Root aphids

Root aphids are not easily seen, as they live on the roots of plants, sucking sap. Sometimes mistaken for nutrient deficiencies, they can cause withered leaves and reduce vigour.

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Close-up of root aphids on <em>Crassula</em> root RHS / Tim Sandall
Close-up of root aphids on Crassula root RHS / Tim Sandall

Quick facts

Common name Root aphids
Scientific names Several species
Host plants A wide variety of plants; weeds, grasses, vegetables, Primula
Main symptoms Wilting, stunting, discoloured leaves, lack vigour
Most active Spring to late Summer

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What are root aphids?

Root aphids are aphid species defined by their habit of feeding on plant roots during part of their lifecycle. In general they are 2-3 mm long, colour varies depending on species, environmental conditions and host plant, they can be yellow, green or brown often with a white waxy appearance. They can be winged or wingless depending on life stage. On some plants a tell-tale sign that you have root aphids is the waxy white secretions that build up on roots, although this can be confused with root mealybugs. Root aphids can be attended by ants feeding on honeydew. The ants can protect overwintering aphid eggs, and in the spring and summer, sometimes carry the root aphids through the soil from one host plant to another.
 
Many root aphids feed on grasses and weeds (secondary hosts), not on garden plants, those often found in the garden or allotment include Rose root aphid, Lettuce root aphid, Cabbage root aphid, Carrot root aphid, Auricula root aphid and Bean root aphid. Large populations are most damaging to young plants and symptoms include wilting, stunted growth, unhealthy colored foliage followed by premature leaf loss. Other factors, especially drought, can have similar effects so examination of the roots may be required. On some plants, such as rose, root aphids have no impact on plant health although eggs laid may be noticed in the autumn.
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Wilting on runner bean (Phaseolus coccineus) caused by root aphids, S M Brown
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Root aphids on runner bean (Phaseolus coccineus), S M Brown
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Wilting on runner bean (Phaseolus coccineus) caused by root aphids, S M Brown
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Root aphids on runner bean (Phaseolus coccineus), S M Brown
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Wilting on runner bean (Phaseolus coccineus) caused by root aphids, S M Brown
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Management

Aphids form the basis of many food chains in the garden and it is not unusual to have some of these animals in a healthy balanced garden ecosystem. Root aphids are difficult insects to eliminate as their life in the soil gives them protection from many control methods. Good plant care will often alleviate the symptoms caused by these aphids.
 
  • Where possible tolerate populations of aphids
  • Be aware that in spring aphid populations often build up before natural enemies are active in sufficient numbers and then give good control. 
  • Check the roots of plug plants or plants in pots or before planting or re-potting. This can help to expose root aphids
  • Keeping plants well watered and fed will help reduce the stress that root aphids can cause plants
  • Washing away old compost from roots with aphids and re-potting may eliminate most of the insects but there is the possibility that some may remain unnoticed
  • Take cuttings of healthy parts and remove as much of the root material with aphids from the soil as possible. Then bury or add to a compost heap
  • Avoid growing susceptible plants in the same soil or site for a year, crop rotation is recommended
  • Cover vegetables with fine insect-proof mesh from May to August to prevent entry from winged aphids
  • Some lettuce cultivars are advertised as tolerant to root aphid: 'Debby', 'Lakeland', 'Beatrice', 'Avon defiance', 'Avon crisp' and 'Salad Bowl' 

Biology

Root aphids are not extensively studied and the status of a number of the species is not known. Most alternate between a woody host and a herbaceous plant. A majority are found on grasses and do not affect garden plants. Some further details of species gardeners may come across are given below.


Genus Pemphigus

Aphids within this genus are small to medium-sized (body length range between 2-3 mm), yellowish green or greyish-green aphids which in spring to early summer cause galls on the leaves, petioles or branches of their primary host Populus (poplar). The differences between species within the genus are small; they are more easily told apart based on the appearance, positioning and shape of the galls they induce and the secondary hosts they feed on. Secondary hosts include plants in the daisy family (Asteraceae) and the carrot family (Apiaceae), whilst some Pemphigus species remain on one host all year. Although the leaf galls on poplar trees may attract attention, they are harmless to the tree. Many species can be attended by ants when present on the secondary host but not on the primary host.

Lettuce-root aphid (Pemphigus bursarius) alternates between its primary host Poplar and members of the daisy family (Asteraceae), especially lettuce. This aphid overwinters as eggs in cracks in bark. In spring the eggs hatch and cause pouch-like galls on the leaf stalks; each gall contains 100-250. By midsummer the winged forms of the aphid develop and disperse to lettuce and other Asteraceae hosts and feed on the roots. The yellow/white aphids become covered in a greyish waxy covering. They reach maximum numbers in August. By late summer, winged forms leave to find the winter host trees to mate and lay eggs. However, some lettuce aphids overwinter in the soil.

Cabbage-root aphid (Pemphigus populitransversus) alternates between Poplar and the roots of plants in the cabbage family (Brassicaceae) e.g. Brassica, Coronopus, Eutrema, Leodium, Rorippa.

Carrot-root aphid (Pemphigus phenax) alternates between poplar and cultivated or wild carrot (Daucus carota). Eggs overwinter on bark and hatch in the spring. Young aphids then move to the unfurling leaves where they induce the formation of midrib galls. Each gall becomes elongate, a somewhat wrinkled, reddish swelling (often tinged with yellow laterally), packed with numerous wax-secreting aphids. In summer, winged forms migrate to their secondary host (carrot) where dense colonies develop on the roots amongst copious amounts of white waxen wool. There is a return migration to poplar in the autumn.


Genus Thecabius

Aphids within the genus Thecabius are small to medium-sized aphids usually 2-3 mm in length. There are about 17 species within the genus, morphological differences between species are slight and they can be very similar in appearance to species in the genus Pemphigus. Most host alternate, inducing galls on leaves, petioles, or branches of Poplar and the secondary host is the roots or the stems of plants such as buttercups (Ranunculus: Ranunculaceae), creeping jenny (Lysimachia: Myrsinaceae), sallow (Salix) and primrose (Primula: Primulaceae). Both primary and secondary hosts are not attended by ants.

Auricula-root aphid (Thecabius auriculae) is a species within this genus that gardeners may encounter. They are off-white to brownish aphids that occur on the roots of Primula auricula and related species throughout the year. Plants in pots or greenhouses are stunted, the leaves turn yellow and in severe cases plants become disfigured.


Genus Smynthurodes

Bean-root aphid (Smynthurodes betae), is the only species in this genus. It is known for feeding on French and runner bean roots, affecting plant vigour. Secondary hosts are often attended to by ants. These aphids are small 1.6-2.7 mm, globular, dirty yellowish white, hairy and dusted with a waxy substance. The primary host is Pistachio (Pistascia) whilst there are a wide variety of secondary hosts; Asteraceae (Artemisia, Arctium), Fabaceae (Phaseolus, Vicia, Trifolium) and Solanaceae (Solanum tuberosum, S. nigrum, Lycopersicon esculentum); they also sometimes occur on Beta, Brassica, Capsella, Gossypium, Heliotropum, Rumex etc.. Sexual reproduction occurs on the primary host, where this is not present or common, such as in the UK, this aphid reproduce asexually on secondary hosts. 

On the primary host (Pistachio trees), Smynthurodes betae induce small red midrib galls. This aphid is common and found worldwide.

Further information and images of many species are available at: https://influentialpoints.com/Gallery/Aphid_genera.htm
 

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