Aphids

Increasing numbers of aphids have been observed in cereal crops over the past few weeks. In canola crops, aphids have been observed in smaller colonies, predominantly around crop margins, trees or where stressed plants are present. The presence of aphids in canola at this time of year is fairly typical, however the infestation in cereal crops is less common in southern NSW.

The types of aphids being observed in cereals are Oat aphid and Corn aphid, while it is Cabbage aphid in canola.

Aphids are sporadic pests which tend to occur in warm dry seasons. As aphids are a sap sucking pest, they are effectively competing with the developing grain for moisture and nutrients. When adequate soil moisture is present, the effect of aphids is minimal, but when moisture is limiting, they may have some effect on crop yield. The effect of aphids on crop yield is not well understood, so it is likely that potential yield loss is often overstated. In many cases the aphids look a lot worse than they really are, due to the fact that their colonies are present in higher numbers around the edge of the paddock.

In cereals, aphids can also be responsible for the spread of Barley Yellow Dwarf Virus (BYDV). BYDV can cause yield loss in cereals when plants become infected at the seedling stage, but infection at this time of year is unlikely to result in a yield penalty.

Where aphids occur in crops that are undersown with pasture, they may effect lucerne survival when moisture is limiting. However under good growing conditions, aphids are unlikely to have any long term effect.

Management of aphids relies quite heavily on populations of beneficial insects such as ladybird beetles, hover flies and lacewings. Their numbers will build up in response to aphid numbers, but there will be a lag time, so it is important for the change in predator numbers to be monitored and taken into account, when considering chemical control of aphids.

Whilst some spraying has occurred in cereal crops, it has generally been limited to situations where the aphicide has been combined with another planned chemical application such as a fungicide, or to undersown crops where aphids were having a noticeable effect on the lucerne.

Where spraying is undertaken, the product of choice is Pirimor (500g/kg Pirimicarb). Pirimor is a specific aphicide which controls aphids and has no effect on beneficial insects. Pirimor works best in warm conditions combined with good coverage.

In the past, other broad spectrum insecticides have been used. However these products do not control aphids very well and they also remove all beneficial insects. Removal of the beneficial insects, means that aphid population can bounce back in the absence of pressure from natural predators.

In summary, aphid numbers should be monitored in conjunction with predator numbers, predominantly ladybird beetles. Any control measures should be given careful consideration, based on very conservative estimates of potential yield loss, as well as weather conditions, product availability, plus the time interval until harvest or windrowing.