Comments on: Are corn earworm a problem in winter cereals?

By: Big Eyed Bug

Big Eyed Bug — Mon, 22 Oct 2007 01:56:00 +0000

Last week Glen Milne asked on the blog "Do you spray (wheat/barley) on numbers of heliothis or wait until there is grain damage?"

My initial response to this question is that if you wait until the grains start getting attacked, then the helis (all armigera in wheat) will also be larger by then and thus more difficult to kill. Once they get above 13 mm in size you only have 2 options left:

1. Disregard the larger (>13 mm) larvae and focus on controlling the smaller larvae in the crop (thus giving you some more options like an SP, NPV or combination of the 2 depending on the situation with mixed heli/armyworm populations).
2. Or if you are going to target these large heli larvae, the only viable control option left is a stand-alone and more expensive methomyl spray .

We haven't heard any reports to date on how methomyl is working in the field against these larger grubs this season, though we understand some methomyl may have gone out. We have heard of mainly SP alone or SP+low rate NPV - I heard one report of only 70% control of helis from an SP spray - not certain what rate, but assume there must have been some larger larvae in there. We'd be interested to hear any news from readers on recent spray performance...

The other key issue is the stage of the crop:
i.e. Some judgements will need to be made about how quickly the larvae will reach damaging size and when this will occur in relation to the crop's development.

For example, if the crop is nearing full maturity/harvest, and the grubs are still small, then there is most likely no need to spray.

The other extreme would be a late crop that is still very green and at early seed fill. In this case, any small larvae present will most likely reach their most damaging size in time to significantly reduce crop yield, and so a spray is likely to be required at some point.

Given the unlikelihood of reinfestation by the helis, an early intervention could be better than a later one (i.e. more options and potentially cheaper).

So that is another factor to weigh up.

Finally, just a reminder to watch the withholding periods if you are contemplating a spray, especially one late in the crop's life.

By: Glenn Milne

Glenn Milne — Wed, 17 Oct 2007 20:20:00 +0000

Very timely. There are heliothis in the wheat crops but I find it hard to see much damage. They seem to eat the awns and a bit of leaf, but little grain damage as yet. Talking to Graham Spackman from Emerald he was saying they did not spray till they saw the damage in the wheat head. DO you spray on numbers of heliothis or wait until there is grain damage? Cheers Glenn

By: BigBug

BigBug — Wed, 03 Oct 2007 22:29:00 +0000

An indirect answer addresses larval development on the crop. H. armigera larvae on wheat and barley usually complete development before ripening of the grain. This is because most eggs are laid on the newly emerged heads (pre-flowering). Larval development will take 30 days at an average temperature of 18.5°C, 20 days at 21.3°C and 16 days at 24.6°C. Add 3 to 5 days for egg hatch, and this gives the approximate time after flowering when larvae will be mature and drop off the plants. This timing should see mature larvae leaving the plants to pupate during the early hard dough stage, and most feeding having occurred during the soft dough stage. The only complicating factor would be if there was uneven flowering in the crop, with larvae on late flowering heads hanging about for longer, and possibly moving onto other heads. Mature grains are not particularly appetising, nor easy to feed on because of the hard seed coat.
Remember that large larvae on the crop have probably completed most of their feeding, so killing them at this late stage may not be of much benefit. Hope this helps.

By: Richard

Richard — Wed, 03 Oct 2007 06:09:00 +0000

Very timely. There have been enquiries from irrigated wheat growers near Emerald re this over the last few days. In some cases, large Helicoverpa grubs that have survived an insecticide application targeted at armyworm.

One question – crops are approaching maturity (starting to ‘turn’) – how mature/dry do wheat grains need to be before they are unpalatable to Helicoverpa?

Richard Routley

By: Anonymous

Anonymous — Wed, 03 Oct 2007 04:50:00 +0000

Marvellous work DPI&F Ento team!