Update on green peach aphids

The widespread frosts in early to mid August initially slowed GPA activity while the more recent warmer weather has hastened development of winged aphids. We provide a simple guide to distinguish winged GPA from other aphids, and provide an update on insecticide resistance issues

 

Where have they been reported?

The widespread frosts in August have slowed green peach aphid (Myzus persicae - GPA) activity while the more recent warmer weather has hastened development, particularly as some crops become moisture stressed. In early August, reports suggested adult aphids and nymphs were on the decline although still easily found in canola near Bordertown in the Victorian Wimmera, and similarly near Tatyoon in the Western district of Victoria. In the last 7-10 days, GPA populations appear to be building up, although still in low numbers in many canola crops throughout Victoria’s Central and Western districts (on 1 per 25 plants) and western parts of the Wimmera (on 1 per 5 plants). In most cases there were very few aphids per plant.

In another report from the NSW Riverina, attempts to find GPA in canola were unsuccessful although the inspected crops had low infestations of turnip aphid, which were initially confused as GPA. Distinguishing between aphid species is not straightforward. It was also noticed that the aphid populations were heavily parasitised (appearing as parasitised mummies).

Our advice

Following many days of warm weather in late August, aphid numbers will have begun to increase. In addition, winged aphids may have started to appear, particularly on moisture stressed plants. In many areas of Victoria and southern NSW, rainfall and cooler weather in early September will again suppress activity. We recommend that growers continue to monitor GPA populations closely. Check for the build-up of numbers and the production of significant numbers of winged aphids in a population, which signal that flights may be about to occur.

If there are neighbouring crops uninfected by Beet Western Yellows virus (BWYV) and judgment about the need to spray aphids are being made, it is important to distinguish winged GPA from other winged forms of aphids. It is pointless or even counter-productive spraying crops if there are local flights of other aphid species, for example winged turnip aphid. To assist in the identification of winged GPA from other aphids, we have developed a simple pictorial guide of the key distinguishing features of aphids in pulse and canola crops. A microscope or hand lens are helpful tools as these distinguishing features are subtle. The Guide illustrates the most characteristic feature of GPA: on the head, between the antennae, are two bumps or tubercles that are turned in. Also, look for the dark patch on the upper abdomen between the wings, and for the longer siphuncles (on the base of the abdomen) than on other species.

Control options

Insecticide resistance among GPA populations continues to be mapped across Victoria, NSW and South Australia. Almost 40 populations have now been tested, with resistance to pyrethroids, organophosphates (OPs) and carbamates found to be widespread.

The use of dimethoate (800 ml/ha) to successfully control GPA was recently reported from a canola crop at the pre-flowering stage near Bordertown in the Victorian Wimmera. This was an area from which GPA were previously found to have resistance to OPs. This follows similar reports from other regions in Victoria and South Australia. The mechanism responsible for resistance to OPs is complex and may confer partial or complete resistance. Hence, the successful control of GPA with dimethoate is not necessarily surprising. See the latest resistance testing results for south-eastern Australia.

An industry-endorsed Resistance Management Strategy for GPA in Australian grains has recently been launched and the downloadble pdf version is available here.

 

* Sources of field reports of GPA

Phil Bowden - Agronomist (NSW South West Slopes)

Rik Maatman – Agronomist, Landmark (Victorian Wimmera)

Simon Mock – Agronomist, Clovercrest Consulting (Victorian Wimmera)

Adam Pearce - Agronomist, Clovercrest Consulting (Victorian Wimmera)

Tim Pilkington – Agronomist, Elders (Victorian South West)