Tag Archives: sycamore

Satiable curiosity – side projects are they worthwhile?

I’ve been meaning to write this one for quite a while.  It was stimulated by two posts, one from the incredibly prolific Steve Heard, the other by the not quite so prolific, but equally interesting,  Manu Saunders.  First off, what is a side project?  To me, a side project is one that is not directly funded by a research council or other funding agency or, in some cases, one that you do in your spare time, or to the horror of some line-managers, is not strictly in your job description 🙂 The tyranny of modern research funding dictates that projects must have specific research questions and be accompanied by hypotheses and very specific predictions; most proposals I referee, even contain graphs with predicted results and almost all have ‘preliminary data’ to support their applications.   This is not necessarily a bad thing but to directly quote Manu Saunders from her blog post

“Whittaker’s (1952) study of ‘summer foliage insect communities in the Great Smoky Mountains’ is considered one of the pioneer studies of modern community ecology methods. The very short Introduction starts with the sentence “The study was designed to sample foliage insects in a series of natural communities and to obtain results of ecological significance from the samples.” No “specific research questions” and the hypotheses and predictions don’t appear until the Discussion” Sounds like bliss.

The central ethos of my research career which began in 1977, can be summed up by this quotation uttered by the character ‘Doc’ in John Steinbeck’s novel Sweet Thursday “I want take everything I’ve seen and thought and learned and reduce them and relate them and refine them until I have something of meaning, something of use” (Steinbeck, 1954).* The other thing that has driven me for as long as I can remember, and why I ended up where I am,  is something I share with Rudyard Kipling’s Elephant Child, and that is a “satiable curiosity”:-) Something that has always frustrated me, is that, in the UK at least, most funded research tends to be of a very short duration, usually three years, often less than that**, and if you are very lucky, maybe five years.  If you work on real life field populations, even if you work on aphids, these short term projects are not really very useful; laboratory work is of course a different matter.

I got my first ‘permanent’ job in 1982 working for UK Forestry Commission Research based at their Northern Research Station (NRS) just outside Edinburgh.  My remit initially was to work on the pine beauty moth, Panolis flammea and finally, on the large pine weevil, Hylobius abietis.  As a committed aphidophile, I was determined, job description or not, to carry on working with aphids. I decided that the easiest and most useful thing to do was to set up a long-term field study and follow aphid populations throughout the year.  My PhD was on the bird cherry-oat aphid, Rhopalosiphum padi, a host alternating aphid, the primary host of which is the bird cherry, Prunus padus, with which  Scotland is very well supplied, and fortuitously, just down the road from NRS was Roslin Glen Nature Reserve with a nice healthy population of bird  cherry trees.  I chose ten suitable trees and started what was to become a ten-year once a week, lunch time counting and recording marathon.  I also decided to repeat a study that my PhD supervisor, Tony Dixon had done, record the populations of the sycamore aphid, Drepanosiphum platanoidis.  In the grounds of NRS were five adjacent sycamore tree, Acer pseudoplatanus, and these became my early morning study subjects, also once a week. I had no articulated hypotheses, my only aim was to count and record numbers and life stages and anything else I might see. Anathema to research councils but exactly what Darwin did 🙂

Although it was a ‘permanent’ job, after ten years I moved to Imperial College at Silwood Park and immediately set up a new, improved version of my sycamore study, this time a once weekly early morning*** walk of 52 trees in three transects and with much more data collection involved, not just the aphids, their natural enemies and anything else I found and on top of all that, the trees themselves came in for scrutiny, phenology, growth, flowering and fruiting, all went into my data sheets.  I also set up a bird cherry plot, this time with some hypotheses articulated 🙂

As a result of my weekly walk along my sycamore transects, a few years later I set up yet another side project, this time an experimental cum observational study looking at tree seedling survival and colonisation underneath different tree canopies. At about the same time, initially designed as a pedagogical exercise, I started my study of the biodiversity of Bracknell roundabouts.

One might argue that most undergraduate and MSc research projects could also come under the heading of side projects, but I think that unless they were part of a long term study they aren’t quite the same thing, even though some of them were published.  So, the burning question, apart from the benefits of regular exercise, was the investment of my time and that of my student helpers and co-researchers worth it scientifically?

Side project 1.  Sycamore aphids at the Northern Research Station, 1982-1992

I collected a lot of aphid data, most of which remains, along with the data from Side project 2, in these two notebooks, waiting to be entered into a spreadsheet.  I also collected some limited natural enemy data, presence of aphid mummies and numbers killed by entomopathogenic fungi.  Tree phenological data is limited to bud burst and leaf fall and as I only sampled five trees I’m not sure that this will ever amount to much, apart from perhaps appearing in my blog or as part of a book.  Nothing has as yet made it into print, so a nil return on investment.

Raw data – anyone wanting to help input into a spreadsheet, let me know 🙂 Also includes the data for Side project 2

 

Side project 2.  Rhopalosiphum padi on Prunus padus at Roslin Glen Nature Reserve 1982-1992

I was a lot more ambitious with this project, collecting lots of aphid and natural enemy data and also a lot more tree phenology data, plus noting the presence and counting the numbers of other herbivores.  I have got some of this, peak populations and egg counts in a spreadsheet and some of it has made it to the outside world (Leather, 1986, 1993: Ward et al., 1998).  According to Google Scholar, Ward et al., is my 6th most cited output with, at the time of writing, 127 citations, Leather (1993) is also doing quite well with 56 citations, while Leather (1986) is much further down the list with a mere 38 citations.  I have still not given up hope of publishing some of the other aphid data.  I mentioned that I also recorded the other herbivores I found, one was a new record for bird cherry (Leather, 1989), the other, the result of a nice student project on the bird cherry ermine moth (Leather & MacKenzie, 1994).  I would, I think, be justified in counting this side project as being worthwhile, despite the fact that I started it with no clear hypotheses and the only aim to count what was there.

 

Side project 3.  Everything you wanted to know about sycamores but were afraid to ask 1992-2012

As side projects go this was pretty massive.  Once a week for twenty years, me and on some occasions, an undergraduate research intern, walked along three transects of 52 sycamore trees, recording everything that we could see and count and record, aphids, other herbivores, natural enemies and tree data, including leaf size, phenology, height, fruiting success and sex expression.  My aim was pretty similar to that of Whittaker’s i.e.   “…to sample foliage insects in a series of natural communities and to obtain results of ecological significance from the samples”  truly a mega-project.  I once calculated that there are counts from over 2 000 000 leaves which scales up to something like 10 000 000 pieces of data, if you conservatively estimate five data observations per leaf. Quite a lot of the data are now computerized thanks to a series of student helpers and Vicki Senior, currently finishing her PhD at Sheffield University, but certainly not all of it. In terms of output, only two papers so far (Wade & Leather, 2002; Leather et al., 2005), but papers on the winter moth, sycamore and maple aphids and orange ladybird are soon to be submitted.  On balance, I think that this was also worthwhile and gave me plenty of early morning thinking time in pleasant surroundings and a chance to enjoy Nature.

The sycamore project – most of the raw data, some of which still needs to be computerised 🙂

 

Side project 5. Sixty bird cherry trees 1993-2012

This project has already featured in my blog in my Data I am never going to publish series and also in a post about autumn colours and aphid overwintering site selection.  Suffice to say that so far, thanks to my collaborator Marco Archetti, two excellent papers have appeared (Archetti & Leather, 2005; Archetti et al., 2009), the latter of which is my third most cited paper with 101 cites to date and the former is placed at a very respectable 21st place.  I don’t really see any more papers coming out from this project, but I might get round to writing something about the study as a whole in a natural history journal. On balance, even though only two papers have appeared from this project, I count this as having been a very worthwhile investment of my time.

All now in a spreadsheet and possibly still worthwhile delving into the data

 

Side project 5.  Urban ecology – Bracknell roundabouts 2002-2012

This started as a pedagogical exercise, which will be the subject of a blog post in the not too distant future. The majority of the field work was done by undergraduate and MSc students and in the latter years spawned a PhD student, so a side project that became a funded project 🙂 To date, we have published seven papers from the project (Helden & Leather, 2004, 2005; Leather & Helden, 2005ab; Helden et al., 2012; Jones & Leather, 2012; Goodwin et al., 2017) and there are probably two more to come.  Definitely a success and a very worthwhile investment of my time.  The story of the project is my most requested outreach talk so also gives me the opportunity to spread the importance of urban ecology to a wider audience.

The famous roundabouts – probably the most talked and read about roundabouts in the world 🙂 Sadly Roundabout 1 i n o longer with us; it was converted into a four-way traffic light junction last year 😦

 

Side project 6.  Testing the Janzen-Connell Hypothesis – Silwood Park, 2005-2012

I mentioned this project fairly recently so will just link you to it here.  So far only one paper has come out of this project (Pigot & Leather, 2008) and I don’t really see me getting round to doing much more than producing another Data I am never going to publish article, although it does get a passing mention in the book that I am writing with former colleagues Tilly Collins and Patricia Reader.  It also gave undergraduate and MSc project students something to do.  Overall, this just about counts as a worthwhile use of my time.

Most of this is safely in a spreadsheet but the data in the notebooks still needs inputting

According to my data base I have published 282 papers since 1980 which given that I have supervised 52 PhD students, had 5 post-docs, and, at a rough estimate, supervised 150 MSc student projects and probably 200 undergraduate student projects doesn’t seem to be very productive 😦 Of the 282 papers, 125 are from my own projects, which leaves 139 papers for the post-docs and PhD students and 17 from the side projects.  Three of the papers published from the side projects were by PhD students, so if I remove them from the side projects that gives an average of 2.3 papers per side project and 2.4 papers per post-doc and PhD student.   So, in my opinion, yes, side projects are definitely worth the investment.

 

References

Archetti, M. & Leather, S.R. (2005) A test of the coevolution theory of autumn colours: colour preference of Rhopalosiphum padi on Prunus padus. Oikos, 110, 339-343.

Archetti, M., Döring, T.F., Hagen, S.B., Hughes, N.M., Leather, S.R., Lee, D.W., Lev-Yadun, S., Manetas, Y., Ougham, H.J., Schaberg, P.G., & Thomas, H. (2009) Unravelling the evolution of autumn colours: an interdisciplinary approach. Trends in Ecology & Evolution, 24, 166-173.

Goodwin, C., Keep, B., & Leather, S.R. (2017) Habitat selection and tree species richness of roundabouts: effects on site selection and the prevalence of arboreal caterpillars. Urban Ecosystems, 19, 889-895.

Helden, A.J. & Leather, S.R. (2004) Biodiversity on urban roundabouts – Hemiptera, management and the species-area relationship. Basic and Applied Ecology, 5, 367-377.

Helden, A.J. & Leather, S.R. (2005) The Hemiptera of Bracknell as an example of biodiversity within an urban environment. British Journal of Entomology & Natural History, 18, 233-252.

Helden, A.J., Stamp, G.C., & Leather, S.R. (2012) Urban biodiversity: comparison of insect assemblages on native and non-native trees.  Urban Ecosystems, 15, 611-624.

Jones, E.L. & Leather, S.R. (2012) Invertebrates in urban areas: a review. European Journal of Entomology, 109, 463-478.

Leather, S.R. (1986) Host monitoring by aphid migrants: do gynoparae maximise offspring fitness? Oecologia, 68, 367-369.

Leather, S.R. (1989) Phytodecta pallida (L.) (Col., Chrysomelidae) – a new insect record for bird cherry (Prunus padus). Entomologist’s Monthly Magazine, 125, 17-18.

Leather, S.R. (1993) Overwintering in six arable aphid pests: a review with particular relevance to pest management. Journal of Applied Entomology, 116, 217-233.

Leather, S.R. & Helden, A.J. (2005) Magic roundabouts?  Teaching conservation in schools and universities. Journal of Biological Education, 39, 102-107.

Leather, S.R. & Helden, A.J. (2005) Roundabouts: our neglected nature reserves? Biologist, 52, 102-106.

Leather, S.R. & Mackenzie, G.A. (1994) Factors affecting the population development of the bird cherry ermine moth, Yponomeuta evonymella L. The Entomologist, 113, 86-105.

Leather, S.R., Wade, F.A., & Godfray, H.C.J. (2005) Plant quality, progeny sequence, and the sex ratio of the sycamore aphid, Drepanoisphum platanoidis. Entomologia experimentalis et applicata, 115, 311-321.

Pigot, A.L. & Leather, S.R. (2008) Invertebrate predators drive distance-dependent patterns of seedling mortality in a temperate tree Acer pseudoplatanus. Oikos, 117, 521-530.

Steinbeck, J. (1954) Sweet Thursday, Viking Press, New York, USA.

Wade, F.A. & Leather, S.R. (2002) Overwintering of the sycamore aphid, Drepanosiphum platanoidis. Entomologia experimentalis et applicata, 104, 241-253.

Ward, S.A., Leather, S.R., Pickup, J., & Harrington, R. (1998) Mortality during dispersal and the cost of host-specificity in parasites: how many aphids find hosts? Journal of Animal Ecology, 67, 763-773.

Whittaker, R.H. (1952) A Study of summer foliage insect communities in the Great Smoky Mountains. Ecological Monographs, 22, 1-44.

 

*

I was so impressed by this piece of philosophy that it is quoted in the front of my PhD thesis 🙂

**

My second post-doc was only for two years.

***

You may wonder why I keep emphasising early morning in relation to surveying sycamore aphids.  Sycamore aphids are very easy to disturb so it is best to try and count them in the early morning before they have a chance to warm up and become flight active.

 

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Ten more papers that shook my world – It pays to move away from home – Janzen (1970) & Connell (1971)

I have long held Dan Janzen in high regard, and not just because he wrote a paper with the memorable title “What are dandelions and aphids?”  (Janzen, 1977).  I have always found his writing enjoyable, and was, and still am, in awe of his ability to straddle whole swathes of ecology, both practically and conceptually. The paper that has, however, had the most impact on me, and perhaps the concept that Janzen is most renowned for, is the one that gave rise to the Janzen-Connell hypothesis/effect (Janzen, 1970).  Janzen was addressing the question of why tropical forests, while generally species rich, have a low density of adult trees of each species when compared with temperate forests (Black et al., 1950).  Janzen states “I believe that a third generalization is possible about tropical tree species as contrasted with temperate ones: for most species of lowland tropical trees, adults do not produce new adults in their immediate vicinity (where most seeds fall).”  He based this statement on his own personal observations, discussions with tropical foresters and on discussions with Joseph Connell.  He then works through several models testing different scenarios, from allelopathy*, different modes of seed dispersal and seed predation. Although allelopathy has been shown to affect seedling recruitment in several tree species (e.g. Webb et al., 1967), his conclusion was that the efficiency of seed/seedling specific predators was the main factor causing the patterns seen in tropical forest structure.  The simple take-home message, and one that I often say to the three of my grown-up sons who still live with us, is that it pays to move away from home; if your parents don’t kill you, then something else will 😊 Easy to remember and understand.

The Janzen model – the further away you are from your parent, the greater the probability that you will survive (Janzen, 1970).

The user-friendly version I use in lectures https://agoutienterprise.files.wordpress.com/2012/09/j-c-diagram1.jpg

So, given that the first mention in print is the Janzen 1970 paper, why is it the Janzen-Connell model/hypothesis/effect and why was a marine biologist,  Joseph Connell, writing about tropical forest diversity (Connell, 1971)?  It could so easily have been a repeat of the Wallace and Darwin contretemps. If you read both papers, it is obvious to see that both men had discusd the subject with each other, and saw their hypothesis as an extension of an earlier paper by the great ecologist, Robert Paine (Paine, 1966).  Connell refers to the Janzen paper as in press, but his ideas saw the light of day in 1970, albeit not analysed fully and referred to as in preparation, at a conference, the proceedings of which did not appear until the following year (Connell, 1971).  The actual data he referred to in his paper, did not appear in journal format until 1984, perhaps one of the longest in preps** ever (Connell et al., 1984).

Although Connell and Janzen continued to address the subject e.g. (Janzen, 1971; Connell, 1978), their names were not linked until Steve Hubbell did so in 1979 (Hubbell, 1979).  This linking of the two names seems to have been the fuse that set off the citation rocket.  As of now, it has been cited over 15 000 times and shows no signs of slowing down.

The Janzen-Connell citation rocket; 15 286 citations to date

So, apart from using it in teaching, how has the Janzen-Connell hypothesis shaken my world? Although I had used the concept in my teaching since the mid-1990s, it was my weekly walk round my 52 sycamore tree transect that got me thinking about it as research topic.  Field work is a great way to keep in touch with your study organism, things go one outside that don’t happen in the lab.  My sycamore transect was set up to monitor the insect herbivores and their natural enemies, but after a few years something else struck me, particularly, during high seed production years (another twenty-year data set for my never going to publish series).  I noticed that although lots of sycamore seedlings emerged underneath my study trees in the spring, by mid-summer hardly any were left; underneath other tree species, they were however, much more common, especially under oak trees.  My first thought was allelopathy, but a quick test using potted sycamore seedling in soil from underneath oak and sycamore trees with standard compost as a control, quickly showed this not to be the case.

Effects of soil type on growth of sycamore seedlings (F = 1.68 2/33 df P =NS).

I then used an undergraduate student assistant (paid I hasten to add) to do a couple of surveys, counting the incidence of sycamore seedlings and saplings underneath different tree species.  This convinced me that there was something going on and I set up twenty permanent plots in 2005, which I monitored until I left Silwood in 2012 (another set of data unlikely to be published), ten under mature sycamore and ten underneath mature oak trees, counting the number of sycamore seedlings that merged every spring and survived or not. After a couple of years I was convinced that there was every possibility of a Janzen-Connell effect going on and persuaded Alex Pigot, then a MSc student that it would be a great project.  To cut a long story short, Alex demonstrated that sycamore seedling survival, was as with tropical tree seedlings, dependent on predation pressure and that this was mainly due to invertebrate herbivores and was greatest underneath their parent trees.

Sycamore seedling mortality highest under sycamore and oak when exposed to invertebrates, vertebrates or both (Pigot & Leather, 2008).

Before anyone accuses me of taking credit for being the first person to demonstrate that the Janzen-Connell effect was also applicable to temperate forests, let me point you at a paper by Douglas Gill (Gill, 1975) who suggested that the spatial patterns of pines and oaks in the New Jersey Pine Barrens might be a result of differential seed predation as suggested by Janzen and Connell.

Despite the undoubted popularity of the Janzen-Connell Hypothesis in ecology, it is still not entirely clear cut; as my colleagues and I pointed out recently “What is clear, is that more studies targeting closed tall forests, and trees from other plant families and their seedlings are urgently needed before we can make sweeping conclusions about the generality of Janzen–Connell effects induced specifically by insects”  (Basett et al., 2019), but nevertheless this is a paper that shook my world and one that is definitely worth reading if you haven’t come across it before or just taken the concept it as gospel.

References

Basset, Y., Miller, S.E., Gripenberg, S., Ctvrtecka, R., Dahl, C., Leather, S.R. & Didham, R.K. (2019) An entomocentric view of the Janzen–Connell hypothesis. Insect Conservation and Diversity, 12, 1-8.

Black, G.A., Dobzhansky, T. & Pavan, C. (1950) Some attempts to estimate species diversity and population density of trees in Amazonian forests. Botanical Gazette, 111, 413-425.

Connell, J. H. (1971) On the role of natural enemies in preventing competitive exclusion in some marine animals and forest trees.  In: den Boer, P. J. and Gradwell, G. R. (eds), Dynamics of Populations. Centre for Agricultural Publications and Documentation, Wageningen, the Netherlands, pp. 298-312.

Connell, J.H. (1978) Diversity in tropical rain forests and coral reefs.  Science, 199, 1302-1310.

Connell, J.H., Tracey, J.G. & Webb, L.J. (1984) Compensatory recruitment, growth, and mortality as factors maintaining rain forest tree diversity. Ecological Monographs, 54, 141-164.

Gill, D.E.  (1975) Spatial patterning of pines and oaks in the New Jersey Pine Barrens. Journal of Ecology, 63, 291-298.

Hille Ris Lambers, J., Clark, J.S. & Beckage, B. (2002) Density-dependent mortality and the latitudinal gradient in species diversity. Nature, 417, 232-235.

Hubbell, S.P. (1979) Tree dispersion, abundance, and diversity in a tropical dry forest. Science, 203, 1299-1309.

Janzen, D.H. (1970) Herbivores and the numbers of tree species in tropical forests. American Naturalist, 104, 501-528.

Janzen, D.H. (1971) Escape of juvenile Dioclea megacarpa (Leguminosae) vines from predators in a deciduous tropical forest. American Naturalist, 105, 97-112.

Janzen, D.H. (1977) What are dandelions and aphids? American Naturalist, 111, 586-589.

Paine, R.T. (1966) Food web complexity and species diversity.  American Naturalist, 100, 65-75.

Pigot, A.L. & Leather, S.R. (2008) Invertebrate predators drive distance‐dependent patterns of seedling mortality in a temperate tree Acer pseudoplatanus. Oikos, 117, 521-530.

Webb, L.J., Tracey, J.G. & Haydock, K.P. (1967) A factor toxic to seedlings of the same species associated with living roots of the non-gregarious subtropical rain forest tree Grevillea robusta. Journal of Applied Ecology, 4, 13-25.

* the chemical inhibition of one plant (or other organism) by another, due to the release into the environment of substances acting as germination or growth inhibitors

**Let me know if you know of a longer one.  I don’t count Darwin, as he didn’t, as far as I know, actually refer to his theory in print before publication was forced upon him by Wallace.

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Not all aphids aggregate in clumps

There is a tendency for people when they do think of aphids, to see them as existing in large unsightly aggregations, oozing sticky honeydew, surrounded by their shed skins and living in positively slum-like conditions. The bird cherry-oat aphid, Rhopalosiphum padi, the black bean aphid, Aphis fabae, the Poa-feeding aphid, Utamphorophora humboldti and the beech wooly aphid, Phyllaphis fagi, being notable examples.

       Damage on bird cherry             Aphids on runner beans 2014           Office aphids compressed           Beech aphid

Whilst this may be true for many pest aphid species, it is far from true for the group as a whole. Yes they may occur in aggregations, but quite often, they look very neat and tidy and well-behaved.

Conker aphids 2013     Aphids on heath

Some aphid species lead quite solitary lives and you often only find them in ones and twos, if at all, e.g. Monaphis antennata.  There is one aphid species, however, that manages to have it both ways, living surrounded by its friends and relatives but managing to exist in splendid isolation at the same time. The exemplar of this phenomenon is the sycamore aphid, Drepanosiphum platanoidis, which exhibits a behaviour termed ‘spaced-out gregariousness’, a term coined by John Kennedy and colleagues in 1967, although the phenomenon was

sycamore aphids on leaf

described and measured by Tony Dixon a few years earlier. Effectively, the aphids like to be in a crowd but to have their own personal space. As proof of this, when the numbers of aphids on a leaf are low, say two to three, they will, instead of spreading out across the leaf, still show the same behaviour, i.e. get to within 2-3 millimetres distance of each other.

Sycamore compressed

Even more extraordinary is when a predator such as a ladybird or lacewing larvae finds its way on to a crowded leaf; the sycamore aphids do a great imitation of the parting of the Red Sea, but still without touching each other and keeping their regulation distance apart. Those finding themselves at the edge, either take wing or move to the upper surface of the leaf. Although a video of this exists somewhere I have been unable to find it so you will have to take my word for it. If anyone does come across the footage please let me know.

Yet another reason to love aphids.

 

References

Dixon, A.F.G. (1963) Reproductive activity of the sycamore aphid, Drepanosiphum platanoides (Schr) (Hemiptera, Aphididae). Journal of Animal Ecology, 32, 33-48.

Kennedy, J.S., Crawley, L., & McLaren, A.D. (1967) Spaced-out gregariousness in sycamore aphids Drepanosiphum platanoides (Schrank) (Hemiptera, Callaphididae). Journal of Animal Ecology, 36, 147-170.

Post script 

You may have noticed that the two references cited spell the species name of the sycamore aphid as platanoides,  It is in fact correctly spelt platanoidis.  To their embarrassment both John Kennedy and Tony Dixon got it wrong.  It wasn’t until 1978, when a very brave (possibly helped by conference alcohol consumption) PhD student (David Mercer) of Tony Dixon’s pointed this out, that the error was noticed and corrected 😉

Reference

Mercer, D.R. (1979) Flight Behaviour of the Sycamore Aphid, Drepanosiphum platanoidis Schr.   Ph.D Thesis, University of East Anglia, Norwich, UK.

 

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Aphid life cycles – bizaare, complex or what?

In a very early post I mentioned that one of the reasons that I love aphids so much is their life-cycles https://simonleather.wordpress.com/aphidology/  and the fantastic jargon that is used to describe them.  Many undergraduates find the jargon off-putting but it was this complexity that really grabbed my imagination.

aphid jargon

Insects are probably the most diverse group of organisms on Earth (Grimaldi & Engel, 2005) and their life cycles range from simple sexual and asexual styles to complex life cycles encompassing obligate and facultative alternation of sexual and asexual components.  Nancy Moran (1992) suggests that in the insect world probably the most intricate and varied life cycles are found in aphids and I certainly wouldn’t disagree.

There are basically two types of aphid life-cycles, non-host alternating (autoecious, monoecious) and host alternating (heteroecious).   Autoecious aphids spend their entire life cycle in association with one plant species as shown below (Dixon, 1985).

autoecious lifecycle

(or group of related plant species), whereas heteroecious aphids divide their time between two very different species of host plant, usually a tree species (the primary host) on which they overwinter, and an herbaceous plant species (the secondary host) on which they spend their summer.

Heteroecious

Approximately 10% of aphid species are heteroecious.  The ancestral aphid life cycle is thought to have been winged, egg laying and autoecious on a woody host plant almost certainly conifers and the oldest families of woody angiosperms e.g. Salicaceae (Mordwilko, 1928; Moran, 1992).

aphid life cycle evolution

Aphid life cycles can also be described as holocyclic, in which cyclical parthenogenesis occurs, with aphids reproducing sexually in the autumn to produce an overwintering egg, in temperate regions and parthenogenetically during spring and summer as shown below for the sycamore aphid (Dixon, 1985).

Holocyclic.png

Some aphids are anholocyclic where the clone is entirely asexual reproducing by parthenogenesis throughout the year. This is often seen in locations where winter conditions are mild, in the tropics for example or as a bit of an oddity around hot-springs in Iceland.

Anholocyclic.png

Parthenogenesis in aphids is coupled with live births and reduced generation times through the phenomenon of telescoping generations.  Parthenogenesis in aphids developed early on but whether the oldest aphids (200 mya) were parthenogenetic is not known.

Host alternation appears to have arisen more than once (Moran, 1988) and occurs in four slightly different forms depending on the taxon in which it occurs.  The main differences being in whether the sexual forms are produced on the primary (winter) host (the host on which the eggs are laid), or as in the case of the Aphidini, the males being produced on the secondary (summer) host and the sexual females produced on the primary host.   The majority of aphids host alternate between unrelated woody and perennial hosts, but some species host alternate between herbaceous plants e.g. pea aphid Acyrthosiphon pisum alternates between the perennial vetches and the annual peas Pisum sativum (Muller & Steiner, 1985) and Urleucon gravicorne alternates between the perennial Solidago and the annual Erigeron (Moran 1983).  Some aphid species such as Rhopalosipum padi, have clones that are holocyclic and some that are anholocyclic, so hedge their bets and also gives me the opportunity to slip in a great slide kindly lent to me by my friend Richard Harrington at Rothamsted Research.

Mixed

One of the things that is rather puzzling is why some aphid species should have adopted a host alternating life cycle which on the face of it, seems to be rather a risky strategy.  You could liken it to looking for a needle in a hay-stack; only about 1 in 300 aphids that leave the secondary host at the end of summer are likely to find their primary host (Ward et al, 1998).  There are a number of theories as to why it has evolved.

1. The nutritional optimization through complementary host growth hypothesis states that heteroecy has been favoured by natural selection because it enables a high rate of nutrient intake throughout the season (Davidson, 1927; Dixon, 1971).  In essence, the clone moves from a host plant where food quality is low and moves onto a herbaceous host that is growing rapidly and thus provides a good source of nutrition.  In autumn, the clone moves back to its primary woody host where leaves are beginning to senesce and provide a better source of nutrition as seen below (Dixon, 1985).

Nutritional changes

On the other hand, non-host alternating aphids such as the sycamore aphid, Drepanosiphum platanoidis, or the maple aphid, Periphyllus testudinaceus, reduce their metabolism and tough it out over the summer months when the leaves of their tree hosts are nutritionally poor, the former as adults, the latter as nymphs (aphid immature forms) known as dimorphs. Mortality over the summer in these species is, however, very high.  In some years I have recorded almost 100% mortality on some of my study trees, so very similar to the 99.4% mortality seen in the autumn migrants (gynoparae) of the bird cherry-oat aphid, Rhoaplosiphum padi.  Other autoecious aphids are able to track resources if they live on host plants that continue to develop growing points throughout the summer.

 Tough it out

Verdict:  No apparent advantage gained

2. The oviposition site advantage hypothesis states that primary woody hosts provide better egg laying sites and provide emerging spring aphids with guaranteed food source (Moran, 1983).  There is however, no evidence that eggs laid on woody hosts survive the winter better than those laid in the herbaceous layer.  Egg mortality in both situations ranges from 70-90% (Leather, 1983, 1992, 1993).

Verdict:  No apparent advantage gained

3.  The enemy escape hypothesis states that by leaving the primary host as natural enemy populations begin to build up and moving to a secondary host largely devoid of enemies confers an advantage on those species that exhibit this trait (Way & Banks, 1968).  At the end of summer, when the natural enemies have ‘found’ the clone again, the clone then migrates back to its primary host, which theoretically is now free of natural enemies.  This is an attractive idea as it is well known that natural enemies tend to lag behind the populations of their prey.

Enemy escape

Verdict: Possible advantage gained

4. The Rendez-vous hosts hypothesis suggests that host alternation assists mate location and enables wider mixing of genes than autoecy (Ward, 1991; Ward et al. 1998).  This seems reasonable, but as far as I know, no-one has as yet demonstrated that host-alternating aphid species have a more diverse set of genotypes than non-host alternating aphids.

Verdict:  Not proven

5.  The temperature tolerance constraints hypothesis which postulates that seasonal morphs are adapted to lower or higher temperatures and that they are unable to exist on the respective host plants at the ‘wrong time of year’ (Dixon, 1985).  I don’t actually buy this one at all, as I have reared spring and autumn morphs at atypical temperatures and they have done perfectly well (Leather & Dixon, 1981), the constraint being the phenological stage of their host plant rather than the temperature.  In addition, there are some host alternating aphid species in which the fundatrix can exist on both the secondary and primary hosts (if the eggs are placed on the secondary host).  This has been experimentally demonstrated in the following species:

Aphis fabae                                 Spindle & bean                                        Dixon & Dharma (1980)

Cavariella aegopdii

Cavariella pastinacea              Willows and Umbelliferae                     Kundu & Dixon (1995)

Cavariella theobaldi

Metopolophium dirhodum       Rose and grasses                                    Thornback (1993)

Myzus persicae                           Prunus spp &  40 different plants       Blackman & Devonshire (1978)

Verdict: Unlikely

6.  The escape from induced host-plant defences hypothesis (Williams & Whitham, 1986), which states that by leaving the primary host as summer approaches, the aphids escape the plant defences being mustered against them.  This is only really applicable to those gall aphids where galled leaves are dropped prematurely by the host plant.

Verdict: Special case pleading?

7.  The constraint of fundatrix specialisation hypothesis is that of Moran (1988), who argues that heteroecy is not an optimal life cycle but that it exists because the fundatrix generation (the first generation that hatches from the egg in spring) on the ancestral winter host, are constrained by their host affinities and are unable to shift to newly available nutritionally superior hosts.  Whilst it is true that some host alternating aphids are however, very host specific as fundatrcies, some aphids are equally host-specific as oviparae at the end of the year the constraints of ovipara specialisation

For example, in the bird cherry-oat aphid Rhopalosiophum padi, the fundatrices are unable to feed on senescent leaf tissue of the primary host, their offspring can only develop very slowly on ungalled tissue and all their offspring are winged emigrants (the alate morph that flies from the primary host to the secondary host) (Leather & Dixon, 1981).  The emigrants are able to feed as nymphs on the primary host on which they develop and as adults on their secondary host, but not vice versa (Leather et al., 1983).  The autumn remigrants (gynoparae, the winged parthenogenetic females that fly from the secondary hosts to the primary hosts on the other hand, feed on the secondary host as nymphs but are unable to feed on the primary host as adults (Leather, 1982; Walters et al., 1984).  The black bean aphid shows similar, but less rigid host specificity and whilst there is a distinct preference for the relevant host plant (Hardie et al., 1989), parthenogenetic forms can occur throughout the summer on the primary host (Way & Banks, 1968), particularly if new growth is stimulated by pruning (Dixon & Dharma, 1980). There are also at least two examples of where both the primary and secondary host are herbaceous (see earlier).  In both these cases the fundatrices could exist on both the primary and secondary host plants

Verdict:  Not proven

So what do I think?  For years I was very firmly convinced that the nutritional optimization hypothesis was the obvious answer; after all Tony Dixon was my PhD supervisor 😉  Now, however, having lectured on the subject to several cohorts of students, if I was forced to pick a favourite from the list above, I would do a bit of fence-sitting and suggest a combination of the nutritional optimization and enemy escape hypotheses. What do you think? There are cetainly a number of possible research projects that would be interesting to follow up, the problem is finding the funding 😦

Sources

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