Armyworms invaded this alfalfa field. Why did they leave the lushest patches?

A few days ago, armyworms stripped leaves from most of our neighbor's 15-acre alfalfa field. But they left several patches untouched — offering clues which may help you avoid pest invasions in the future. 

September 25, 2021  By Jerry Carlson Alfalfa had regrown to about 14 inches high after a third cutting. In two days, armyworms devoured 90% of the field. But you can see in these photos how they avoided certain patches. On Monday Sept. 20, we scouted the field, which adjoins our 20-acre research farm, and found full-grown armyworm larvae clinging to leaf-stripped alfalfa stems, while others were moving on the ground. The "army" had stopped along the border of the untouched patch in the nearby photo.

We farmed this field several years for WakeUP-related field trials. But with our trials complete, we quit renting it and another farmer planted alfalfa here three seasons ago. The renter's home farm is six miles away, so he wasn't immediately aware of the invasion. He checked it on the Monday we scouted the field, and as a courtesy alerted us that he was sending someone to spray the field. By the next day, the worms had quit feeding. But as we saw it, they would have continued to avoid the patches that repelled them, even without the insecticide.  

Thus, we know the soil variations in every part of the field, and know its cropping history.

Here's one clue: Armyworms avoided any patch of lush alfalfa where the leaves tested a Brix level of 12 or above. In effect that's 12% sugar and other dissolved nutrients.

Organic growers have long observed that most chewing or sucking insect pests avoid high-sugar leaves. Highly fertile soils with excellent biological life readily convert mineral nutrients for plant health, rapid growth and highly efficient photosynthesis. In such alfalfa, sugars can build up to 12% or higher on sunny days. When the sun recedes, sugar accumulation slows and Brix levels drift lower as plant sap is pumped through the phloem system to roots and growing points. However, armyworms feed mostly during the day.

Organic crop literature treats insects as nature's scavengers whose job is removing weak and sick plants. Sick or stressed plants typically emit ammonia, which flying insects like armyworm moths detect and home into as an appealing place to lay eggs. Their larvae emerge among low-brix greenery. Here's an Iowa State University extension link describing the fall armyworm life cycle.

The healthiest acre of alfalfa — which escaped the attack — is a rectangle in the northeast corner which overlays a one-acre "prairie preserve." Our neighboring landowner kept that acre of wild prairie for years, and we farmed around it. With multiple species in that acre year-round, the soil built up a good reserve of plant nutrition.

We also left cover crops on a 20-foot border on north and south sides of the field. 

The new tenant tore up the prairie and farmed within inches of the property line.

Several other patches across the creek also escaped. See the nearby photos for our theory on why the brix levels in these untouched patches was at least 11 brix. We tried to measure brix levels in the stripped alfalfa, but all the leaves were gone and the stems wouldn't yield any juice.

 

 Here's our theory on why each of these green, surviving patches didn't get eaten.

1. The green splotch on the northeast corner: This ground is wet, fed underground by runoff flowing downslope from the east. Until we ran a tile line just north of the fence line, the "green patch" was a favorite place to get stuck with a fertilizer spinner wagon. 

2. Border strips, both north and south. We kept these strips in cover crops for several years, and ran our test rows north and south. The strips replaced endrows and made flagging strips easier. Thus, multi-species covers were building soil life. The new tenant farmed through them.

3. Signs of a north-south "fertile strip" about 36 rows of 30-in. crops emerged here. This was a high-fertility experiment, where we applied extra potassium, some by foliar feeding.

Bottom line is that good biological life and fertility leads to high chlorophyll content in leaves, high sugar production and thus insect resistance.

Update September 26: We investigated a smaller patch of untouched alfalfa, about 100 square feet surrounded by alfalfa eaten down to the bare stems — not a leaf remaining. About noon on this sunny day, we pulled a sample of leaves and checked the brix level.  Result was about 12 brix, same as in the other untouched patches. (All of the untouched alfalfa within the field was surrounded by barren, stripped plants.) We managed to take a slightly out-of-focus image of the brix level with an iPhone camera -- see photo below.

Update September 28: Today, grandson Lane Carlson and I told several soil conductivity readings for more bit of evidence on why armyworms bypassed certain sections of the field. The complete report and a photo are below.

Soil types and fertility levels clearly made the biggest differences in alfalfa survival. Our grandson, Lane Carlson, and I checked several points in the western section of the alfalfa field with a soil conductivity probe. 

In the most totally chewed part of the field, an east-facing hillside, the soil is sandy and very compacted. In this mid-field area, the soil temperature at 4 inches deep on this sunny afternoon was 84 degrees. There was almost no foliage, as alfalfa stems were completely stripped and drying out.

The soil conductivity was a miserably low 22.2 Microsiemens (µS).

That's a combination of sandy soil, lack of moisture and presumably a low cation exchange capacity.  

Lane and I went into the lush patch of alfalfa in the northwest corner — the acre which was historically a long-established prairie until about three seasons ago. The conductivity probe sank easily, showing decent moisture and soil temperature of about 65 with conductivity ranging from 222 to 284 Microsiemens. Some of the alfalfa was blooming. Brix level: 12.5.

A few years ago, I measured the conductivity of a "second bottom" layer of soil east of the creek because our soybeans were dark green and massive in that patch. The readings ran 700 to 800 Microsiemens then, under the soybeans. I was reminded of that today — because that's where some of the thickest, greenest alfalfa is today after the armyworm attack.


 

 


Example of a simple refractometer. The horizon line in our samples
was around 12. As shown in the live refractometer image below, 
the horizon when measuring alfalfa sap was blurred, indicating
high calcium content in the sap.
Live image from the most recent sample of surviving alfalfa,
taken Sept. 26 about noon. It shows about 11% sugar.
The horizon line from white to blue is hazy, indicating
high calcium content.