatica) plants to complete their life cycle: they’re uncommon plants around here, so this could be the end of the road for these aphids, depending how far they can travel. The damage the aphids are doing looks impressive, but is really of no consequence.

Honeydew : aphid poo

For the last few weeks, the weather here has been very calm, dry and increasingly warm. Conditions are ideal for aphids and their numbers are booming.

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An aphid on juniper – if you look very closely you can see a drop of honeydew coming out of the aphid’s rear
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This juniper has quite a heavy infestation on the undersides of the branches (but the colour variation is natural – it’s a variegated type).

Sometimes I hear people say that particular trees (especially lime trees) or shrubs create honeydew. And people don’t like that, especially if it drips down onto cars, or if it grows mould and mars ornamental plants. However, it isn’t really (not directly) the trees or shrubs that make honeydew. Honeydew is aphid poo (it can come through a few other bugs too). The aphids pierce through into the vascular system of the plant and the plant’s sap more or less flows out through the aphid – in at one end, a little bit is digested by the aphid, and a slightly thicker stickier ‘honeydew’ continually seeps out of the other end…

After the aphids

Through May and June black aphids were sucking the life out of the little wild cherry trees in the garden (funnily enough, they completely ignored the cultivated varieties).

The cherry aphids infest the underside of young leaves. They will keep on multiplying under the leaves and sucking them until they turn brown and crisp.
The cherry aphids infest the underside of young leaves. They will keep on multiplying under the leaves and sucking them until they turn brown and crisp.

These little suckers are almost certainly the black cherry aphid, Myzus cerasi. Their scientific name is apt as it more or less means a cherry sucker.

It is difficult to watch the destruction these aphids wreak. And it’s hard to believe that they won’t completely kill the trees they infest; especially if they are just young saplings. But, by and large, although these aphids make an unsightly and worrisome mess, cherry trees will recover.

By the end of June or early in July, the aphids will move on to their herbaceous wild plant hosts and leave the cherries with time to recover.

Now, in the third week of July, the aphids have left and the same cherry tree is putting on healthy new growth.
Now, in the third week of July, the aphids have left and the same cherry tree is putting on healthy new growth. The curled and crisped leaves damaged by the aphids are clear to see lower down the stem.

The bad news is that another generation of the aphids will come back in the autumn to lay their eggs on the cherry trees. Those eggs will sit there through the winter, ready to start the whole saga again the following spring.

Lacewings – aphid assassins :)

Lacewings look so fragile and fly so feebly, it’s hard to imagine them as predatory killers. But that’s just what they are: they love to eat aphids. As grubs they suck the life out of aphids and as adults they spend their nights chewing them up.

Lacewing on birch twig
Lacewing (Chrysopa carnea, I think) on birch twig

There are several species of lacewing  in the UK, some are green, some are brown, but all are aphid assassins:)

Lacewing on birch leaf
Lacewing on birch leaf

Extrafloral nectaries. Pardon?

Lots of plants have glands that secrete nectar, the sweet liquid so attractive to bees and other insects. However, these glands aren’t always in the flower. When they’re outside the flower, they’re called extrafloral nectaries.

Ants feeding from the extrafloral nectaries (the red bumps at the leaf base) on a young cherry tree.
Ants feeding from the extrafloral nectaries (the red bumps at the leaf base) on a young cherry tree.

The function of these extrafloral nectaries has been conjectured for years and years, but still no one is really certain what their role is. Back when Charles Darwin was writing On the Origin of Species he noted that:

Certain plants excrete sweet juice, apparently for the sake of eliminating something injurious from the sap. … This juice though small in quantity is greedily sought by insects; but their visits do not in any way benefit the plant.

This remains one of the hypotheses for the role of the extrafloral nectaries – that they are helping with them elimination of undesirable compounds.

The other hypothesis, which is more popular these days, is that the extrafloral nectaries attract beneficial insects which help in defending the plant from “pests”.

I have to say, in my experience the glands attract mainly ants who, as Darwin noted, feed greedily from them. Meanwhile the aphids still come and begin feeding on the plant leaves. Then the ants, which are there in abundance having been dining at the extrafloral restaurants, set up their aphid farms, defending the aphids from would be predators like ladybirds. The ants then live a particularly happy life, lounging drunkenly around the extrafloral nectaries and milking their aphids.

New growth on a young cherry heavily infested with black aphids which are being farmed by ants
New growth on a young cherry which is heavily infested with black aphids (causing characteristic leaf curl) which are being farmed by ants

There must be more to this than meets the eye though, otherwise it would be a bit of a suicidal strategy.

Sometimes it’s tough being a tree…

This leaf is heavily infested with gall mites. But wait until you see how bad things are underneath...
This leaf is heavily infested with gall mites. But wait until you see how bad things are underneath…
Underside of sycamore leaf with felt galls and aphid infestation.
Underside of sycamore leaf with felt galls and aphid infestation.

The red galls on the top of the leaf are caused by the leaf’s reaction to damage inflicted by gall mites feeding. In response to the grazing, the leaf makes a little dome (the gall) and later on the mite lays its eggs inside that gall, where the larvae will hatch out and continue feeding and growing, perpetuating the infestation.

The white fluffy looking coating between the major veins on the underside of the leaf is also caused by the leaf reacting to a gall mite. This time, it is a felt gall mite. Apparently a chemical in the mites’ saliva somehow stimulates the tree to produce lots of tiny hairs: which are what the fluffy stuff actually is – tiny little hairs. Within the hairs the mites can feed and breed with a degree of protection.

Perhaps surprisingly, the galls will have very little impact on the tree’s overall health and growth. Both the felt and the red blister galls are caused by Aceria mites.

As well as the galls, this leaf is host to a huge number of aphids at various stages of growth: the little creamy egg shapes are very tiny aphids, then there are all sizes up to fully grown adults, plus there are the ghosts – the white skins that have been shed as the aphids grew.

It seems tough for this tree, but it should be fine in the long run:)

Pine aphids

This young pine tree has a light infestation of pine aphids (Cinara spp – possibly Cinara confinis). The aphids are being “milked” by ants: basically, the ants are looking after the aphids and eating their excrement or “honeydew” to put it more politely!

Pine aphids (Cinara spp) clustering round the pine needle bases
Pine aphids (Cinara spp) clustering round the pine needle bases
Pine aphids with ants
Pine aphids with ants
It's hard to pull one of the aphids off without squishing it, but here's one so you can better see what it looks like.
It’s hard to pull one of the aphids off without squishing it, but here’s one so you can better see what it looks like. The aphids on the shoot are at various stages in their growth cycles, this is one of the larger ones.

At this low level of infestation (the aphids were only present on a couple of shoots throughout the whole tree), there isn’t a particular problem. The aphids haven’t been shown to spread any viruses, unlike many other aphids, and they provide a food source for many beneficial insects, like ladybird and hover fly larvae.