Are these insects on my plant pests, or are they eating the pests?
Identifying the predator larvae that get squashed by mistake, and why spraying a plant that already has them usually makes the infestation worse.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 4 min
Short answer
Before treating anything, look for a full minute. Slate-grey spiky larvae with orange spots are ladybird young; small translucent maggots sliding among the aphids are hoverfly larvae; flat brown things with curved jaws are lacewing larvae; bronze swollen aphids with a round hole are parasitised. If any are present, wash the aphids off with water and leave it alone.
The most damaging thing an anxious gardener does is spray a plant that had already been colonised by its own pest control. Broad-spectrum sprays kill predators reliably — they are exposed on the leaf surface, they are larger, and they do not breed as fast — while killing pests only partly. Two weeks later the aphids are back and nothing is eating them.
Step by step
- Learn the ladybird larva, because it looks nothing like a ladybird.Long, tapering, slightly spiky, slate grey to black with orange or yellow markings, and moving quickly over the shoots. It is routinely mistaken for a pest and squashed. One larva eats hundreds of aphids before it pupates.
- Learn the ladybird pupa too.A stationary orange and black lump glued to a leaf, faintly ribbed, that does not move when touched. It is the next generation, three days from emerging.
- Learn the hoverfly larva.A small legless translucent maggot, greenish, brownish or pinkish, sliding slowly across a leaf among the aphids. It is unglamorous and it is probably the most effective aphid predator in a British garden. It looks like something unpleasant, which is why it gets wiped off.
- Learn the lacewing larva.Flat, brown, tapering, with long curved jaws held out in front like a pair of tongs. Some species stick the drained husks of their prey onto their backs as camouflage, so it can look like a walking piece of debris.
- Look for aphid mummies.A swollen, papery, bronze or straw-coloured aphid glued to the leaf, often with a neat round exit hole. A parasitic wasp has bred inside it. A colony full of mummies is already collapsing and needs nothing from you.
- Recognise the harmless bystanders.Ground beetles, centipedes, spiders, harvestmen, earthworms, woodlice on dead material, most flies, and adult hoverflies feeding at flowers. Earwigs are genuinely both — they damage dahlia and clematis flowers, and they eat aphids and codling moth eggs in fruit trees.
- Then identify the actual pests by shape.Aphids are pear-shaped with two small tubes at the rear, clustered on soft tips. Whitefly are tiny white moth-like insects on leaf undersides that rise in a cloud. Capsid bugs are fast, green and shield-shaped and drop the moment a leaf is touched. Adult vine weevils are dull black, long-snouted, and only found after dark.
- Choose the least destructive control that works.A jet of water removes most of an aphid colony and harms nothing else. Squashing by hand is targeted. Removing badly infested tips takes the colony with it. All three leave the predators in place, and the predators are the reason the problem does not come back.
- Plant for the predators.Adult hoverflies and lacewings feed on nectar and pollen from open, shallow flowers — fennel, dill, poached egg plant, marigolds, umbellifers. A garden with those in it recovers from aphid outbreaks without intervention.
Tips
- A hand lens costs very little and turns this from guesswork into identification. It is the single most useful diagnostic tool in a garden.
- Aphid numbers in a garden usually peak and then crash without any help, because the predators are one generation behind them. Waiting a fortnight is a legitimate strategy.
- Ants running up a stem mean sap-suckers above, because ants farm them for honeydew and drive predators off. Deal with the ants and the predators return.
- Do not judge an insect by whether it looks unpleasant. Almost all the useful ones do.
Common mistakes
- Squashing ladybird larvae — They are the single most commonly killed beneficial insect in gardens, purely because they do not look like ladybirds.
- Spraying at the first sight of aphids — It removes the food source before the predators arrive, so the predators go elsewhere and the next outbreak has nothing eating it.
- Treating every small creature in the compost as a pest — Worms, woodlice, springtails and most compost life are decomposers. The exceptions worth knowing are vine weevil grubs and root aphids.
Questions people ask
Should I buy ladybirds or lacewings?
Released adults usually fly away. Bought biological controls are worth it in the enclosed conditions they were designed for — a greenhouse or conservatory — where predatory mites and parasitic wasps work well. Outdoors, planting for them beats buying them.
Are the black insects on my flowers pests?
Pollen beetles, common on yellow flowers and on cut sweet peas, are a nuisance rather than damaging. Leaving cut flowers in a dark room with one light source draws them out.
What about ants — are they doing harm?
Not directly to a healthy plant. They matter because they protect and move aphids, mealybug and scale, and because their nests can dry out the roots of a plant in a pot.