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Cabbage aphid

With ongoing warm weather, numbers of cabbage aphids (Brevicoryne brassicae) in canola crops are continuing to increase, and there is some uncertainty among growers and advisors as to whether it is economically viable to spray crops prior to windrowing. Agronomist, Greg Toomey (Landmark), says aphids are particularly high in one canola crop near Elmore, in the Northern Country district of Victoria. Approximately 5-10% of the plants are infested with aphid colonies that range from 1-20 cm in length. Given the current weather conditions and the fact the crop will not be windrowed for atleast 2 weeks, control is considered necessary.

Agronomist, Matt McLoughlan (Mr Agronomy), says cabbage aphids are present in low-moderate numbers in canola crops around Warracknabeal, in the Wimmera district of Victoria. Matt says it is unlikely that these crops will require spraying for aphids before they are windrowed in approximately 2-3 weeks.

Researcher, Aston Arthur (University of Melbourne), says cabbage aphids have also recently been observed in many late-flowering and early-podding canola crops around Shepparton, in the Northern Country district of Victoria. The number of aphids observed in these paddocks does not warrant chemical control.

Cabbage aphids are about 3 mm long and dull grey-green in colour. Colonies appear bluish-grey and are covered with a fine, whitish powder. They are usually found on the terminal flowering spikes, and the distribution within a paddock is often patchy. In a trial we conducted in spring 2008 near Elmore, Victoria, cabbage aphid numbers increased by approximately 10-fold in only 14 days within a canola crop. In this instance the weather conditions were very favourable for aphid development with daily maximum temperatures generally between 22-28°C.

It is important to be aware that aphids are largely driven by local climatic conditions and populations will develop much faster in warm weather compared with extremely hot or cold conditions. Rainfall may slow population growth but it is unlikely to significantly reduce numbers unless there is heavy rainfall for a sustained period. Greg reports that good rainfall received at the end of September did not seem to reduce numbers of cabbage aphids in canola crops in the Elmore region. Click here or refer to PestFacts Issue No. 9 for more information on canola aphids.

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