Battling Armyworms - Nozzles & Yield Loss Potential

Our urban neighbours now know what armyworms look like. They are dead on the roads, they can be seen marching across fields and lawns but they like the taste of our winter wheat. First you have to scout your fields to see if they are there in sufficient numbers to spray. Previous issues of CropPest have the threshold numbers. If you are planning to spray you should make sure you setup your sprayer to get the job done.

Keep Water Volumes High

Unlike spraying herbicides to control weeds, insecticides generally require higher water volumes to get improved spray coverage. These higher spray volumes are needed because you are spraying insect pests that are moving in a crop canopy and not just spraying weeds that don't move in a field. The crop canopy also intercepts the spray material on the plant surfaces. Most insecticides should be applied using 200 l/ha or 20 GPA. Always check the product label to get specific water volume recommendations

Medium Droplets Required

The droplet size required for the application of insecticides is MEDIUM or MEDIUM to FINE. This spray quality is likely considerably smaller than what you are using for herbicide application with low drift or air-induction nozzles. There are a number of nozzle designs or nozzle configurations that can be used to apply the desired MEDIUM or MEDIUM to FINE spray quality. The types of nozzles available for the job include:

• flat fan
• extended range flat fan
• drift reducing flat fan
• Turbo TeeJet flat fan
• Low pressure air induction (40 to 100 PSI depending on brand and size)
• High pressure air induction (100 PSI or higher depending on brand)
• twin flat fan
• double nozzle caps with flat fan nozzles
• double nozzle holders with flat fan nozzles

These nozzle designs and the various nozzle sizes will need to be operated at different pressures to produce a MEDIUM quality of spray. Consult the manufacturer's catalogue to determine operating pressures required for your chosen nozzle to produce the desired spray quality. Spray operators that travel at higher speeds may need to consider double nozzles to deliver the spray volumes desired and also produce the droplet sizes required for insecticide application.

Spray Drift

Because of the fact you will be spraying insecticides with a MEDIUM or MEDIUM to FINE spray, your spray window may not be as wide as it is for herbicide application. The main reason for this is that herbicide sprays are typically much coarser, in the COARSE to VERY COARSE spray quality. Insecticide spray that drifts off can result in off-target deposition and significant amounts of the spray evaporating before it gets into the crop canopy. Both situations are undesirable.

Yield Loss

"Six weeks from heading to harvest" is the sage wisdom from long term farmers. This period of rapid grain fill is critical to high yields. So what happens when armyworms leave nothing but stalks and stems in the field? The answer totally depends on when the defoliation occurs.

Of the six week period, the final (sixth) week is essentially dry down before harvest. Wheat is physiologically mature around 30% moisture, so we can subtract the 6th week in our discussion. The fifth week of grain fill is primarily translocation. Photosynthesis has slowed, and the plant is finishing grain fill by robbing the rest of the plant. Losing leaves at this stage has little impact on yield. It will reduce kernel size slightly, but the impact is small. Of course, it is by keeping the plant healthy and green through this last period
that we achieve top end yields, but controlling armyworm at this stage is not economical. This actually lines up well with the shortest pre harvest interval of available insecticides: Sevin XLR has a 14 day PHI.

Weeks 1 to 4 are critical to grain fill and yield. The closer to heading the armyworm defoliates the crop, the greater the yield loss. Total crop defoliation is hopefully rare, as growers stay ahead of the problem. In fields where parts or whole leaves remain, the amount of compensation is incredible. Glumes, awns, stalks, and remaining leaves all increase in photosynthetic activity, to make up for the lost leaf tissue. So yield loss is dependent both on timing and amount of leaf tissue lost.

Research in North Carolina shows 45% yield loss with total defoliation at head emergence. Older research at Ridgetown College (Smid, 1979) shows an upper end of only 20% yield loss. These numbers do not account for head clipping, which could increase loss dramatically. If armyworm totally strip the field two weeks after heading, a maximum of 20% yield loss would be expected. In a field with partial (50%) defoliation three weeks after heading, yield loss will only be 5%. Of course, test weight and protein are likely to drop in damaged fields, without a full canopy to fill the grain. In damaged fields, the impact of any Staganospora (septoria glume blotch) on yield will be much more significant than normal, with photosynthate from the glumes critical to yield. Fields that were sprayed with a fungicide for fusarium control will have a definite advantage here, if Stagonspora infections occur.

Armyworm is definitely another frustration, but not a total disaster. It is certainly NOT to the point of making MY wheat crop into HAY!! Of course, armyworm hit the best fields with the highest yield potential first. If those fields had 125 bu/ac (3.4 t) yield potential, 5% is still over 6 bu/acre loss. At $7.00/bu, that is another $42.00 out of your pocket, plus $20/ac to control them. It's shaping up to be a LEAP YEAR…….!!!!!!!!