Garden Q&A: Why are our gardens being invaded by aphids?

Irish Examiner

2 days ago

The RHS was right about an invasion of sap-sucking aphids thanks to a warm spring, if my garden is anything to go by.
In the past three weeks, I've seen wafts of greenfly smother the stems and buds of pretty flowering perennials and roses, while trademark trails of blackfly have appeared on my clematis.
Back in May, the RHS reported that the warm spring has encouraged rapid and early colony growth, with many aphids to be found now on acers, roses and honeysuckle. In addition, some previously glasshouse-bound species are now thought to be venturing into beds and borders and finding new plant hosts to feed on.
(Georgi Mabee/RHS/PA)
'This is the time of year when you'd expect to see more aphids. Aphids will overwinter or there will be eggs which will hatch out, and then populations build up,' says Dr Stephanie Bird, RHS senior plant health scientist.
While last year, the slug population exploded in the warm, wet weather, 2025 is the year of the aphid, the RHS has reported, and the number of inquiries it has received up to May about these tiny sap-sucking insects has risen.
'A lot of the aphids we have on our garden plants will have a different summer host that they'll start moving to, and this is the point in the year when predators haven't actually started controlling populations,' says Bird.
You'll see aphids in shades of red, yellow, black, green, brown or pink, some of which feed on specific plants, others which will suck the sap out of a wide host range.
They are weather-dependent
(Alamy/PA)
The weather will affect populations, as aphids will thrive in a warm, dry spring and summer.
'Aphids' life cycles are temperature dependent, and the slightly warmer weather means they are able to reproduce and build up their numbers slightly quicker,' she explains. And warmer winters lead to earlier activity, she adds.
Aphids don't need a partner to reproduce
'They don't need to find a partner and can reproduce asexually. It's a bit like Russian dolls. Inside one aphid, there is another aphid, and inside that aphid, there's another one,' says Bird. Their populations can bloom rapidly.
How much damage can aphids do?
Many plants tolerate their feeding with no ill effects, but colonies can cause a lack of plant vigour, distorted growth, and they often leave a coating of their sticky excretions called honeydew on which sooty moulds can grow. Some aphids also transmit plant viruses.
'They're unlikely to kill plants, especially established ones,' says Bird. 'For instance, if you've a cherry tree which has distorted leaves which are black and sticky, and is covered in cherry black aphids, you will still get cherries off that tree.'
Nor should they kill general garden plants — they just make them look unsightly, she adds.
Who are their natural enemies?
(Alamy/PA)
Parasitoid organisms, such as wasp larvae, develop inside the aphids and eventually kill them. Other predators include birds, predatory midges, earwigs, ladybirds and their larvae, lacewing larvae and hoverfly larvae.
'Hoverflies are beneficial pollinating insects and their larvae feed on aphids,' says Bird.
'Having aphids in your garden is part of the natural garden ecosystem. You need these kinds of insects to feed things higher up in the food chain,' she adds.
Hayley Jones, principal entomologist at the RHS, continues: 'Understanding how they can be managed through simple planting choices or encouragement of natural predators will help minimise damage and potential plant losses while maintaining a healthy garden ecosystem.'
How can we deter them?
Encourage natural predators (Alamy/PA)
The RHS advises gardeners to tolerate them where possible, but if you can't, use your finger and thumb to squash colonies and check plants frequently before the damage has developed. Don't use pesticides, it says, because they reduce biodiversity, including natural enemies, and impact soil health.
Encourage natural enemies of aphids into your garden by planting pollinator-friendly plants.
Susceptible plants
Aphids are attracted to many plants. Some are specific to which plant they will invade, such as rose aphids. Others will inhabit many different plants.
Aphids on a rose bud. Picture: Rachael Tanner/RHS/PA
'They generally inhabit the same sorts of places, so you'll find them a lot of the time on the undersides of leaves, but they also like the developing softer tissues around flowers and buds,' says Bird.
You won't notice aphids on plants such as ornamental grasses and ferns, Bird points out.