Common garden pests and natural solutions

Most garden pests can be controlled well without resorting to broad-spectrum insecticides. In fact, resorting to them usually makes the problem worse in the medium term, because they also kill the beneficial insects — ladybirds, lacewings, hoverflies, parasitic wasps — that keep pests in check naturally. Without those allies, the pest bounces back stronger and unchecked, and you enter a cycle of spraying more and more.

The strategy that works is called integrated pest management and it's simple: identify the pest as early as possible, act with the mildest selective product that solves the case, and encourage the natural predators so they do 80% of the work the rest of the year. This guide has a table to identify the pest by the symptom you see; the detailed natural treatment of the eight most common pests; a month-by-month monitoring calendar; how to set up a garden that defends itself; and what to do when natural control isn't enough. At the end, twelve frequently asked questions.

How to identify which pest you have

The sticky "honeydew" and the black fungus (sooty mould) that grows on leaves and fruit almost always means you have aphids, scale or whitefly above: those insects excrete a sugary liquid and the black fungus lives on that sugar. If you get rid of the insect, the sooty mould dries out and washes away on its own with the rains.

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Aphids

The most common pest, especially in spring and on the tenderest shoots (roses, broad beans, fruit trees, oleander, hibiscus, viburnum). It sucks sap, deforms leaves, weakens the plant and transmits viruses. Step-by-step treatment, from mildest to strongest:

1. A jet of water under pressure onto the colonies. Repeated 2-3 days in a row, it's sometimes enough, and it doesn't harm the predators.
2. Insecticidal soap at 1-2% in water, sprayed at dusk, wetting the underside of the leaves and the shoots very well. Repeat after 5-7 days, 2 or 3 times.
3. Neem oil if the soap doesn't control it in two applications; its effect lasts longer.
4. Check the feeding: excess nitrogen gives soft, juicy growth that attracts aphids like a magnet. Switch to a more balanced fertiliser or one with more potassium.
5. Encourage the predators: plant marigold, fennel, dill, alyssum and coriander nearby; their flowers feed ladybirds, lacewings and hoverflies, which devour aphids by the hundred. A single ladybird larva eats 50 aphids a day.
6. Control the ants with a band of sticky glue on the trunk: if they stop "herding" and protecting the aphids, the predators can get to them.

Scale

Hard to treat because the shell (armoured scale, "San José scale") or the waxy coating (mealybug) protects the insect from almost anything you spray. It attacks citrus, bay, oleander, olive, houseplants put outdoors and many shrubs. Solution according to the size of the plant:

• Small plants: go over them with a cotton bud soaked in 70° alcohol, scale by scale. Laborious but very effective.
• Medium plants: spray neem oil, or insecticidal soap with a splash of alcohol, insisting every week for a month. The key is to reach the mobile larvae ("crawlers") as they hatch, which are vulnerable before forming the shell.
• Heavily infested branches: prune them and burn them or put them in the bin.
• Winter treatment: a winter (paraffin) oil on dormant deciduous fruit trees and shrubs smothers the eggs and overwintering forms.
• Biological control: the beetle Cryptolaemus montrouzieri ("mealybug destroyer") is very effective against mealybug in a greenhouse or enclosed garden.

Whitefly

Typical on tomato, hibiscus, verbena, geranium and houseplants. The adults and nymphs suck sap and excrete honeydew. Its cycle is fast (2-3 weeks) and populations explode in summer, so you have to persist. Combine three fronts:

• Yellow sticky traps hung among the plants at foliage height: they capture adults and act as an early warning and to see whether the population is going up or down.
• Insecticidal soap or neem oil on the underside of the leaves, 3-4 applications every 4-5 days to catch the ones that are hatching (the fixed nymphs are the most vulnerable).
• Ventilation: it's much worse in enclosed corners, poorly aired greenhouses and crowded plants. Space them out and air them.
• Biological control: the parasitic wasp Encarsia formosa in a greenhouse, widely used in tomato production.

Spider mite

It's a mite, not an insect, so many insecticides do nothing to it. It appears with heat and a dry atmosphere (summer, plants next to a wall that reflects heat, heated interiors, greenhouses). It causes fine yellow stippling that dries the leaf, and in serious cases webs covering the shoots and dozens of tiny red dots moving. Solution:

• Raise the ambient humidity: spray water over the plants on dry days, especially the underside, in the morning. Spider mite hates humidity and it greatly slows its reproduction.
• Wettable sulphur (which is also a miticide) or neem oil on the underside, repeating every 5-7 days for 2-3 weeks to break the cycle.
• Remove and destroy the most affected leaves and those with visible webbing.
• Biological control: the predatory mite Phytoseiulus persimilis, very effective in a greenhouse or enclosed garden.

Snails and slugs

They eat at night and in the wet, and wreak havoc on seedlings, hostas, dahlias, lettuces and tender flowers. One snail can devastate a row of freshly transplanted seedlings in one night. Combine several methods:

• Hand collection at dusk or after rain, with a torch. It's the most effective in the first few weeks, when you reduce the population all at once. Check under pots and stones.
• Barriers around sensitive plants: dry ash, crushed eggshell, coffee grounds, or a copper strip (it gives them a small galvanic shock and they won't cross it). Barriers need topping up after rain.
• Beer traps: a container sunk flush with the soil with a finger of beer; they fall in attracted by the yeast and drown. Empty and refill every 2-3 days.
• Ferric phosphate bait: approved in organic farming, low toxicity to pets, hedgehogs and birds (unlike metaldehyde, which is toxic and heavily restricted).
• Water in the morning, not at dusk: soil that is dry at night slows them down a lot.
• Favour their predators: hedgehogs, toads, shrews, birds, glow-worms and ground beetles. A pile of stones, an old log or a shallow pond gives them shelter.

Caterpillars

Butterfly and moth larvae that devour leaves: cabbage white, pine processionary (dangerous, don't touch it!), box tree moth, geranium bronze, tomato hornworm. For those that eat the leaves, the perfect selective treatment is Bacillus thuringiensis var. kurstaki (Bt): a bacterium that only affects lepidopteran larvae and is completely harmless to bees, ladybirds, pets and people. Spray it on the leaves when you spot the first holes; the caterpillars eat it, stop feeding within hours and die in 2-3 days. Repeat after rain or after a week, because the sun degrades it. For the box tree moth, also check the inside of the shrub with a torch and treat with Bt as soon as you see the webs and eaten leaves.

For the pine processionary, act in autumn with Bt on the tree when the caterpillars are small, remove and burn the nests in winter with thick gloves and respiratory protection (the urticating hairs are dangerous for people and very serious for dogs), and place pheromone traps and collar traps in spring.

Thrips, leaf miners and others

Thrips: tiny elongated insects that scrape the surface of leaves and petals, leaving silvery patches and tiny black dots of excrement. Worse in dry, hot weather, and on pale flowers (roses, gladioli, hibiscus). Use blue sticky traps and insecticidal soap or neem, insisting every 4-5 days. Remove the most affected flowers, where they shelter.

Leaf miner: the larva of a small fly makes winding tunnels inside the leaf (citrus, chard, chrysanthemum, tomato). Remove and destroy the tunnelled leaves, favour the parasitic wasps and, if it's serious on a young tree, apply neem when the first mines appear.

Vine weevil: the adult notches the leaf edges at night; the larva, white and curved, eats the roots underground and can kill the plant. Night-time collection of adults and, against the larvae, entomopathogenic nematodes (Heterorhabditis) watered into the soil in autumn.

Prevention: a garden that defends itself

1. Flower biodiversity. Have flowers all year round, especially umbellifers (fennel, dill, carrot in flower, coriander, celery in flower), herbs and small flowers, which feed the adult beneficial insects, whose larval forms are the ones that devour pests.
2. Shelters. An insect hotel, a pile of stones, a rotting old log, an area with uncollected leaf litter, a shallow pond with a stone to perch on.
3. Rotation and variety. In the veg garden, don't plant the same thing in the same spot every year. In the garden, avoid monocultures: a hedge of a single species is a buffet for its specific pest.
4. Weekly check. Look at the underside of the leaves of the most prone plants. Half a pest caught in time is solved with a single treatment; an established pest, with three or four and a bit of luck.
5. Strong plants. Water stress and excess nitrogen feeding attract pests. Correct watering and a balanced soil prevent more than any spray.
6. Quarantine. Check each new plant thoroughly before putting it in the garden, leaves top and bottom, and the soil in the pot. Most new pests get in this way, in a bought plant.
7. Hygiene. Remove very diseased plants, don't leave rotting remains and disinfect the secateurs between plants.

Monitoring calendar

• February-March: winter treatment with paraffin oil on fruit trees (aphid and scale eggs). First pheromone traps.
• March-May: aphids on tender shoots, snails after rain, cabbage caterpillar, first thrips on flowers.
• June-August: spider mite, whitefly, scale (peak crawler activity), thrips, leaf miner, adult vine weevil.
• September-October: second generation of aphids, autumn caterpillars, snails with the rains, processionary (apply Bt when the caterpillars are small).
• November-January: remove processionary nests, nematodes against vine weevil larvae, general clean-up and removal of infected leaf litter.

Frequently asked questions

Do home remedies like garlic, nettle or chilli work?

Nettle liquid feed (fermented 1-2 weeks) and garlic or chilli extracts have some repellent effect and strengthen the plant, useful as a support and prevention, but they rarely stop an established pest. Dishwashing liquid does NOT: it contains degreasers and perfumes that damage the leaves; use insecticidal soap, which is formulated for plants.

I have a lot of ants, are they a pest?

Ants themselves don't harm plants, but they "herd" and protect aphids and scale to milk their honeydew, and even move them from plant to plant. If you control the aphids, the ants lose interest and go away. A band of sticky glue around the trunk cuts the supply and leaves the aphids at the mercy of the predators.

When should I call a professional?

With notifiable quarantine pests (red palm weevil, Xylella fastidiosa, Popillia japonica beetle), with processionary in large trees near footpaths, or when a large tree is affected and the treatment requires height, equipment and sometimes an administrative permit.

How often do I need to repeat an insecticidal soap treatment?

Every 5-7 days, 2 or 3 times. It only kills by direct contact and leaves no residue, so you have to persist to reach the individuals hatching from the egg. If after three well-done applications (wetting the underside) it doesn't drop, switch to neem oil, which has a longer-lasting effect.

Can I treat edible plants with these products?

Yes: insecticidal soap, neem oil, Bacillus thuringiensis and sticky traps are approved in organic horticulture. Respect the safety interval stated on the label before harvesting (usually 0 to 3 days with these products) and wash the vegetables well.

Is neem oil dangerous for pets?

At garden doses and once dry on the plant, the risk is low. Even so, apply it at dusk, let it dry before dogs and cats access the area, and store it well sealed. With cats you need to be especially careful: don't apply it on plants the cat nibbles.

How do I tell a pest from a nutrient deficiency?

If you see insects, sticky honeydew, webbing or bite marks, it's a pest. If the leaves yellow uniformly (starting with the old or the young ones depending on the nutrient), with no spots or bugs, it's usually watering or soil pH and nutrients. If you see spots, white powder or pustules, it's a fungus: go to fungi on plants.

Is it any use releasing bought ladybirds in the garden?

Not much, if there are no flowers or shelter: released adult ladybirds fly off looking for a better spot. It's more effective to create the conditions (flowers for pollinators, no spraying, shelters) so they come and stay on their own. Predatory larvae bought for a specific outbreak in a greenhouse do work.

Can I prevent aphids without products?

To a large extent, yes: don't over-feed with nitrogen, have flowers for the predators from the start of spring, control the ants, and check and clean with pressurised water the first outbreaks before they explode.

Do ultrasonic devices and electronic repellents work against garden pests?

There is no evidence that they work against insects. For moles and rodents, the results are very erratic. The money is better invested in traps, barriers and favouring the natural predators.

And against the birds that eat the fruit?

Bird netting over small fruit trees and berry bushes during ripening, reflective tape, or simply plant extra and share. Never products or methods that harm the birds.