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Aphid numbers increasing

Aphid populations have remained relatively low in many crops so far this spring, according to reports from several regions in Victoria and southern New South Wales. This pattern is likely to be reflective of cooler winter temperatures and relatively mild spring temperatures. In addition, significant spring rainfall events may have helped suppress or prevent aphid numbers building up as quickly as they have in previous years.

In recent weeks there have been some increases in aphid numbers, however in most cases the crops are growing well and aphids are unlikely to impact yield, and hence will not warrant spraying. Agronomist, Ben Batters (Elders), has reported blue green aphids (Acyrthosiphon kondoi) in a lentil crop north of St. Arnaud, in the Wimmera district of Victoria. Aphids were mostly found on the upper part of the plants, with approximately 20-30 found per 10-sweeps. There was no visible feeding damage observed.

Over the past fortnight, green peach aphid (Myzus persicae) numbers have increased by 2-4 fold in a canola crop north of Marong, in the Northern Country district of Victoria. Researcher, Samantha Strano (cesar), says aphids were easily found last week on most plants in a corner of the paddock adjacent to a shelterbelt and a grassy roadside, which was also quite weedy. In other sections of the paddock aphids were very difficult to find.

At a canola research trial site near Charlton (in the Wimmera district of Victoria), cabbage aphid (Brevicoryne brassicae) numbers have increased from zero about 4 weeks ago, to a low-moderate level last week. In the most recent samples, up to 40 aphids were collected in 6 sweeps, and colonies of up to 10-15 cm in length were observed on a low number of plants. The canola is growing well at present and is not under moisture stress, and hence the aphids are unlikely to impact yield. 

Consultant, Sandy Biddulph (Biddulph Rural Consulting), reports finding very few aphids in many crops inspected around Cootamundra, in the South West Slopes district of New South Wales. In addition to this, Sandy has observed finding quite a high number of some beneficial insects such as hoverflies, when checking these crops. High numbers of hoverflies have also been collected in sweep net samples at the Charlton trial site.

Despite the relatively low aphid numbers observed so far, it is important to be aware that populations can increase very rapidly when conditions are favourable. Aphid development is strongly influenced by temperature, with reproduction occurring faster at warmer temperatures (up to an optimum). Crops are at greater risk of yield loss when they are already moisture stressed, as aphid feeding will exacerbate this further by sucking sap from plants.

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