Your garden's ecosystem: how to encourage helpful insects

Val Bourne / 07 June 2019

Gardening expert Val Bourne explains why biodiversity in the garden is so important, and how your garden ecosystem can encourage beneficial insects.



As a lifelong organic gardener, I’ve always gardened for wildlife and in the mid-1990s I realised that many gardeners had been persuaded to use toxic chemicals, chiefly insecticides and slug bait, in their gardens. I also realised that insect life - the bedrock of the natural world - was in decline and this was having a knock-on effect on other wildlife, including many birds. I wrote a book called The Natural Gardener in 2005 to explain that it was perfectly possible to have a lovely garden without using chemical props because naturally run gardens self-regulate themselves.

In 2004 I moved to a new garden at Spring Cottage in The Cotswolds. It was in a run-down state and it had been heavily treated with chemicals, including DDT. I set about making a garden designed to attract and sustain wildlife and it worked. Within ten years the garden was full of insect life and I wrote about the plants and the wildlife in a new book called The Living Jigsaw, published by Kew in 2017. Chris Packham very kindly wrote a forward.

The Living Jigsaw explains why it’s important for gardeners to support wildlife by building up a self-sustaining ecosystem that regulates the garden through a series of tiny interactions between ALL living creatures. In the past gardeners have tried to separate wildlife into saints and sinners and keep one and get rid of the other. They want hedgehogs, but not slugs, or they’d like a thrush and yet they abhor snails. Natural gardening doesn’t discriminate, it embraces the good, the bad and the ugly. It’s impossible to separate God’s creatures into good and bad, because they all rely on each other.

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The catch 22 situation

The saintly 7-spot ladybird, for instance, feeds on aphids, an insect that many gardeners believe wrongly to be a mini-locust. One newly emerging female will lay an average of 500 eggs, in batches of 30, during the growing season. The instars (or larvae) hatch from their mustard-yellow eggs with a source of aphids close by. This is the catch 22 of natural gardening. You have to have aphids in order to attract aphid-eating ladybirds, you can’t have one without the other. If you’re the sort of gardener who jumps for an insecticide at the first sight of an aphid every spring, you’re unlikely to see any ladybirds.

If I spray?

The biggest problem with insecticides, is that they kill across the board and knock off your predators as well as your pests. Predators are hunters by nature and they’re bound to come into contact with your insecticide. With them gone, you’ll end up with more of an aphid problem and not less of one. The aphids will bounce back quickly, unchallenged, but you predators have longer life cycles and they may be gone for the year.

The aphid is a master at reproducing clones

Take the 7-spot ladybird. A female will only produce one generation and her offspring will not be able to breed until the following year. In contrast aphids could produce forty generations per year, without even breeding, through a process called parthenogenesis. With no mate involved, all the offspring are genetically identical to the parent. Should that parent aphid show resistance to your insecticide, this will resistance be passed on through the generations. Given that a newly born aphid becomes a reproducing adult within about a week and then can produce up to five offspring per day for up to 30 days, it’s obvious that they multiply quickly. The French naturalist René Réaumur writing during the late 18th century calculated that if all the descendants of a single aphid survived during the summer and were arranged into a French military formation, four abreast, their line would extend for 27,950 miles, which exceeds the circumference of the Earth at the Equator!’ Pesticides are definitely speeding up the evolutionary process and producing a super race of resistant aphids.

Use the flying squad instead

If you leave a colony of alone aphids in spring, they’ll soon disappear. Most garden birds need small invertebrates to feed their chicks. Blue tits, for instance, have to collect 10,000 small invertebrates to feed their fledglings over a three week period and they’ll collect your aphids and small caterpillars for you. Wasps will also collect large numbers of aphids and carry them back to their nests. Tiny parasitic wasps (species of Aphdius) looking rather like black flying aphids themselves, will target your aphids. They will lay one egg inside each body and the affected aphid will turn into round, brown blobs. After seven days, a new parasitic wasp will emerge from a perfectly round hole. The new parasitic wasp will live for between 15 and 27 days and probably parasitise 200 aphids, as well as eating another 40 or more. Parasitic wasps, which are often used to control pests on commercial crops, come free to the gardener.

Hoverfly larvae are also voracious predators of aphids. They lay single, cream, oval eggs in the middle of colonies and their maggot-like larva consume aphids. Most garden hoverflies have insect-eating larvae.

Lacewings also have aphid-eating larvae. They’re known as lacewing lions because they have hairs surrounding their faces. Standing back and allowing your predators to devour your aphids makes sense.

If your aphids really bother you, just rub them off with your fingers, because they have very vulnerable feeding tubes called stylets so are easily destroyed.

Should I use and green or organic insecticide or soft soap?

No, so called organic and green sprays still kill across the board - knocking out predator and prey alike. They’re designed to look eco-friendly, but they’re just as damaging to an ecosystem.

What about slugs?

The same type of interactions are happening at ground level. The gardener’s enemy, the slug, is being devoured by a number of creatures. Ground beetle adults and their larvae are the biggest predators of slugs and gardens can be full of them. You may not see them, because these creatures are active at night. They’re also devoured by birds, hedgehogs and some small mammals. In any case I don’t want to target all my slugs and snails, because some larger slugs eat detritus, rotting plant material, thereby tidying the garden and leaving fewer hiding places for more-destructive slugs. Many of these detritus-eating slugs are large and naturalist Bill Oddie says - if it’s big and black put it back.

The three most destructive slugs

Netted or grey field slug (Deroceras reticulatum)

A light fawn slug with a glistening body looks almost as though it has been through a car wash due to the abundant milky mucous – which is a deterrent to predators. This is the plump, pinkish slug found in your hearting lettuces and cabbages and it feeds mostly on seeds and plants above ground. It’s active for many months of the year, even when temperatures are close to freezing.

Garden slug (Arion hortensis group)

This group, which consists of Arion hortensis, A. owenii and A. distinctus, are known as roundback slugs and they have wrinkled backs and no raised keel (prominent ridge on its back). They are smaller than the grey field slug, typically 2.5–3.5 cm (around 1–1.5 in).The body is dark and the foot (underside) ranges from yellow to orange. All the species in this group produce characteristic yellow-orange mucus and, like all Arion species, they are active only at temperatures above 5°C.

Common keeled slug (Tandonia budapestensis)

This is the dark brownish-grey that penetrates your potatoes and root crops. If seen, it is usually curled into a sickle shape, but most of the time it’s invisible because it’s underground. It has a keel (ridge) and a clearly visible yellow line along its body. Keeled slugs (Milax, Tandonia and Boettgerilla species) are more or less subterranean, coming to the surface only during the breeding season.

Avoid all slug pellets - they pollute water supplies

They kill every slug, even the detritus eaters, and the blue metaldehyde pellets are very toxic to dogs. Metaldehyde is also very motile in damp conditions, so it soon leaches into water courses and at the moment there is no known way of extracting it from drinking water. According to the Environment Agency (EA), between 2009 and 2011 concentrations of metaldehyde used by farmers to protect their crops from slugs were found in 81 of 647 (that is, one in eight) reservoirs, rivers and groundwater in England and Wales from which drinking water was being sourced. In summer 2013, Natural England and the EA recorded that homes in Essex and Suffolk, drawing supplies from the River Stour, had to drink tap water containing a hundred times the recommended level of 0.1ppb! Ferric phosphate pellets stop slugs feeding. Evidence shows that they attract rats.

If slugs are targeting your plants, go out at dusk and pick them off your plants and big-boot stem. Wear rubber gloves if you’re squeamish.

Why should it matter?

It matters because once a species disappears it disrupts a food chain. That species can never be recreated, so it’s gone forever. Climate change, intensive farming and loss of habitat have prompted steep declines within the last 40 years, but it can be reversed. I would like my great grandchildren to be able to see a hedgehog, watch a thrush and perhaps hear a cuckoo too - just as I did. It definitely matters!

Six wildlife strategies

Don’t use any chemicals in your garden. They dismantle food chains, build up toxicity and many have been shown to harm human health. So called ‘green insecticides’ and ‘green slug pellets’ are equally harmful to an ecosystem.

Don’t be too tidy. Have wilder, undisturbed edges to your garden - this will provide a refuge for slug-eating ground beetles - particularly in winter. Try to include some rotting wood and an insect hotel. The RSPB website shows you how to make your own.

Make a mini-meadow by allowing some grass to grow. Give this a cut in early autumn. This will boost the number of brown butterflies, meadow browns, ringlets and gatekeepers, and also shelter small mammals.

Make a pond, if you can, to encourage amphibians. They are in decline due to loss of habitat and the wetting agents used in weed killers have harmed them in the past.

Have a supply of flowers for as many months as possible. Early flying bees are need in of nectar from February onwards. Late nectar also helps pollinators.

Grow lots of different flowering plants in a range of colours and types. Umbellifers (cow-parsley-like plants) and orange flowers attract hoverflies. Long tubes and pallid colours suit moths and some butterflies. Bees are attracted to blue. A mixture of trees, shrubs, perennials, annuals and grasses encourages biodiversity.


Ten useful allies

Ground beetles

Large beetles with rounded bodies, also known as carabids, are abundant in chemical-free gardens. They’re voracious predators and their predatory larvae are totally carnivorous, although the adults are omnivorous, sometimes eating plant material and carrion. The violet ground beetle (Carabus violaceus) can devour five slugs in four days.

Ladybirds

Eco-friendly gardens could have 5 aphid-eating species. A 7-spot adult could consume 5,000 whole aphids over its lifetime, according to the UK Ladybird Survey (also Rothamsted Research).

Spiders

These soft-bodied creatures need a chemical-free environment. They thrive in grassy areas. A single acre of hay meadow is estimated to contain 2.25 million spiders. Each of these will eat two insects a week for six months, which works out at about 108 million insects!

Earthworms

The third preferred food of hedgehogs, after beetles and caterpillars. Worms tunnel in soil and eat organic matter, pulling down leaves for instance. They excrete mineral-packed castings and maintain healthy soil.

Hoverflies

Hoverflies have tiny mouthparts and they need simply shaped flowers or clusters of tiny flowers as in scabious and astrantia. Their larvae are more effective at finding aphids than ladybirds, even though they are blind. They eat aphids more quickly and can reach into tighter spaces than ladybirds.

Parasitic wasps

Parasitic flies target pests, by laying eggs inside their bodies. The cabbage white butterfly is predated by Cotesia glomerate. This takes place as the caterpillars emerge from their eggs. Eventually sulphur-yellow cocoons rupture the adult caterpillar’s body.

Social wasps

These meat-eating bees are great predators and you shouldn’t persecute them. Plant nectar-rich, late-summer flowers (such as crocosmia and kniphofia) to satisfy their sweet tooth.

Mason, mining and solitary bees

There are more than 275 species of solitary bee in Britain and over 90 per cent of those are not social and do not live in colonies. Some resemble small wasps, others look like small bees and some look almost ant-like, to me anyway. They are fantastic pollinators.

Lacewings

Lacewing adults often feed on honeydew in trees. However the female green lacewing lays eggs in batches on stalks, sticking them on with a drop of gummy fluid. We’ve seen them suspended from scabious flowers, like swings on a carousel. Lacewing larvae are particularly voracious creatures.

Woodlice

Lots of creatures eat woodlice and they generally cause little damage to plants. They break down plant material, into compost, and overturn the soil.

Val Bourne is the author of The Living Jigsaw, Kew Publishing

Living Jigsaw 

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