Birth and death on a birch tree

On Saturday 9th July I spent some time in my garden looking at an aspen tree (Populus tremula) I planted there about 10 years ago. As I was doing so, I noticed little spots of liquid on some of the leaves that I recognised as being the honeydew that is secreted by aphids when they feed on the tree’s sap. This prompted me to start looking at the leaves higher up on the tree, in search of the aphids themselves. Almost immediately, I found some large aphids on a few of the aspen’s leaves, but they didn’t look like the aphid species that I’m familiar with that feed on aspen.

This only puzzled me for a few moments though, because the aspen is growing right next to a silver birch (Betula pendula), and when I turned slightly, I brushed against the leaves of the birch and at least half a dozen aphids fell on to my clothes. These were winged aphids and I recognised them from aphid surveys we’ve had done at Dundreggan as being in the genus Euceraphis, which feed on birch trees. I sent a few specimens to Ed Baker, a specialist who carried out the first aphid survey at Dundreggan in 2012, and he confirmed them as being the silver birch aphid (Euceraphis betulae). Ed has written an excellent overview about the ecology of aphids, which can be found here.

Because so many of the aphids had fallen on to me, I looked at the birch tree more closely and found that it was covered in aphids – they were literally falling out of the tree as I explored it and turned over the leaves. Thus began an exciting saga of discovery and photography for me, that continued for the following two weeks, and has resulted in this blog. As regular readers will know, I’ve had an ongoing and developing interest in aphids for several years, and this remarkable group of insects have featured in several previous blogs – some examples can be read here, here and here.

As I looked at the tree, the sheer numbers of aphids on it were almost overwhelming. In addition to the large silver birch aphids (Euceraphis betulae), there were other smaller aphids on the leaves. These were bright green in colour, and often clustered together in dense groups. They were the small downy birch aphid (Betulaphis quadrituberculata), which as its name suggests is mainly found on downy birch (Betula pubescens), but also occurs on silver birch. While some leaves had one or other of these two species on them, a number of leaves had both together, with there sometimes being a dozen or more aphids on a  single leaf.

In addition to the living aphids, I soon began finding dead ones as well, which is very common in large aphid colonies, as aphids have a lot of parasitoids and predators. On the leaves of this birch tree, there were quite a lot of what are called aphid mummies. A mummy is an aphid that has been parasitised by a tiny wasp, resulting in a distinctive change in the aphid’s appearance and its death. The wasp lays an egg inside the aphid, and when the wasp larva hatches, it begins eating the aphid from the inside.

The aphid’s body swells up in size to accommodate the growing larva inside, and becomes desiccated and brown in colour, as the wasp larva literally sucks the juice out of it.  In the case of the first mummies I found, of silver birch aphids, the wasp larva cuts a hole in the bottom of the aphid and attaches it to the leaf with silk, forming a distinctive silver disc. This gives away the wasp’s identity as being a species called Praon flavinode, which is a common parasitoid of both the silver birch aphid and its close relative, the downy birch aphid (Euceraphis punctipennis).

When the wasp larva is fully grown, it pupates and the resultant adult wasp makes an exit hole in the aphid mummy, flying off to mate, so that the female can lay an egg in another aphid and thus the life cycle repeats itself. I collected some of these aphid mummies to send to Ed, as he has a particular interest in aphid parasitoids. I also also found some other aphid mummies, of the small downy birch aphid. These were light brown in colour, and lacked the distinctive disc-shaped cocoon of the other ones. These had probably been parasitised by a different wasp (Aphidius aquilus), and I sent some samples of these to Ed as well, for rearing, so that he can confirm the identity of the parasitoid.

I continued turning over some of the birch tree’s leaves, looking for more aphids, and on the underside of one leaf I found a blackish insect that I recognised as being the larva of a beetle – and probably of a ladybird, which are specialist predators of aphids. A quick search on the Internet later suggested to me that it was the larva of a two-spot ladybird (Adalia bipunctata), a common species which occurs throughout Europe and also in North America.

The general shape of the larva and its distinctive pattern of yellow spots are the key distinguishing features, and my provisional identification was subsequently confirmed by Sholto Holdsworth, a specialist who helps me with beetle IDs. Discovering this larva was very interesting for me, as I suspected there would be more of them on the tree, taking advantage of the abundance of aphids. Although I’ve been fascinated by aphids for several years now, I’d not seen any ladybird larvae actually predating on them before, so I was quite excited when I found another of the same larvae in the process of making a meal of a hapless small downy birch aphid.

Aphids are very vulnerable to predators such as ladybird larvae because of their method of feeding. Aphids suck the sap of trees and plants, and have a specialised proboscis called a rostrum that they insert into the plant or tree to access the sap. This means that they are literally fixed to their feeding spot, and when a predator appears they are effectively ‘sitting ducks’, being unable to free themselves quickly enough to avoid being eaten.

I took a series of photos of this larva eating an aphid, and they were especially symbolic of the enemies that aphids have, because there was a mummy of an aphid right beside the one being eaten. Several other small downy birch aphids were feeding there as well, seemingly oblivious to the risk that they might become the next meal for the ladybird larva. On another leaf, I found a silver birch aphid that had met a different fate. Its wing had seemingly got stuck to the larval case of a micro-moth (Coleophora serratella) and, unable to escape, it would die there.

As I continued looking at different leaves on the birch tree I made another interesting discovery. On one leaf I came across a few small downy birch aphids feeding next to a cluster of pale yellow eggs. My immediate intuition was that these must be eggs of the two-spot ladybird, so I photographed them and then later compared the photo with images on the Internet of ladybird eggs – it looked like I was correct. I made a careful note of where this leaf was on the tree, so that I could check on it over the next few days.

For several days the eggs didn’t seem to change at all, but in the early evening of the 14th of July, five days after I had found the eggs, I was excited to discover that they were a different colour, and that two of them had begun to hatch. The eggs are tiny, so I had to use my macro lens at its highest magnification, five times life size, to take the photograph here. Looking like a scene from Ridley Scott’s science fiction film, ‘Alien’, these minute, transparent larvae were struggling to emerge from their eggs.

I watched the larvae for quite a while, and although two of them had the upper portion of their bodies out of their eggs, they seemed unable to free the other half easily. Their legs were able to move, but couldn’t reach the egg case to be used as levers. I had hoped to see one of the larva being fully born, but when it got dark, none of them had freed themselves. The remaining eggs had become transparent, and by looking closely I could see the shapes of the larvae still fully contained in them – again this reminded me of a scene from ‘Alien’.

In between photographing the eggs and the partially-emerged larvae, I looked at some other nearby leaves on the tree, and the small downy birch aphids in particular were very abundant – ready prey for the ladybird larvae.

 

 

 

 

 

 

 

The next morning, 15th July, I got up early and spent some time looking at the leaf before I went to work. There had been a big change overnight, as all the larvae were partially emerged from their eggs now. None of them had freed themselves fully yet,  but those that had been first to emerge had turned black in colour, and contrasted with the more recently emerged ones, which were still a pale green transparent colour. Presumably the larvae need time for their exoskeletons to harden, signified by the darkening of the colour, before they can become mobile. Interestingly enough, according to information I found on the Internet, most ladybird larval hatching from eggs takes place at night. Perhaps this is an adaptation to reduce the risk of predation, as fewer predators will be able to find the emerging larvae in the darkness, when they are still unable to move.

As I looked at this cluster of partially-emerged larvae I couldn’t help but think again about the ‘Alien’ film, and I realised that the makers of that had done quite a good job of basing their creature on the biology of insects like this!

 

 

 

 

 

 

When I returned home late in the afternoon, there was another change in the larvae. Some of them were clearly now free from their egg cases, and had visibly moved around on the cluster of larvae. As had now become usual for me,  I watched them for a little, and as I looked from different angles, I saw that one of them was eating an aphid.

 

 

 

 

 

 

 

 

 

I suspect that this was the aphid that had been feeding on the leaf right next to the eggs when the larvae had hatched out, and it had been picked off by one of the first ones to emerge fully.

On a nearby leaf, I saw an aphid that had met a different fate. It was a winged adult silver birch aphid and was almost completely covered by a brown fungal growth. I was unable to tell whether the fungus had killed the aphid, or if it had just grown over the aphid after it had died from a different cause. In either case, it was another example of the fact that many aphids die prematurely, and in doing so they sustain a wealth of other organisms, such as this fungus.

On the next day, Saturday 16th July, it was exactly a week from when I had first found the ladybird eggs, and by that morning only a few of the larvae were still there. Some had obviously already dispersed, and those that remained were  all completely black in colour, and looked like they had grown somewhat overnight. I’ve read that ladybird larvae can exhibit cannibalistic behaviour, but I saw no evidence of that having occurred here. The shrivelled, empty egg cases were very visible in the middle of the cluster, and it may be that the larvae had been feeding on them after they hatched.

As I watched, some of the remaining larvae began to move away from their birthplace and spread out over the leaf the egg cases were on. The aphids that had been there when I first found the eggs were all gone – I had seen one being eaten, and I assume that the newly-emerged larvae ate the others as well. I looked at some nearby leaves on the same twig of the birch tree, and I found a solitary young larva crawling around on one. There were no aphids on that leaf though, so the larva would have to look elsewhere for a meal.

On another different leaf, just a few centimetres away from the one with the eggs on it, I spotted a couple of the newly-emerged larvae beginning to feed on a small downy birch aphid. At about 3 mm. long, the larvae were of a similar size to the tiny aphid, but would grow quickly as they fed. Ladybird larvae have been reported as eating up to 100 aphids per day, but that is likely to be when the larvae are almost fully grown.

Like many insects, the ladybird larvae go through a number of stages of growth, called instars, which are separated by moults, during which the larva sheds the exoskeleton it has outgrown. The larvae have been described as looking like miniature crocodiles or alligators, because of their overall shape and the pattern of spikes on their bodies. Given the reputation of those large reptiles as fearsome predators, this comparison is perhaps quite fitting, as ladybird larvae are voracious in their predation of aphids. This has led to them being sold as biological control agents for aphid infestations in commercial plant nurseries etc.

As I continued to look at the birch tree I found another fully grown larva of the two-spot ladybird that was eating a small downy birch aphid. The comparison in size with the newly-emerged ones was quite remarkable, and I could see how this larger larva could easily eat a lot of aphids in a single day. Despite that, and the effects of all the other ladybird larvae on this birch tree, there were still plenty of aphids on many of the leaves. Aphids are able to reproduce and multiply in numbers very quickly, and are obviously able to withstand the high levels of predation they experience.

In one stage of their lifecycle, female aphids are able to reproduce without mating with males. They also give birch to live young aphids at this stage, instead of laying eggs, and the newborn aphids each already have the developing embryo of another aphid inside them. This is called telescoping of generations, and this combination of reproductive traits enables aphid numbers to build up very quickly. The birth of each ladybird larva may well spell the death of a considerable number of aphids, but the birth of more aphids continues throughout the summer, and is part of the overall cycle of life and death that occurs in  Nature.

Although aphids are given very little consideration by humans, and when they are noticed it is usually as garden ‘pests’, they are a vital and fundamental part of the food webs in Nature. By sucking the sap of trees and plants they make the nutrients contained within those  available to other organisms, both through the aphids becoming food for ladybirds, midge larvae and hoverfly larvae, and also via the number of other creatures that consume the honeydew that aphids secrete as a waste product. This includes various ant species that tend aphids, and also insects such as hoverflies, wasps and flies that consume the honeydew that is left on leaves, and which had first alerted me to the presence of these aphids, as I described at the start of this blog.

I continued to check on the birch tree for several more days, and on the 18th of July, 9 days after I had found the ladybird eggs, and three days after the larvae had started to hatch, the only reminder of their presence on the leaf was the shrivelled up remains of the egg cases. The larvae must have all dispersed over the tree, as there were plenty of aphids for them to feed on. Most of that would be taking place where I couldn’t see it, as the tree is about 8 metres tall, and it’s only the comparatively few leaves on the lower branches that I can reach.

On one of those days though I came across another different example of predation on aphids – I found the bright orange larva of a midge (Aphidoletes aphidimyza) feeding on a small downy birch aphid.

 

 

 

 

 

 

 

These midge larvae are surprisingly mobile and agile, given that they don’t have any legs, and they also take advantage of the aphids’ relative immobility when they are feeding. The midge larvae do not actually eat the aphids, but instead pierce an aphid’s exoskeleton and suck out its liquid interior, leaving a wrinkled, deflated-looking aphid corpse behind. On another leaf, I found an example of the other main group of aphid predators – the larva of a hoverfly (Syrphus ribesii). This is a very common species, and the adults feed on the nectar of flowers in summer.

From the simple observation of some drops of honeydew on my aspen tree on 9th July, this has turned into a major journey of discovery for me, and it has also become something of a marathon blog! Thanks for staying with it if you’ve made it this far, and I hope you’ve found it as fascinating as me. Perhaps the next time you see aphids, or their predators, you’ll look at them with different eyes. To close with, here’s a sequence of photos, cropped and rotated to give the same perspective in each one, of the development of the ladybird larvae from their eggs over a period of 9 days.