Seed dressings have run their race

Recent reports of aphids in cereals and legumes suggest that we are outside the protection window of neonicotinoid seed dressing.

Early season reports of Russian wheat aphid (RWA, Diuraphis noxia) in cereals indicated crops grown from untreated seed or found on volunteers were the hardest hit. However, there have been more recent reports of RWA appearing in crops grow from neonicotinoid-treated seed. Similarly, cowpea aphid (Aphis craccivora) and other cereal aphids (oat aphid, Rhopalosiphum padi and corn aphid, Rhopalosiphum maidis) have now been reported in crops grown from treated seed.

Is this a case of seed dressing efficacy waning with time and crop growth? Likely. But another important factor to consider in this equation is the behaviour of aphids.

Seed dressings offer early season crop protection against virus transmission and direct feeding, a time which typically coincides with autumn migration of aphids from ‘green bridges’ to establishing crops. Fortunately, with the onset of winter, aphid activity generally stagnates through reduced migration, reproduction and generation time but also mortality from adverse weather (e.g. frost and rain). This means that as we leave the window of efficacy for seed treatments, we often enter a time where the threat of invasion or damage by aphid pests is reduced by seasonal conditions.    

But keep in mind that the decline in aphid activity over winter will not always be in sync with seed dressings wearing off. For example, earlier sown crops can fall outside the seed dressing window of efficacy during periods of moderate to high aphid activity, especially in the event of milder winter weather. As we move out of winter, it will be important to keep an eye on paddocks harbouring aphids as spring conditions will bring weather conducive to rapid increases in aphid populations.

The decision on the need for and timing of a spray for crop aphids is complex and involves trade-offs. Spraying early will manage populations before they cause too much feeding damage. But the use of broad-spectrum insecticides, will destroy beneficial insects and may result in a secondary, potentially more destructive infestation later on.

If chemical control is warranted, selective insecticides (e.g. pirimicarb) are recommended as they are less harmful to beneficial insects and therefore unlikely to induce a secondary pest outbreak. Beneficials invariably play an active role in keeping aphid populations in check, even though their presence is often overlooked.

Have you spotted oat aphid or cowpea aphid?

We are currently seeking samples of oat and cowpea aphid for upcoming baseline insecticide resistance screenings. To send us a sample, please follow these instructions.

For further information contact Dr Siobhan de Little on (03) 9349 4723 or via email at sdelittle@cesaraustralia.com. 

Cowpea aphid (left) and oat aphid (right) (Source: cesar)

 

Field reports

Ben Brennan – AGRIvision (Mallee, VIC)

Lincoln Harris – Landmark (Albury, NSW)

Andrew McMahen – Landmark (Mallee, VIC)

Rick Rundell-Gordon – Grounded Agronomy (Mallee, VIC)

Matthew Witney – Dogshun Medlin (Mallee, VIC)