Pasture root aphids

Agronomist, Jake O’Brien (Agricom), says pasture root aphids have been found in established ryegrass paddocks in various locations across the Northern Tablelands district of New South Wales. Pasture root aphids have also recently been found in various parts of Victoria. Product Development Manager, Rob Salmon (PGG Wrightson Seeds Australia), says aphids have been observed in perennial ryegrass pastures near Alexandra and Daylesford, in the North Central district, south of Mortlake in the Western district and also near Traralgon, in the West and South Gippsland district.

Pasture root aphids (Aploneura lentisci) are recognised as important pasture pests in New Zealand, where they are widespread and have been shown to significantly reduce the growth and survival of ryegrass and tall fescue. The importance of pasture root aphids in Australia has only recently been realised since new grass endophytes have been developed. To date pasture root aphids have been located in New South Wales, Victoria, South Australia and Tasmania. The full extent of their distribution and pest status remain unclear.

As the name suggests, pasture 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 pastures, and the effects are amplified when plants are stressed by other factors such as drought, heavy grazing or attack from other insects. The pasture root aphid is also capable of damaging wheat. Insecticide applications are largely ineffective against root aphids due to their subterranean habits.

Aphids may be present on pasture roots all year round and populations can build to high numbers at any time under favourable conditions. Symptoms of damage are often associated with disappointing pasture recovery after drought, a loss of vigour and poor persistence. Within a drill row, individual plants hosting high levels of root aphid will often be small, weak, pale and carry dead material around their base.

Adult root aphids are oval bodied insects that grow to approximately 2.5 mm in length. They are usually cream or pale yellow in colour and have very small antennae. They can be confused with the pasture mealybug. Root aphids are most easily detected by the presence of a white fluffy wax that is exuded from their body. To check pastures, use a spade to dig up soil to a depth of approximately 30 cm, and then break up the soil over a plastic sheet. Look for the white waxy exudates and examine this wax for aphids.

Thresholds for damage are difficult to determine because they depend on environmental conditions and other plant stresses. Pasture root aphid numbers will also vary widely across a paddock. As a rough guide, an average of 20 root aphid colonies per spade is sufficient to cause a reduction in ryegrass productivity. One colony is defined as a small clump of white wax with at least one aphid that is visible. The use of particular grass endophytes, which are commercially available, remains the most effective control strategy against pasture root aphids.

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