What Effect Does Prolonged, Heavy Rain Have On Fall Armyworm?

Fall Army Worm

By David Jones, CropNuts BroadAcre Agronomist

Fall Armyworm pressure is notably lower so far this season. That does not mean for a moment that we should slacken our approach, and there are undoubtedly areas of the country that are still seeing serious damage, but the early and heavier than usual rains have clearly played a role..

So what can we learn from this, and are these assumptions actually correct?

We know that the adult moth activity is greatest in warm, humid conditions, but at what point does it become detrimental to their spread and survival?

Fall Army worm

There is very little evidence that larvae actually ‘drown’ in the maize whorl in extreme wet conditions – even in light rain the whorl can remain saturated overnight until evaporation takes place.

The more likely explanation is firstly that flight conditions are far less suitable for the adult moths, and secondly that the eggs are washed off the plant before they hatch after 2-3 days. I have also noticed on plants that I have marked prior to heavy rain, that after 20mm overnight rain the protective scales that the moth rubs from her abdomen to protect the eggs also appears to be washed off.

It is also important to remember one of the key differences between Fall Armyworm and other types of moth larvae which is the inability to diapause. This means that regardless of weather conditions the pupae will hatch to a new moth after 8-9 days.

Therefore a period of 8-9 consecutive wet days across a large geographical region will see a large reduction in egg laying activity… in theory.

Yellow Cereal Fly

I have seen a number cases of what appears to be Yellow Cereal Fly in wheat and barley. These cause ‘deadheart’ symptoms on a single tiller where the youngest inner leaf is yellow and withered.

They appear to be a Diptera (fly) larvae rather than a moth because of the presence of wing markings and absence of legs or a distinct head, but if anyone can help with a full identification it would be much appreciated!

Fall Army Worm

Lupins

Many growers are considering lupins as an alternative pulse break crop. It is important to remember that there are a few key differences between growing lupins and growing peas or beans:

  Beans (faba) Peas Lupins
Rhizobium inoculant Often nodulate quite successfully without inoculant In most soils here, nodulation appears poor even when inoculated The bradyrhizobium inoculant used on lupins is the most acid tolerant of all rhizobium strains so well worth applying
Seed dressing Metalaxy (e.g. Apron Star) useful for reducing Damping Off and Downy Mildew Avoid Thiram if inoculating seed
Herbicides Wide range available Wide range available Do NOT use Sencor

Do NOT use Basagran

 

Glyphosate and burndown trials

We often assume that different brands of fallow herbicides such as glyphosate are all equally effective and reliable, and that there are costs to be saved by shopping around.

I have previously shown that this is not the case with other chemicals such as the fungicides tebuconazole and chlorothalonil, so how do different glyphosates, 2,4-D’s and various spray mixes stand up together?

Herbicide Spraying

We trialled a range of products with the Agventure Centre of Excellence for Crop Rotation to look at how best to control difficult weeds including Commelina, Amaranthus, Mallow and Setaria Grass.

The lessons are in summary:

  1. On Commelina in particularly, use at least 3 l/ha of a quality glyphosate such as Touchdown or Roundup Turbo. Even at high rates with less expensive generic products, control was never equal to the well formulated products.
  2. It goes without saying to put 0.5% Ammonium Sulphate into the spray tank BEFORE the glyphosate to stop Magnesium, Calcium etc locking up the glyphosate.
  3. Acidify the water on difficult weeds (be careful if you are using 2,4-D as well as glyphosate – that can react badly below pH 5.5). Use a product such as ‘Lower 7’.
  4. Wetters or adjuvants help improve the speed of control with both glyphosate and 2,4-D where this is important.
  5. Mixes of paraquat and 2,4-D were surprisingly good on a range of weeds – as a ‘double knock’ 7-10 days after glyphosate this appeared to be a very useful (and less expensive) mix.

 

Separately I have been looking at Glyphosate mixed with glufosinate ammonium on Conyza. This is a real problem weed in many areas and once the weeds get to 4-5 leaves you often find hand rogueing is the only option.

Glufosinate ammonium is a very useful product where quicker burndown is required, but it appears to add very little on Conyza, and if anything it almost looked as though it encouraged larger Conyza to shoot from the base before the glyphosate had time to really hit the plant.

It is a very useful product, but for me Roundup Turbo with Lower 7 is not much slower and does a very good job, which can be followed 5 days later with paraquat where an equally quick burndown is required.

 

Center for Excellence for Crop Rotation Agventure

Center for Excellence for Crop Rotation

Farming for the future requires a change of approach. Monoculture, soil degradation and climate change and soil degradation are threats to the future of how we feed the planet.

Agventure Ltd set up the Center of Excellence for Crop Rotation to help farmers diversify cropping systems and introduce techniques which have a long-term outlook to improve soil health.

The Center of Excellence for Crop Rotation works extensively with Crop Nutrition Laboratory Services.

About David

 

David Jones is the Broad Acre Specialist at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). David has a keen interest in soils and no till farming systems where he has undertaken work looking into weed levels and changes in soil structure, and has extensive experience in field trials and in the development of precision farming techniques. In his spare time he enjoys playing rugby.