Aphid biology and barley yellow dwarf virus

There have been many reports of aphids on various crops this season both in Victoria and NSW. Agronomist, Phillip Bowden (NSW DPI), has reported aphids in barley crops near Cootamundra, in the South West Slopes of NSW. Due to the favourable growing conditions and health of the plants, Phillip says the paddocks will not be sprayed, but closely monitored over the coming months. Jim Cronin (Landmark) says several species of aphids have been observed around Forbes and West Wyalong, in the Central West Slopes and Plains district of NSW. The oncoming cooler weather will slow the rate of aphid development and many crops should be able to outgrow the damage.

Aphids have complex life cycles, with populations that are predominately female. Adults can be wingless or winged (alates). Nymphs (immature forms) look similar to adults in appearance but are wingless, smaller and go through 3-4 instars (growth stages) before they reach maturity. Aphid survival, development and population growth is influenced by environmental factors, particularly temperature. Spring triggers a rapid increase in their life cycle and numbers build-up quickly.

Winged aphids move into crops in autumn from alternative host plants, usually weeds along roadsides and verges where they over-summer. Aphid numbers build-up on the edges of paddocks and tend to form dense colonies on the growing points of a single plant before moving onto surrounding plants. Hence, aphid infestations are usually patchy across the paddock. All aphids are sap-suckers and when in large numbers direct feeding damage that removes the sap causes yellowing and wilting of plants. Heavy infestations, especially at flowering, can lead to large amounts of honeydew and black sooty mould. In addition, many aphid species transmit serious plant viral diseases, which can be detrimental and yield limiting, especially in young plants.

Phillip says barley yellow dwarf virus (BYDV) has been detected in trials near Cootamundra, NSW. BYDV is transmitted by aphids from infected grasses and can have a greater impact on cereal crops than direct feeding by aphids. Low densities of aphids can transmit BYDV, with earlier infection resulting in higher levels of damage. Using chemical control of keep a crop free of aphids is difficult. Research in WA indicates a strategy of chemical control at 3 and 8 weeks post emergence to minimise BYDV infection and spread within a crop. Again, chemical control would only be warranted in high yielding crops. Selecting cereal varieties with greater tolerance to BYDV is an important part of minimising losses in BYDV-prone regions.

Monitoring of crops should be performed throughout the season and involve the inspection of at least 20 sites within each paddock. A workable management threshold guide is around 20% of plants heavily infested. If this is exceeded, control measures to avoid yield loss should be considered. However, it is important to remember that if plants are stressed, the level of damage may be more substantial.

Crops should also be monitored for natural enemies, and these should be considered prior to spraying any insecticide. Beneficial insects include parasitic wasps, ladybirds, hoverflies and lacewings. These are a very reliable form of control during the warmer days of spring and when low to moderate numbers of aphids are present.

If spraying is necessary, the insecticide to be applied should be chosen carefully. Many chemicals have a detrimental effect on beneficial insects and thus increase the likelihood of subsequent pest outbreaks. Many aphid predators are generalists and feed on other pests, such as native budworm and diamond-back moth. Killing natural enemies early in the season may have implications for the control of these pests later in the season. The use of “soft” chemicals, such as pirimicarb, will preserve many beneficial insects.