Is green peach aphid resistance to seed dressings increasing virus spread?
- Grass Patch
Quenten Knight (Agronomy Focus) reports that aphids are becoming very widespread in canola crops throughout the Esperance region. Quenten is finding some large mixed colonies of turnip and green peach aphids (GPA) on the underside of canola leaves. In some areas around Munglinup and Jerdacuttup the aphids are severely stunting canola plants over large areas, particularly where the crop has been recently moisture stressed.
The GPA will be sent for resistance testing as it is suspected they may be resistant to imidacloprid, the canola plants will be tested for the presence of turnip yellows virus (TuYV) (formerly referred to as beet western yellows virus).
Andrea Hills (DPIRD) reports finding green peach aphid in a canola trial near Gibson.
Plant virologist Ben Congdon (DPIRD) commented that early GPA colonisation of young canola plants (beginning at the 3-5 leaf stage) has recently been detected around Munglinup, Coomalbidgup, Gibson, Scaddan and Grass Patch, which is consistent with his earlier surveillance during May and June when flying GPA carrying the virus were detected in these areas. Ben said one site which had received an imidacloprid seed dressing is already at 70% infection with TuYV. It is possible that this extremely high rate of spread has been amplified by limited efficacy of imidacloprid seed dressing where GPA has been able to establish and spread virus through the crop from earlier growth stages. Furthermore, plants at these growth stages are the most vulnerable to TuYV-induced yield losses.
Overseas research into imidacloprid resistance in GPA has shown that the aphids which survive low doses in imidacloprid-treated plants were encouraged to reproduce faster and survive longer during periods of food/water stress. This further complicates the issue if GPA are growing to higher numbers and potentially spreading viruses more on imidacloprid seed-treated crops than those without seed dressing at all.
GPA has a wide host range, including oilseeds, lupins, pulse crops and some broadleaf pastures. Non-crop hosts include capeweed, marshmallow, wild radish, wild turnip, Lincoln weed and other cruciferous weeds. GPA is an important vector of a number of plant viruses including TuYV), cauliflower mosaic virus (CaMV), turnip mosaic virus (TuMV) and cucumber mosaic virus (CMV), all of which damage canola.
Although suspected for some years by growers and consultants, resistance to neonicotinoid insecticides was last year confirmed in Australian populations of GPA including populations from Western Australia. This discovery means that Australian GPA populations are known to have resistance to four different chemical groups – synthetic pyrethroids (for example, alpha-cypermethrin), organophosphates (for example, dimethoate), carbamates (for example, pirimicarb) and now, neonicotinoids. Neonicotinoids, such as imidacloprid, are applied as seed treatments in the grains industry and have become commonplace in canola pest management across Australia.
Spray GPA only if you have to. GPA feeding damage can lead to plant death but this is usually associated with moisture stressed crops and high numbers of GPA on the plant with the plant unable to outgrow GPA feeding damage. To reduce the risk of resistance to any insecticide group, it is important to rotate insecticides with different modes of action, avoid the use of broad-spectrum insecticides and apply appropriate insecticides only after careful monitoring and correct identification of species. Transform®, a sulfoxaflor foliar insecticide, remains an effective means to control GPA in canola crops and should be used judiciously.
Growers and advisors are encouraged to download and follow the GPA resistance management strategy to minimise the further development of insecticide resistance.
For more detailed information on pesticide resistance in green peach aphid refer to Pestfax Issue 4 May 2017 - Green peach aphid control in canola: Are we running out of options?
For more information on seasonal virus risk refer to Aphids and virus in canola - a risk guide in Pestfax Issue 9.
For more information on canola aphids refer to DPIRD's Aphid management in canola crops.
For a list of insecticides registered for use on insect pests in canola see DPIRD's Autumn Winter insecticide guide 2018.