Life cycle and identification of crop damage caused by the willow–carrot aphid
Willow–carrot aphids affect carrot, celery, parsnip, and parsley. Severe infestations can distort foliage, stunt growth, and kill very young plants. This aphid can also transmit Potato virus Y (PVY) and Potato virus A (PVA), although it is less important as a vector than the peach-potato aphid.
Risk factors in carrots and potatoes
- Dry, sunny weather in late May/June favours a large-scale migration to host crops. Conversely, cold, rainy weather inhibits migration
- Nearby umbelliferous plants can act as an alternative host
- Increased risk if virus transmission coincides with an early crop growth stage
- Insecticidal control of the aphids may have little effect on virus transmission
Scientific name: Cavariella aegopodii
Wingless adults are 1–2.6 mm long, green, or yellowish–green, elongate oval and somewhat flattened. There are two tubes (siphunculi) at the rear end, swollen towards the tips. A small outgrowth is present above the tail.
Winged adults are darker and have a black patch on the upper surface of the abdomen.
Willow–carrot aphid life cycle and crop damage
Dec–Jan: The aphid mainly overwinters as eggs around willow buds. In warmer conditions, some overwinter as mobile stages on crops in field storage or on wild hosts.
Feb–Mar: Eggs hatch.
Mar–Apr: The aphids feed and reproduce on willow.
May–Jul: Winged aphids migrate to other host plants, including carrot, over a 5–6week period, usually with a peak in early June. Late seasons can delay migration for 2–3 weeks.
Aug–Sep: Further winged generations disperse to wild hosts.
Oct–Nov: Winged aphids migrate to willow to mate and lay eggs.
Typically, aphids infest carrots at the cotyledon stage, but can also invade older plants. When many are present, the leaves may be discoloured, distorted and shiny from honeydew excretion. The crop may become covered with cast skins.
Feeding damage may be confused with carrot fly damage and, sometimes, drought stress.
The aphid is the vector of Parsnip yellow fleck virus (PYFV) and the two viruses that cause carrot motley dwarf disease. PYFV can cause stunted plants and blackening of the central core. Carrot motley dwarf disease produces a yellow mottling of the leaves and stunts the plants.
It can also transmit Carrot red leaf virus (CtRLV), Parsnip mosaic virus (ParMV) and Celery mosaic virus (CeMV).
Non-chemical and chemical control
Natural enemies, including ladybirds and parasitoid wasps, attack mobile forms of this pest.
The efficacy of several biopesticides, but not biological control, is under evaluation in the UK.
A day-degree forecast has been developed to predict the start of the migration from willow to carrot and other host plants.
Resistance to pyrethroids has been confirmed in the UK.