Fall armyworm pesticide

By David Jones, Broad Acre Agronomist, MSc. Agriculture

10th August 2018

Earlier in the year in #ThinkAgronomy I presented the results from a study of Fall Armyworm that I undertook, examining the effectiveness of different insecticides. One of the major challenges I found last year was how to obtain reliable and meaningful results from a randomized, replicated field trial with an insect that is highly sporadic through the crop. This means that there is a danger that we confuse good control with plots or treatments where Fall Armyworm happen to be absent. 

Fall army worm

So, I went back a few steps and designed an experiment in which I collected larvae from the field, put them into batches according to size and placed them in identical containers with maize leaf and applied a known concentration of insecticide. This achieved the following:

  1. We know that the larvae were exposed to the chemical and not protected under the leaves.
  2. We know exactly how many larvae were exposed, and at what stage they were when treated.
  3. There is no uncertainty as to whether a larva hidden inside a leaf whorl did or did not receive a lethal exposure to the chemical.
  4. We can assess mortality accurately, rather than making assumptions about the numbers of dead larvae present, or in most field experiments, the absence of larvae which makes a big assumption about whether the larvae were killed / migrated / pupated / not there in the first place!

All of the major approved insecticides were examined in a fully replicated (three times) with six larvae at 1-2 instar, six at 3-4 instar and six at 5-6 instar. The results after 5 days were as follows:

fall armworm control

Note that the 5% Confidence Interval is 27%. So anything that has achieved between 73% and 100% control is not statistically different. In other words, pyriproxyfen, emamectin benzoate, indoxacarb and acephate containing products are all very effective.

Lufenuron performed poorly even on small larvae which is what our experience in the field suggests, while flubendiamide and chlorantraniliprole + abamectin showed poor performance on larger larvae. Nonetheless the later two are still important chemicals when applied at the correct stage, and bring a different mode of action to the control program so do not dismiss them.

Of course there are limitations. Some chemicals may be slightly more mobile in the leaf whorl because of locally systemic or vapour activity, so performance in the crop might vary. But the object of this study was to look at Fall Armyworm control as objectively as possible.

A few closing remarks:

My job as an independent agronomist is not to deliberately seek controversy, but to lay out the facts in an objective and scientific manner without fear or favour. I have NO relationship or commercial agreement with any manufacturers or suppliers of farm inputs, and doubtless the above results may not be well received by some individuals in the agricultural supply industry. Yet I feel that for the greater advancement of our knowledge as farmers and society it is important to research and to lay out our findings in an open and transparent way.

Understanding Desiccation to Finish a Good Potato Crop

Good haulm desiccation is crucial to lifting and storing good potatoes. Number 1, getting rid of the foliage stops tuber blight by removing the source of zoospores. Secondly, stopping the plant growing and achieving good skin set means improved storage life and less damage at lifting, no matter how gentle you are with the tubers.

Potato harvesting

Thirdly, a good skin set means a bright potato with better marketability, and greater resilience against storage diseases such as Pink Rot and Gangrene. It is essential for controlling the tuber size for specific markets, and lastly detaching the stolons from the tubers means gentle lifting and easier sorting on the harvester.

There are many ways to achieve a fast and effective burndown. Growers are often told that there is a ‘right way and a wrong way’ but in reality if it works for you that is what matters. What are the options available?

  Advantages Disadvantages
Flail Safe, zero harvest interval

Essential in dense canopies in varieties such as Asante, Melody or Shangi

Needs to be set up well to avoid ridge and tuber damage

More wheelings through crop

Tends to be less effective at detaching stolons.

Avoid if Blackleg present in the crop.

Diquat Fast, removes Blight risk.

Good stolon detachment

A small, early dose makes failing far easier and more effective.

Kills weeds and prevents them seeding

Can mix a fungicide such as fluazinam to reduce Tuber Blight.

 

Dry soil around tubers can shock the plant and cause vascular browning.

Not available in every country.

Glufosinate ammonium Given time, it is very effective on prolific varieties.

Provides good weed control.

Only approved in some countries, NOT Kenya.

Need to wait until crop is naturally senescing. Less relevant for smaller seed or salad crops.

 

Carfentrazone Far less regrowth than Diquat. Not available in Kenya as yet.

Slow acting

 

 

Stopping indeterminate varieties such as Markies can be quite a task in some years, and it starts with not over fertilising the crop.

But when the canopy is thick, a low dose of desiccant is a good start followed by the flail to cut the haulm. In crops that are smaller but still growing rapidly I prefer to spray first, then top to reduce the shock of cutting off the foliage. This is particularly important where you are looking for smaller tubers.

Where there is blight active in the crop, spraying diquat or paraquat first with a fungicide with zoospore activity such as Infinito or fluazinam is more sensible.

In dry conditions when bruising risk at lifting is particularly high, flailing early in the day is ver\y important. This allows the tubers to rehydrate overnight, so they are less prone to physiological shocks which can cause internal browning.

when to harvest potatoes

In wetter conditions regrowth is often rapid, so hitting with paraquat or diquat will likely need two and possibly three sprays to stop the crop.

Carfentrazone is quite effective on stems but less so on dense foliage and is relatively slow acting.

Flailing requires accurate driving to avoid ridge damage, which is why on ware crops many growers open the crop up with a low dose of diquat/paraquat with the final fungicide. The flail also needs to be set correctly – it is essential to leave 25cm of stem for the follow up chemical to work on, and make sure that the haulm is thrown into the furrow and not left on the ridge where it will stop the chemical getting to the stems.

Above all, keep monitoring – it will be different in every variety each season so stay flexible.

New Wheat Varieties Show Promise in Nakuru

Check Soils now to finish 2018 on a high

Farmers are warned not to lose sight of the substantial amount of rain and the impact that it will have had on soil structure and nutrients. Get out now, dig holes and take soil samples to set your fields up for the coming planting season.

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 Ltd (CropNuts).

Till next time,

Happy farming!

David

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.