Clusters of greenfly on a rose shoot with ants running up and down the stem is one of the most familiar sights in a UK garden, and it often prompts a search for pest control. The Royal Horticultural Society (RHS) publishes separate advice pages on aphids and on ants, and they explain why the two are connected and why the RHS generally recommends tolerance. This article summarises those two pages. It covers gardens in the UK, describes RHS advice rather than legal requirements, and is general information. Ants inside a building are a different problem, covered in the site’s article on ants indoors and why identification matters before treatment.
What aphids are
The RHS describes aphids as sap-sucking true bugs, 1 to 7 mm long. Some are the familiar greenfly or blackfly, but there are yellow, pink, white and mottled species. It says there are more than 500 aphid species in Britain, that some feed on only one or two plant species while others use many, and that almost any plant can host them, including ornamentals, vegetables, fruit, greenhouse plants and houseplants. Most feed on foliage, stems and flowers, but some feed on roots. A few species, such as those on poplar, cause galls, and woolly species can be confused with scale insects, mealybug or whitefly.
What the damage looks like
Colonies can usually be seen with the naked eye, often on shoot tips, flower buds and the underside of younger leaves. Symptoms the RHS lists include stunted growth, curled or distorted leaves, sticky honeydew on which black sooty mould grows, and white cast skins on the upper leaf surface. The RHS says sooty mould can be wiped off with a damp cloth and that aphids very rarely kill plants. It does flag that some aphids transmit plant viruses, which is a particular problem on soft fruit such as strawberry and raspberry, on some vegetables including tomatoes and members of the cucumber and marrow family, and on ornamentals such as dahlias, lilies, pelargoniums, tulips and sweet peas. Virus-affected plants should be destroyed to prevent spread.
Where ants come in
The RHS ant page says ants collect the honeydew that aphids and some other sap-feeding insects excrete. Ants can protect aphids from ladybirds and other predators to secure the supply, and the aphid page adds that ants tend the aphids and remove aphid predators. On the ant page the RHS notes that increased aphid numbers may mean more damage to plants. The aphid page adds that ants found climbing a plant may be tending an aphid colony.
Ants have their own effects. According to the RHS they do little direct damage, but they can disturb soil around roots and push excavated soil to the surface during nest building. That can be an issue on lawns and where low-growing plants are buried by soil. Ants may also disturb roots in pots and containers, and this disturbance can make plants more prone to wilting when dry at the roots. Nests are frequent in lawns, flower pots, compost bins and among plant roots. The RHS says an ant nest in a compost heap or bin is not causing damage in that situation. It adds that some ants, mostly Myrmica or “red ants”, can sting, though for most people this is no more than a minor irritation. More than 30 species are found in Britain, and the black garden ant Lasius niger is the familiar one in gardens.
The RHS position on control
On aphids, the RHS says to tolerate them where possible, that it is usual to have some in a healthy garden, and that they are a vital food source for wildlife. It says gardeners do not have to kill or control them. If control is wanted, its answer to whether to use pesticides is no, because most pesticides, including organic types, reduce biodiversity including natural enemies, affect soil health and have wider environmental effects. The steps it lists are:
- check plants frequently so that action can be taken before damage develops;
- squash colonies with finger and thumb;
- encourage natural enemies such as ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs, noting that spring aphid populations rise before enough natural enemies are active, so waiting often lets them do the work; and
- not bother trying to control aphids on trees, where natural enemies normally reduce numbers.
The RHS adds that some natural enemies, including hoverfly larvae, lacewing larvae and several parasitoid wasps, can be bought as biological control for greenhouses.
For ants, the RHS says they should be tolerated wherever possible and that unless nests are particularly troublesome they are best left alone. It also warns that if a colony is destroyed, incoming queen ants are likely to take over the territory and may establish even more new nests.
Lawns, pots and nematodes
For lawn ant heaps the RHS advises brushing the excavated soil away on a dry day before mowing, because otherwise the mower smears the soil on the surface. If years of ant activity have made the lawn uneven, it suggests peeling back turf on the raised areas, removing excess soil and relaying the turf, which is easier in winter when ants are less active. Where control is judged necessary, it says a pathogenic nematode, Steinernema feltiae, is available from some suppliers of biological controls for treating nests in lawns and flower beds. The nematodes are watered into the soil where ants are bringing soil to the surface, and the RHS says they may also affect non-target insects.
The bottom line
The RHS treats aphids and ants as connected parts of a garden’s wildlife rather than problems to be eliminated: ants farm aphids for honeydew, aphids seldom kill plants and pesticides do more harm than good in its view. Squashing colonies, brushing away lawn ant heaps and letting natural enemies build up are its first-line options for UK gardens.
Sources
- Royal Horticultural Society, “Aphids” (rhs.org.uk)
- Royal Horticultural Society, “Ants in the Garden: Helpful or Harmful?” (rhs.org.uk)