Why Are There Aphids on My Rhubarb Leaves?

Finding colonies of small, soft-bodied insects on the undersides of rhubarb leaves — usually accompanied by sticky honeydew deposits and possibly a dark sooty mould on the upper surfaces — is an aphid infestation. Several aphid species can infest rhubarb, including the dock aphid and peach-potato aphid, which visit rhubarb as a secondary host. The good news is that aphid infestations on rhubarb are rarely a serious problem for the plant, and the harvest (the stalks) is not directly affected.

Do aphids damage rhubarb stalks?

Aphids feed on the soft leaf tissue and may cause some leaf distortion and yellowing in heavily-infested leaves, but they do not infest the stalks directly. Since we eat only the stalks, an aphid infestation on the leaves does not affect the edibility or quality of the harvest. A plant with very large leaf area can tolerate considerable aphid pressure without any impact on stalk production.

Natural predators keep populations in check

Aphid populations on rhubarb are typically brought under control naturally by ladybirds, lacewing larvae, parasitic wasps and hoverfly larvae. If you observe a large aphid colony in spring or early summer, check again two weeks later — natural predators will often have significantly reduced or eliminated the population without intervention. Avoid broad-spectrum insecticides that kill these beneficial insects.

When to intervene

If the infestation is very large and causing significant leaf distortion on multiple plants, a strong jet of water from a hose applied to the undersides of leaves physically displaces aphids. Insecticidal soap spray is the organic standby and is effective. Treating small colonies before they build up to damaging levels — particularly in early spring — is easier than managing large late-summer populations.