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Coolendel - How to control garden pests without using chemicals


Gardens How to control garden pests without using chemicals

You can squash them, hunt them — or let predatory insects such as dragonflies and ladybirds do the dirty work for you

Jane Owen SEPTEMBER 29 2023

A 7cm southern hawker dragonfly is eyeballing me or, more accurately, eyeing me with 30,000 of its lenses. As I wade further into the pond to pull out long green strands of blanket weed waving over the bale of barley straw, which is supposed to kill off the wretched stuff (just saying), the green, black and blue entomological biplane lunges at me.

I am not a wimp. I have stood up to Fleet Street editors, braved Royal Horticultural Society judges and dangled from the end of a rope out of a Royal Navy helicopter. But the southern hawker’s close attention is unnerving.

“I can see you’re trying to defend your territory so your mate can lay her eggs. But please, can you back off?”

The beast lunges at my face. “Your attitude to garden insects stinks,” it says. Or seems to — of course, it might all be a product of my guilty conscience. “You need predators like us.”

In the interest of saving the planet, I have bought toe-curlingly expensive biological controls to use against red spider mites, chafer grubs, slugs and other irritants. I rarely use chemical control, which means I have to put up with censure and ridicule for letting weeds thrive: all so insects such as this southern hawker can feed, shelter and party.

But then, I have pulled out nettles, which are food for valuable pollinators red admiral, small tortoise-shell, painted lady and comma butterflies. All year long, I’ve had to squash lily beetles, aphids, vine weevils — I could go on and, unfortunately, insect and other garden pests do go on and on through our warm and fuggy autumns and winters.

The southern hawker is right. The time when many UK gardeners could depend on chilly winters to kill most garden insect pests is long gone. Instead, increasingly, we have to depend on alternative controls, including predator insects.

A rose garden at RHS Wisley, in Surrey, where chemical pest control is discouraged © Jason Ingram Dragonflies have been on the earth roughly 300mn years, long before dinosaurs, who managed only 165mn years, and long before we started forming up through the primordial soup to screw up the planet. They eat 10-20 per cent of their bodyweight in greenfly, whitefly, flies, midges and just about every other garden insect every day.

Right now until late October, dragonflies are doing their last rounds before leaving their larvae and eggs in readiness for next year’s pest feast.

Like lacewings, ladybirds and many other beneficial insects, dragonflies are excellent pest predators, says Andrew Salisbury, head of plant health at the Royal Horticultural Society. Among his responsibilities is pest control at the 240-acre RHS Wisley garden in southern England. Some of its planting includes lilies, which have been under attack from the lily beetle since the 1940s. “Some patches of lily are stripped of leaves, but only occasionally does a patch die out, and this may not be due to the beetle,” he says. “Staff try to check lilies, but they don’t have time to pick off the beetles and their larvae.”

Picking them off is not for the faint- hearted. The larvae cover themselves in poo to discourage predators.

Luckily, Salisbury explains, three species of tiny parasitic wasp (Tetrastichus setifer, Lemophagus errabundus and Diaparsis jucunda, since you ask), undeterred by poo, kill the larvae by laying their eggs inside the creatures. These predators are widespread in New England and increasingly active in the UK.

With 1.4bn insects for every person on earth, the total weight of insects is 70 times more than all the people  

Chemical controls are available but not particularly effective. In my experience of battling the lily beetle for 20 years or so, it’s best to leave the dirty work to birds, frogs, beetles and tiny wasps. This chemical-free approach may sound effortless, but while it’s easy enough to leave fallen branches — or even trees, if you have the space — to form natural “bug hotels”, hand weeding takes a lot of back-breaking work. And although I spend less time mowing (which itself increases food and shelter for pest predators), I dig out hogweed, dock, nettle and brambles in the grass visible from the house, for inexcusable aesthetic reasons.

At least I’m not flying in the face of RHS advice on pesticides. “[The RHS] recommend avoiding pesticide controls,” says Salisbury. “For a healthy garden, it’s best to tolerate some damage, grow vigorous plants and encourage natural predators. Only then should you consider using biological control.”

Insecticides, which had a global market value of nearly $84.5bn in 2019 according to the Heinrich Böll Foundation’s 2022 Green Atlas, create one of many pressures on insects. Some of these pressures are outlined by the State of Nature Report 2019, produced by more than 50 organisations involved in recording, researching and conservation of nature in the UK and its Overseas Territories. It found that the abundance and distribution of UK species has declined since 1970. The report raised concerns about pollinators and other beneficial insects, largely related to the use of pesticides such as neonicotinoids.

A report in Science from April 2020 confirmed these worrying trends. It calculated that between 1925 and 2018, the average decline of terrestrial insect abundance measured about 9 per cent per decade.

Threatened insects include earwigs, which eat codling moth, the cause of maggoty apples, pears and even walnuts (as the RHS head of science Alistair Griffiths explained to me at the FT Weekend Festival, when I made the mistake of moaning about earwig attacks on dahlias). Since the noughties, orchard owners have reported plummeting earwig numbers.

In the past 20 years, UK flying insects have declined by 60 per cent, according to research from the government’s Insect Decline and UK Food Security Committee 2023.

Know your enemy

Vine weevil © agrofruti/Alamy Chafer grubs and leatherjackets can be left to badgers, crows, jays and foxes, although they do reveal the bare patches where the pests have eaten the grass roots.

Vine weevils When a pot plant suddenly droops, turn it out and pick out any maggot-like vine weevil grubs from the root remains. Leave the revealed root remains overnight and birds will remove the remaining pests. Nematodes kill them but prevention is better: hunt the beetle-like weevils on summer evenings after dark.

Aphids Squashing them is effective. Birds love eating aphids as do ladybirds, lacewings and not forgetting dragonflies. A powerful jet of water works too. Sap-sucking scale insects tend to stick to the underside of leaves. Scraping them off works but ladybirds, lacewings and parasitic wasps do a better job.  

Red spider mites Like so many insect pests, they hide on the underside of leaves. Greenhouse plants are vulnerable but the mite is active outside too, as I know from bitter experience. They killed a purple-flowered Tibouchina urvilleana given to me by the great gardening author Christopher Lloyd. Natural predators include lacewings and minute pirate bugs, which can be encouraged with nectar-rich flowers. High humidity will also keep these pests at bay. 

Without chemical control what is left in the garden defence tool box? Rothamsted Research, which gave evidence to the same government committee, does at least offer an early warning system about aphids. Its weekly forecasts, intended for farmers, are accessible to everyone. Faced with dates for a predicted leap in aphid population, gardeners have time to buy and apply biological control, or just set aside more time for squashing or hosing the creatures. RHS members can also access a free gardening advice service.

While it may seem contradictory to harness insect power to kill insects, at least insect power leaves no harmful residue or negative after-effects. The RHS, the RSPB, the Natural History Museum and many other respected institutions advise against the use of pesticides and are in favour of insects and other biological controls as the best long-term options. So that means that we need insect-friendly gardens created by everything from low mowing to planting nectar-rich flora to bug hotels for overwintering beneficial insects such as lacewings and ladybirds.

Red spider mites © Tomasz Klejdysz/Alamy It’s a message trumpeted by Pestival, the brief-lived interspecies celebration of music, art and science, supported by insect specialists, as well as comedians such as Stewart Lee; Blur’s Graham Coxon and musician Robyn Hitchcock. Founder Bridget Nicholls, who did a three-year London Zoo residency, explains its genesis: “I was recovering from serious illness and living near the London Wetland Centre [in Barnes] when I had my Dorothy moment as I slowed down and noticed the myriad insects around me,” she says. “I was captivated by common insects like cabbage white butterflies. I saw ants under threat busily carrying their luminous eggs to safer positions. That summer, those insects saved me, so I vowed to save them. After all, no bugs, no us.”

True, but however fragile and enthralling many insects are, box tree moth caterpillars are destroying my extensive box topiary created over 25 years. When they appeared a couple of years ago, I tried pheromone traps, which instantly filled with the moths. The traps couldn’t keep up thanks to several 7-metre box-caterpillar-infested box trees in the lane beside our garden. I have admitted defeat, ripped out and burnt all my box, and vowed to start again when a reliable natural predator gets to work in this area.

The transition time needed for a garden to settle into a pest and predator balance is challenging and frustrating. And removing pests as they appear, rather than giving them a chemical blast when numbers overwhelm, takes time.

According to the Royal Entomological Society, there are about 1.4bn insects for every person on earth. In fact, the total weight of insects is about 70 times more than all the people.

And, without them, the planet is stuffed. Get rid of insects and there would be precious little pollination, vastly poorer soil health and less disease control — as I imagine that southern hawker would remind me, with a roll of its 30,000 lenses:

“Insects might be pests, but you lot are the most destructive pests of all.”

Can anything deter slugs and snails?

© Sabena Jane Blackbird/Alamy Slugs and snails, the bane of every gardener’s life, continue to munch through plants in all but the coldest UK winters. They lurk in and above the soil, ready to snaffle up winter salad seedlings, newly planted winter-flowering pansies and any other tender plant they detect.  

Here’s my experience of battling the blighters over many years. Physical barriers such as sand, eggshells and copper wire sometimes work but they’re a pain to maintain.

Grapefruit halves attract slugs and snails in droves, to be picked off every morning. Problem is that they attract so many into the garden that vegetables seem to do better without them! Nematodes, if applied at just the right temperature and moment, are highly effective, keeping slug damage to a minimum for several months.

‘Nature-friendly’ pellets use ferric phosphate as opposed to the regular slug pellets that include metaldehyde, but aren’t such good news, according to the Royal Society for the Protection of Birds. “Although ferric phosphate is less toxic than metaldehyde,” it says on the RSPB website, “the other ingredients in the tablets can also affect earthworms and . . . can poison pets.”

As far as I’m concerned, the most effective method is to go out after dark with a head torch and catch the creatures, particularly after rain when slugs and snails come out to play — and eat. 

Jane Owen is an FT contributing editor