Root aphids

Aphids on the roots of canola plants are unusual and have the potential to weaken young crops

Where have they been reported?

Clusters of distinctive and particularly rounded or globular aphids were found on the roots of a canola crop (stem elongation) near Albury, in the NSW Riverina. The plants showed no above soil symptoms but seemed to have amassed extra fine root growth in response to the feeding. The aphids were abundant but did not appear to be holding back the crop.

The aphid causing this problem is a root aphid, an insect with distinctively different habits to those aphids more commonly associated with canola. The species is possibly the bean root aphid (Smynthurodes betae), a pest that occasionally causes problems in cotton, but rarely in other crops. Reports of root aphids in broadacre agriculture are generally uncommon, and more often confined to aphids colonising grasses and cereals (e.g. pasture root aphids (Aploneura lentisci) and cereals (Rhopalosiphum rufiabdominalis).

Habits of root aphids

As the name suggests, root aphids live underground on plant roots, and colonies can develop anywhere in the root system from the crown to deep within the soil profile. They cause damage by sucking sap from the plant tissues, which weakens the plant. Feeding can gradually thin and weaken plants, and the effects are amplified when plants are stressed by other factors such as drought, disease or attack from other insects. Within a drill row, individual plants hosting high levels of root aphid may be smaller or paler, and carry dead material around their base.

It is not uncommon for root aphids to be tended by ants, which protect and sometimes transport the aphids to different plants and, in return, feed from their excreted honeydew. The full extent of the distribution and pest status of the bean root aphid is unclear.

Control options

To check crops, use a spade to dig up soil to a depth of approximately 20 cm, and then break up the soil over a plastic sheet. Insecticide applications are largely ineffective against root aphids due to their soil-inhabiting habits. Thresholds for damage are difficult to determine because they depend on environmental conditions and other plant stresses.

Description

Adults of bean root aphids are 3 mm long. They are usually cream or pale yellow in colour and have very small antennae and no visible siphunculus (see I-SPY manual, Section 4). Colonies of these aphids may be tended by ants that construct small chambers to allow the aphids movement around the roots. These chambers are covered with a white waxy dust from the aphids.

*Source of field reports of root aphids

Lincoln Harris – Agronomist, Elders (NSW Riverina)