The Sceptical Gardener E-Book

Ken Thompson

• Herausgeber: Icon Books
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 327

Veröffentlichungsjahr: 2015

Beschreibung

How are birds linked to house prices? How can a gardener improve the flavour of their vegetables? Do wildflowers really thrive in poor soil? In this collection of articles from The Telegraph, biologist and gardening columnist Ken Thompson takes a scientific look at some of the greater – and lesser – questions faced by gardeners everywhere in a bid to sort the genuine wisdom from the hokum. What is the ideal temperature for a compost heap? What do bees do that improves strawberries? Why are gardeners in literature always such dummies? This is an expert's gardening miscellany, aimed at making you not necessarily a better gardener, but probably a far more thoughtful one.

Leseprobe

The Sceptical Gardener

BY THE SAME AUTHOR

Where Do Camels Belong? (Profile, 2014)Do We Need Pandas? (Green Books, 2010)The Book of Weeds (Dorling Kindersley, 2009)No Nettles Required (Eden Project, 2007)Compost (Dorling Kindersley, 2007)An Ear to the Ground (Eden Project, 2006)

The Sceptical Gardener

The thinking person’s guide to good gardening

KEN THOMPSON

Published in the UK in 2015

by Icon Books Ltd, Omnibus Business Centre, 39–41 North Road, London N7 9DP email: [email protected]

www.iconbooks.com

Sold in the UK, Europe and Asia

by Faber & Faber Ltd, Bloomsbury House, 74–77 Great Russell Street,

London WC1B 3DA or their agents

Distributed in the UK, Europe and Asia

by TBS Ltd, TBS Distribution Centre, Colchester Road, Frating Green, Colchester CO7 7DW

Distributed in Australia and New Zealand by Allen & Unwin Pty Ltd,

PO Box 8500, 83 Alexander Street,

Crows Nest, NSW 2065

Distributed in South Africa by

Jonathan Ball, Office B4, The District,

41 Sir Lowry Road, Woodstock 7925

Distributed in India by Penguin Books India, 7th Floor, Infinity Tower – C, DLF Cyber City, Gurgaon 122002, Haryana

Distributed in Canada by Publishers Group Canada,

76 Stafford Street, Unit 300

Toronto, Ontario M6J 2S1

ISBN: 978-184831-933-2

Text copyright © 2015 Telegraph Group Limited

The right of Ken Thompson to be identified as the editor of this work has been asserted under the Copyright, Designs and Patents Act 1988

No part of this book may be reproduced in any form, or by any means, without prior permission in writing from the publisher

Typeset in Adobe Caslon by Marie Doherty

Printed and bound in the UK by Clays Ltd, St Ives plc

CONTENTS

About the author

Introduction

Garden wildlife

Birds and house prices

Threats to birds

Homes for birds

Fear of cats

Plants for bugs

Neonicotinoids and bees

Plants for bees

The tree bumblebee

Urban birds

Amphibians in gardens

Bat boxes

Birds smell trees

Butterflies and birds

Earwigs

Native and alien plants – and animals

Plants old and new

The rise of the harlequin ladybird

Native and alien trees

New Zealand flatworm

The lynx effect

Parakeets

Spanish bluebells

Not worth doing

Painted heathers

Buying bees

Grafting

Planting by the moon

Permaculture

Top 10 don’ts

Plants worth growing and plants worth buying

Plug plants

Compost tea

Panic in the garden

Strange fruit

Names of plants – and people

Flower names for girls

Plant names – common or Latin?

DNA and plant names

It’s Hesperantha, not Schizostylis

Hyphens and capitals

Taxonomy of Meconopsis

Spelling mistakes

Artichokes or cardoons?

Botany and Latin

The European Garden Flora

Herbaceous borders

Growing food

Allotment soil

Cabbage patch, part I

Cabbage patch, part II

Growing food to reduce your carbon footprint

The raspberry manifesto

Students growing their own

Bees and strawberries

Nutritious vegetables

Breeding for flavour

Interesting things about plants

Colour change

CSI plants

Floral clocks

Nicotiana: how not to poison your pollinators

How plants flower in winter

The rat-race to get out of the shade

The secret life of spores

A temperate Tarzan?

Variegation

The Swiss cheese plant: why holes are an advantage

England and Scotland – going their separate ways?

Dioecious imports

Tall or short, and not much in between

Practical gardening

Compost – getting the temperature right

Conifers and soil pH

Crocks in pots

Drought – not all bad news

Flooding – leave well alone

Gardeners’ Question Time, part I

Gardeners’ Question Time, part II

Meadows and nutrients

Meadows and sugar

Mulch

Soil type

Sunburned leaves?

Wildflowers and poor soil

Hot beds

Real meadows

Sweet peas

The value of trees

Cacti in Britain – outdoors

Eccremocarpus and the virtues of self-seeders

On being a gardener

Why you should join the BSBI

Gardeners on Desert Island Discs

Gardening and literature

Gardening makes you a better person

Careers in horticulture

Status of gardeners

ABOUT THE AUTHOR

Ken Thompson was for twenty years a lecturer in the Department of Animal and Plant Sciences at the University of Sheffield. He writes regularly on gardening for the Daily Telegraph. His previous book, Where Do Camels Belong? (Profile, 2014), was described as ‘lively and punchy’ by TheSunday Times.

INTRODUCTION

For nearly all my adult life I’ve been a university academic; specifically a plant ecologist. For almost as long I’ve also been a gardener; the two periods would coincide exactly, were it not that it took me a few years to acquire a garden. But it took me a surprisingly long time to realise that the two strands of my life were really quite closely related. I’m not sure whether there was an actual eureka moment, but if there was it was probably some time in the late 1990s.

As an academic, I spend a lot of time reading papers in scientific journals – in my case, botany and ecology journals. The great majority of these papers are of interest only to a small number of specialists, and a significant proportion are of no interest to anyone, sometimes not even their own authors. But hidden in there are a tiny number of gems that recount findings of interest to gardeners. Or at least, they would be of interest to gardeners if someone bothered to tell them. I looked around and realised that there was no ‘someone’ doing that, so maybe I should have a go.

But where to start? My eye fell on the magazine Organic Gardening, and I emailed its then editor, Gaby Bartai Bevan, who agreed to let me write a monthly column on science for gardeners.

The first column appeared in June 2001, and I wrote one every month for over three years, each inspired by a scientific paper. Gaby paid me £30 per article, which doesn’t seem like much now, and frankly didn’t seem like much then either. But I didn’t mind; the practice was good for me, and demonstrated that there was a market out there for ‘gardening science’. It also gave me the confidence to write my first gardening book, An Ear to the Ground. This wasn’t a collection of published articles, but was in much the same spirit as the Organic Gardening column: random bits of gardening science that I found interesting, and that I hoped others would too.

Fast forward to September 2010, when Joanna Fortnam, gardening editor of the Daily Telegraph, emailed me to ask if I would be interested in writing a column for her. I said yes, and I’ve been writing them ever since. Our arrangement, which appears to suit both of us, is marvellously informal – I write something whenever I feel like it, and Joanna prints it whenever she feels like it. In practice many articles are inspired by recent papers in scientific journals, which may be on anything from harlequin ladybirds to the genetics of artichokes and cardoons. Some of the stuff I write about may crop up elsewhere in the media, but much of it doesn’t. Several pieces start not with a particular scientific paper, but with a question, such as: is buying bumblebees a good idea, does compost tea do anything, and do wildflowers really like poor soil?

My approach in trying to answer such questions is essentially the one expressed by the motto of the Royal Society, Nullius in verba (Latin for ‘take nobody’s word for it’). That is, I always start by going back to the scientific literature to try to find some actual evidence; I am not (or only rarely anyway) particularly interested in airing my own opinions. Thus, even if I express what looks like an opinion (for example, that forest gardening is unlikely to feed many people beyond those who write books about it, that the panel’s advice on Gardeners’ Question Time isn’t always right, or that planting by the moon is – literally – for lunatics), I hope the facts are on my side.

Of course, given a soapbox and an invitation to stand on it, I do occasionally stray quite a long way from the objective, evidence-based ideal. Thus I have, over the years, pondered the relative scarcity of gardeners on Desert Island Discs, the waxing and waning of flower names for girls, and why gardeners in literature are always such dummies. I have also expressed my total bafflement at why anyone would bother spraying heather plants bright orange, and why anyone else would be willing to buy plants thus treated. In the spirit of ‘all work and no play …’ etc., I hope readers will bear with me.

It might have occurred to me eventually, but it was Joanna who first suggested that the columns should be collected together in a book, so here they are. I have not attempted to update them. Most don’t need it, and in any case the updating itself would soon be out of date. In a very few cases, where the article is perhaps a bit of a cliff-hanger, I have added a brief footnote that explains where we are now (in mid-2015). In even fewer cases, a footnote clarifies a topical reference that isn’t obvious from the context. Otherwise, the columns are reproduced here in exactly the form in which they were originally written.1

Cerys Hughes at the Telegraph undertook the thankless task of selling the idea of a book to publishers, and Duncan Heath was kind enough to recognise that it might fit with Icon Books’ aim of publishing ‘thought-provoking non-fiction’. So to Cerys and Duncan, my enormous thanks for getting the idea off the ground. To everyone else at Icon, my thanks for seeing the project through to fruition. Special thanks to Gaby Bartai Bevan for giving me my start in writing for gardeners, to Joanna Fortnam for being so consistently (and surprisingly) interested in what I choose to write about, and my wife Pat for putting up with me while I write it. Finally, thanks to you, the reader. If these columns are new to you, I hope you like them, and if you’ve read them before, I hope you enjoy them all over again. I certainly enjoyed writing them.

Footnote

1 The great majority of these articles first appeared in the gardening pages of the Daily Telegraph, but a few, for one reason or another, did not. In fact, they have never appeared anywhere.

GARDEN WILDLIFE

Birds and house prices

Birds are like people. At least, when it comes to choosing a congenial urban neighbourhood to live in, birds and people share remarkably similar tastes. Ecologists have known for 50 years that many birds like a layered, semi-wooded landscape. That is, they like well-spaced trees, with plenty of smaller trees, shrubs and herbaceous vegetation in the gaps between the big trees; essentially plants at every level from the ankles upwards. The reason is simple – this multilayered vegetation is best at providing everything birds want out of life: food, nesting sites, protection from predators, and somewhere to perch and sing.

At the same time, students of the human condition have shown that this kind of landscape – wooded but not too wooded, partly open, partly shady, diverse, and above all, interesting – appeals to people too. The measure of that appeal is simple: we are prepared to pay more for houses surrounded by that sort of landscape. Not for nothing is the ‘leafy suburb’ the standard cliché for a desirable place to live, unless you’re allergic to gardening, or too poor to afford a gardener. Partly that’s because to the human eye it just looks right, but we probably also realise, perhaps only subconsciously, that such landscapes also deliver a range of benefits: reduced noise, wind, dust and air pollution, better rainwater management and less need for air conditioning in summer.

Pleasant as the leafy suburb is, it’s quite hard to quantify exactly what it is we like about it. Birds don’t worry about such things – they know what they like, and vote with their feet (or wings anyway). This is demonstrated by a recent study in the city of Lubbock, Texas, published in the journal Urban Ecosystems, in which a team from Texas Tech University took the logical step of asking: if birds and people like the same kinds of neighbourhoods, can we link birds directly to house prices? They identified and counted the birds in a sample of contrasted urban neighbourhoods, paying particular attention to the less common and more interesting species. That’s because some birds, such as European imports like house sparrows and starlings, but also some native birds like the great-tailed grackle, tell you nothing about the quality of a neighbourhood – they’re just everywhere. But other birds, such as American robins, blue jays and mockingbirds, are both less common and more discriminating; in a British context, think birds like woodpeckers, nuthatches, siskins and tree sparrows.

They asked a simple question: after we allow for obvious factors like house size and age, total plot size and presence of a garage, can birds help to predict any of the remaining unexplained variation in house prices? Turns out they can, and I’ll tell you how much in a moment. But what the analysis also reveals is that while, in theory, the bosky paradise preferred by birds could be provided by public open space or by gardens, or by some combination of the two, in Lubbock (and, I suspect, in many other places too) it’s left to gardens to do all the heavy lifting. The researchers checked to see if the presence of a park within half a mile had any effect on either birds or house prices, and found no effect whatsoever. In short, ‘green space’ is not all the same, and birds are very good at spotting the good stuff. Lubbock’s private gardens (or some of them, anyway) provide the varied, multi-layered canopy that birds like, but Lubbock’s public parks do not.

So, for those of you who have already concreted over the front garden and are considering paving over the back too, what’s the likely damage? Well, the researchers found that on average, the presence of just one more species of less-common bird is linked to a house-price premium of $32,028.1

Threats to birds

You’ve probably never heard of the Conservation Evidence Project, and there’s no particular reason why you should have. In its own words, it ‘summarises evidence from the scientific literature about the effects of conservation interventions such as methods of habitat or species management’. Basically it looks at reports of such ‘conservation interventions’ and asks: did they work? All this is freely available from www.conservationevidence.com, but also, once they have accumulated enough evidence on a particular subject, they produce a synopsis, which ‘lists all the possible actions you could take to conserve a given species group or habitat, or to tackle a particular conservation issue’, together with how well they worked.

It’s early days, and there are just three synopses so far. The first, on bees, tells you for example that artificial nests for solitary bees (essentially an object or container full of holes) work rather well, and that nest boxes for bumblebees usually don’t work at all. The second synopsis, on birds, is enormous – 704 pages long. Fortunately, hardly any of this is of any relevance to gardeners, but some of it is, and I have extracted the following nuggets for you:

Bird collisions with windows

Birds can be injured or even killed by flying into windows. Does marking windows with wind chimes, silhouettes of falcons, stickers of eyes or model owls reduce bird collisions? No it doesn’t. However, fewer birds fly into windows if they are tinted or largely covered with white cloth. An American study, definitely to be filed under ‘interesting but useless’, found that fewer birds collided with windows angled at 20° or 40° from the vertical, compared to vertical windows. So if birds flying into your windows is a problem, and you don’t want tinted glass, or to have your house rebuilt at 20° to the vertical, it looks like your only option is to keep the curtains closed.

Another American study found that placing bird feeders close to windows reduced the number of collisions. But other studies also found that feeders are used more if they are further from the house, so you pays your money and takes your choice.

Reducing predation by cats

Ultrasonic cat deterrents are devices that emit high-pitched noise above the hearing of humans, but audible to cats. But do they work? A definite ‘maybe’ here; one study found that an ultrasonic cat deterrent in gardens reduced the number of visits by cats, but another one didn’t. There’s no evidence, either way, for the effects of ultrasonic cat deterrents on bird populations.

What about fitting something to the cats themselves? Another mixed bag I’m afraid. One trial found that fewer birds (and mammals) were caught by cats fitted with a collar and bell or a collar with a CatAlert™ sonic device. The sonic device worked no better or worse than a bell. But a second trial in the following year found no effect of wearing a CatAlert™ sonic device, or one bell, or even two bells. The successful trial also showed that, clever as cats are, they don’t seem able to figure out a way of hunting that makes bells or sonic devices less effective. At least, over the five months of the trial, bells and sonic devices worked as well at the end as they did at the beginning. On the other hand, cats are very good at losing collars, and in fact they managed to lose many of them during the experiments.

An Australian study found that wearing a CatBib™ ‘pounce protector’ (a neoprene flap that hangs from a collar in front of a cat’s front legs, acting either as a visual warning or as a barrier to pouncing) reduced the number of cats catching birds by a massive 81 per cent. Adding a bell had no additional effect. No one asked the cats what they thought of this.

Homes for birds

More on birds from www.conservationevidence.com.

Not surprisingly, lots of studies across the world (though not all) show that songbirds readily use nest boxes, and most also find that nest boxes increase numbers of birds, or breeding success, or both. So nest boxes are definitely a good thing, but that still leaves plenty of other interesting questions about how to get the best out of them.

For example, is colour of nest boxes important? Yes and no, but mainly no. An old American study compared use of black and green nests by American robins and mourning doves. Robins made more nesting attempts in green nests than in black nests, but there were an equal number of successful attempts in each, so the success rate was higher in black nests. The behaviour of the doves was exactly the opposite: more nesting attempts in black nests than green nests, but the same number of successful attempts in each colour, i.e. a higher success rate in green nests. A British study found that blue and great tits preferred green boxes to brown ones. So it looks like every species of bird is different, and since nest boxes are usually a plain neutral colour, there seems no compelling reason not to leave them that way.

On the other hand, orientation probably is important. A British study found that tits avoided nest boxes facing south-west, and that fewer pied flycatcher chicks fledged from south-west facing boxes. So it looks like the official RSPB advice to site nest boxes facing between north and east is right.

Does nest box material matter? Apparently it does. In one American study, warblers strongly preferred empty milk cartons to wooden boxes. In another, eastern bluebirds showed an overwhelming preference for woodcrete (concrete reinforced with wood fibre or sawdust) nest boxes over those made from wood. Meanwhile, a British study found that four species of tits all preferred woodcrete boxes over wood, while an American study also found that tree sparrows preferred woodcrete boxes, and suggested that this may be because they’re warmer, allowing the birds to start nesting earlier. So if you’re buying a nest box, it looks like the extra expense of a woodcrete box may be worthwhile.

Is it worth cleaning out nest boxes every year? On the one hand, old nest boxes may contain parasites, but on the other, old nesting material may provide a nice comfy base for building a new nest, so it’s not obvious whether cleaning them out is a good idea or not.

The evidence from bird preferences is mixed. Five studies found that birds preferred clean nest boxes, one study found birds avoided dirty nest boxes but only if they were really grotty, another study found no preference either way, and two studies found a preference for used nest boxes. In one Canadian study, tree swallows preferred clean, empty boxes, but also liked those where the old material had been left, but sterilised by microwaving. So there’s a suggestion that most birds prefer clean nest boxes, but the evidence is not overwhelming. Among the five studies that checked whether nest cleanliness affected nesting success or parasitism levels, none found any effect. On balance, if you currently don’t bother to clean out your nest boxes every year, the scientific evidence doesn’t offer any very urgent reason to change your behaviour.

Does it help to provide extra nesting material? Two Scottish studies here. In one, blue, great and coal tits strongly preferred empty boxes to those containing a layer of wood shavings. In the other, wood pigeon feathers were put out for songbirds to collect during the nesting season over three years. Not many were used, and when surrounding nests were searched, only 2.8 per cent of the marked feathers turned up in them. The study authors concluded that nest construction is not limited by the availability of nesting material, and therefore providing extra is basically a waste of time.

Finally, tits also strongly prefer nest boxes with round entrance holes to those with a wedge-shaped entrance, but since I never saw a nest box with anything other than a circular entrance, I think that’s another observation that goes in the ‘interesting but useless’ category.

Fear of cats

Some gardeners adore cats, and may also be cat owners, some loathe cats with fanatical intensity, and most of us are somewhere in between. What’s not in doubt is that cats kill an awful lot of birds every year in Britain (55 million according to the Mammal Society), plus plenty of other things, from snakes to bats. Surprisingly, however, no one knows if this has any effect on bird numbers. The RSPB, for example, thinks not (for perfectly good reasons), but they don’t really know any more than I do. But new research here in Sheffield, published in Journal of Applied Ecology, sheds some completely new light on the relationship between birds and cats.

Ecologists have known for a while that predators don’t just kill their prey, they also alter their behaviour, and this study looked at the impact of cats on nesting blackbirds. Real cats, of course, are extremely poor at obeying orders, so the researchers used a stuffed tabby, which they placed about 2 metres from a blackbird nest for fifteen minutes before removing it. To check what effect any stuffed animal might have, they did the same with a stuffed grey squirrel (a possible nest predator) and a stuffed rabbit (in which blackbirds should have no interest at all).

The effects of the cat were more or less what you would expect. Parent birds responded with loud alarm calls and aggressive behaviour, diving on the cat model and sometimes striking it. But in addition, food delivery to the nest was reduced by more than a third, an effect that persisted for at least 90 minutes after the cat was taken away. This kind of reduction is very likely to lead to lower chick growth and survival. Parent blackbirds took much less notice of the stuffed squirrel, and none at all of the rabbit.

The researchers also checked the fate of each nest after a further 24 hours. During that time, nests exposed to squirrels or rabbits suffered negligible predation, but almost a quarter of nests exposed to the cat were predated, mostly by crows and magpies. The most plausible explanation for this result is that the alarm calls and aggressive behaviour by the parent birds alert predators to the presence of nests that they would otherwise have failed to discover. But it’s always possible that responding to the threat of cat predation also reduces the ability of the parent birds to defend against subsequent predation attempts.

Whatever the cause, the results demonstrate that our understanding of predation of songbirds, and of nest failure in general, in places where there are lots of cats (i.e. in towns and cities) is far from complete. In particular, we may sometimes blame crows and magpies for nest predation when the real culprit is already dozing in front of the fire several streets away. It also suggests that well-meaning efforts to reduce the number of birds killed by cats may be misplaced at best, or even counterproductive. Collars fitted with bells or sonic devices are widely recommended, but their effectiveness is suspect, and if they serve to make cats even more obvious to birds, they may simply provoke even more alarm calls and aggressive behaviour, making the problems described above even worse. Research shows that the best predation deterrent is a CatBib™ ‘pounce protector’, but even this would have no effect on the problem identified here. The only certain solution is to keep cats permanently indoors, which most cats find perfectly acceptable, and is normal practice in the USA. Harder work for cat owners though, so I can’t see it catching on.

For what it’s worth, it’s still hard to argue with the RSPB’s verdict that cats don’t have much effect on numbers of birds in gardens, if only because – despite cats – there are such large numbers of birds in towns and cities. But the new research suggests that if there were fewer cats, there might be even more.

Plants for bugs

If you aim to try to keep the wildlife in your garden happy, you probably already know most of what you ought to be doing: grow plenty of flowers that provide pollen and nectar; grow a variety of trees, shrubs and climbers, or a mixed hedge; leave a pile of dead wood in a shady spot; provide water for birds and other animals, or better still, dig a pond; make and use garden compost; feed the birds, go easy on garden chemicals, and don’t be too tidy.

Something you’re probably less sure about is how far you should endeavour to grow native plants. A few native plant-eating insects are real generalists, able to eat most of what comes their way, but most are more choosy to various extents – some confined to a single plant family, some to a genus, some to a single species. For example, among British butterflies, the grizzled skipper will eat a wide range of species from the rose family, several fritillaries will use any violet, while the white admiral can eat only honeysuckle. Thus it seems only natural that if you want to please as many native insects as possible, you should grow lots of native plants. To some, this is such a self-evident proposition that it scarcely needs testing – in fact ‘native is best’ has, for many, assumed the status of an axiom of good wildlife gardening practice.

And yet, the evidence stubbornly refuses to conform. Several major studies have shown no large effect of native plants on the diversity of wildlife in gardens. You may also be uneasily aware that you have grown all the plants mentioned above for years, but the butterflies that are supposed to eat them have so far declined to show up. But for some people (for example Stefan Buczacki in the New Naturalist volume on Garden Natural History), the value of growing native plants is so obvious that it simply must be true. The difficulty is that, good as gardens already are for wildlife – despite being dominated by alien plants – there’s always the possibility that with a few more native plants they could be even better. Personally I don’t think it makes much difference how many native plants you grow, but could I put my hand on my heart and say I have the definitive evidence to support that view? No, I could not.

So how do we find out which plants the bugs prefer? The obvious answer is to ask them, or in other words a good, old-fashioned controlled experiment, in which we manipulate plant origin and see what happens. Three cheers then for the Royal Horticultural Society, who are behind the Plants for Bugs project. Replicate plots (each 3 × 3 m) at the RHS’s flagship garden at Wisley contain matched sets of plants of three different levels of ‘nativeness’. Some plots are planted entirely with British native plants. Other plots take advantage of the fact that all the northern hemisphere’s land was stuck together in a single continent, Laurasia, until quite recently, and therefore shares a distinct ‘northern temperate flora’ that’s essentially the same everywhere. Thus all British natives have fairly close relatives in North America and Asia, and often even closer ones in mainland Europe, and it’s these close relatives (Plants for Bugs calls them ‘near natives’) that populate a second set of plots. Finally, a third set of plots contains the ‘exotics’: plants that are as remote as possible, both taxonomically and geographically, from the natives; in practice these hail from the southern hemisphere.

To try to make sure that the degree of ‘nativeness’ is the only thing that varies between the three sets of plots, plants are carefully matched so that all three in a matching set fill the same ‘niche’. For example, each set has a ‘tall herbaceous perennial’: native Eupatorium cannabinum (hemp agrimony), near native E. maculatum (Joe Pye weed), and exotic Verbena bonariensis. Another plant type is ‘slow-growing, small-leaved evergreen shrub’: here the native is Buxus sempervirens (box), the near native is Sarcococca confusa (sweet box) and the exotic is Pittosporum tenuifolium. Because differences between any one set of natives, near natives and exotics could be the result of chance, there’s more than one set of each, and since location might affect the outcome, the whole experiment is repeated at a second site, also at Wisley but distant from the first site.

The plots were planted up in 2009 and RHS entomologists spent 2010 going over them with a fine-tooth comb to see what the wildlife thinks. They’re observing flying insect visitors such as bees and butterflies, trapping slugs, pitfall trapping for ground beetles, woodlice and other ground-living fauna, and sucking leaf-dwelling insects off the foliage with a Vortis suction sampler, essentially an entomological Dyson. All this is working pretty well, and one thing we can say already is that the plots contain plenty of wildlife. For example in 2010 over 2,500 flying insects were observed visiting the plots, including seven species of bumblebee and twelve species of butterfly, while the pitfall traps caught over 8,000 invertebrates consisting of over 130 species, including 30 species of ground beetle and four different woodlice.

The Plants for Bugs project is planned to run throughout 2011 and 2012, so no answers yet I’m afraid,2 but for more information including a full plant list, visit www.rhs.org.uk/plants4bugs. Better still, visit Wisley and see the plots for yourself.

Neonicotinoids and bees

Victorian gardeners were familiar with the alkaloid nicotine as a pesticide, and very good it is too at killing almost anything that moves. Unfortunately that includes people – the nicotine in three or four cigarettes would kill you if you absorbed all of it. As a result nicotine has not been available to amateur gardeners for some time, and approval for professional use was withdrawn in 2009. But in the 1970s chemists developed a new class of insecticides that, although not closely related chemically to nicotine, share the same mode of action and were thus christened neonicotinoids. Like nicotine, neonicotinoids are extremely effective nerve poisons, but unlike nicotine they are really only toxic to insects and are very safe chemicals to use. Neonicotinoids have several other desirable features. Their mode of action is different from other major classes of insecticides such as pyrethroids or organophosphates, which means that even if insects had already evolved resistance to those earlier chemicals, they would have to start from scratch with neonicotinoids. They are also highly systemic, that is, easily and rapidly moved around inside the plant. This means that they can be applied as seed dressings, which are then absorbed by the young plant when the seed germinates, removing the need to spray and more or less eliminating the risk to non-target organisms.

This combination of effectiveness and safety, both to humans and other animals, has resulted in neonicotinoids becoming the fastest-growing type of insecticide in the world, worth €1.5 billion in 2008. Nowadays, 99.8 per cent of maize seed sown in the USA is treated with neonicotinoids (the other 0.2 per cent is organic). Several members of the neonicotinoid family, such as imidacloprid and thiacloprid, are familiar garden insecticides in the UK.3 Thiacloprid, for example, is the active ingredient of Bayer Provado.

But no sooner was the crop protection industry congratulating itself on discovering the pesticide equivalent of the philosopher’s stone, than problems began to emerge. Because neonicotinoids are so effectively transported around the plant, they can turn up anywhere, including in pollen and nectar. Admittedly, the quantities involved are minute: in lab studies, the single LD50 dose (i.e. that kills 50 per cent of dosed individuals) is about one hundred times the amount a honey bee might acquire from a day’s nectar-foraging. But a single bee might visit a field of treated oilseed rape every day for several weeks, eventually consuming quite a large dose. Not only that, there’s always the possibility of so-called ‘sublethal’ effects, reducing bee lifespan or impairing foraging ability; these subtle effects are much harder to detect than straight mortality.

In recent years, numerous studies have tried to estimate how dangerous neonicotinoids are for bees. Most have used imidacloprid, although there’s no reason to believe the other types behave very differently, and most have also used honey bees, largely for practical reasons, although they are not always the most important pollinators, in gardens or elsewhere. So what did these studies find? Well, generally they confirm the original, back-of-the-envelope impression that neonicotinoids don’t actually kill bees at the sort of doses they would normally experience. Therefore it also seems unlikely that neonicotinoids are responsible, at least on their own, for so-called Colony Collapse Disorder (CCD), in which whole hives suddenly expire.

The sublethal effects are more interesting. We’re talking here about bees’ abilities to perform a variety of tasks, including learning and remembering the location of good nectar sources, and ability to return successfully from a remote nectar source. The effects are subtle, but James Cresswell, from the University of Exeter, found that if he analysed the data from all the studies together, a consistent picture emerged. The performance of bees exposed to the sort of dose they might receive from foraging on treated oilseed rape or sunflowers was reduced by anything from 6–16 per cent. That may not sound like much, but it’s like waking up with a hangover every day. Except a hangover won’t kill you, but a bee that loses its way is a dead bee.

Those results come from studies in which bees were fed realistic doses of neonicotinoids. In contrast, several studies have failed to find much effect on bee colonies under field (or close to field) conditions. The reason seems to be that, as any beekeeper will tell you, honey bee colonies are naturally very variable in the sorts of things measured, such as worker bee lifespan and honey yield, and no study so far has used a sample size big enough to detect the small effects of neonicotinoids with any real confidence.

What about other bees? Bumblebees are extremely abundant in gardens, where they are probably more important pollinators than honey bees, and several studies report effects of neonicotinoids that are very similar to those in honey bees. Bumblebee expert Dave Goulson’s team at the University of Stirling have shown that bumblebee colonies exposed to low doses of imidacloprid grew more slowly than control colonies, and crucially produced far fewer queens, potentially reducing the number of colonies in subsequent years. This result is consistent with other studies showing that neonicotinoids make bumblebees lethargic and reluctant to forage.

Extrapolating all this work to the real world is complicated by various factors. On the one hand, especially in gardens, bees are likely to forage on a wide range of plants, only some of which will have been treated with pesticides, thus diluting the effect. The bees most at risk may be those that forage on mass-flowering agricultural crops like rape. On the other hand, the effects of pesticides may be combined with those of other stresses, such as starvation or disease, and the combination may be much worse than either on its own. Since one effect of neonicotinoids is reduced foraging ability, affected bees may often be short of food, which could explain the poor queen output in bumblebees. In one French study on honey bees, a dose of imidacloprid too low to cause any harm on its own had a much worse effect when combined with infection by Nosema, a common microsporidian parasite of bees. Nosema has been suggested as a cause of CCD, and together with neonicotinoids it looks like as good an explanation as any.

Finally, it’s worth noting that in many ways gardens represent the most benign scenario for bees and pesticides, with a great variety of flower sources and probably rather low pesticide use. Out in the agricultural landscape things are far worse, and bees may be exposed to neonicotinoids in surprising ways. At sowing time, contaminated soil dust and talc (used to stop treated seeds sticking together) spread far and wide, coating wildflowers growing nearby and persisting in soil for at least two seasons. To receive a high dose of neonicotinoids, bees don’t need to visit a treated crop, or even visit flowers at all for that matter – just being around when the crop is being sown can be fatal.

Plants for bees

What are the best plants to grow for bees and other pollinators? Observant gardeners already know at least part of the answer to that question. In my own garden, off the top of my head, I guess foxgloves, culinary sage, lavender and an unidentified cotoneaster must be near the top of the list. Nor is there any shortage of advice online. The RHS ‘perfect for pollinators’ initiative provides a long list of good plants. Yet the RHS list also illustrates the problem; such a list must include plants that are really good and also those that are merely OK, with no obvious way of telling them apart.

To try to introduce a little quantitative rigour to the subject, Professor Francis Ratnieks and his student Mihail Garbuzov at the University of Sussex have come up with a deceptively simple protocol. In the ‘snapshot’ method, pollinating insects on patches of flowers are counted more or less instantaneously by eye. Measuring the area of each patch means the attractiveness of each plant can then be expressed as the total number of insects per snapshot per square metre. Each single snapshot counts for little on its own, but if you keep doing it, on different plants, in different places and at different times, a picture starts to emerge of the winners and losers in the battle to attract pollinators.