8.1 Category 01: Agricultural Plant Pest Control & Soil Fumigation (01A)

8.1 Category 01: Agricultural Plant Pest Control & Soil Fumigation (01A)

[!NOTE] Category Scope & Regulatory Authority: Nebraska Commercial Category 01 governs the application of pesticides to agricultural crops, grasslands, rangelands, pastures, and non-crop agricultural land (such as field borders, farmstead windbreaks, and irrigation ditch banks) for compensation or hire. Individuals holding Private Applicator certification may only apply Restricted-Use Pesticides (RUPs) to land owned, rented, or leased by themselves or their employer for the production of agricultural commodities. Any custom application performed on the property of another for a fee requires active Category 01 commercial certification under Title 25 Nebraska Administrative Code (NAC) Chapter 2.

---

Major Agronomic Insect Pests: Biology, Scouting & Economic Thresholds

Successful pest management in Nebraska field crops requires systematic field scouting, accurate pest identification, and strict adherence to science-based economic thresholds developed by University of Nebraska–Lincoln (UNL) Extension.

1. Western Corn Rootworm (Diabrotica virgifera virgifera)

The western corn rootworm is the most destructive insect pest of continuous corn production across Nebraska. Overwintering occurs exclusively in the egg stage within field soil. Egg hatch begins in late May and continues through June, directly coinciding with rapid early vegetative corn growth.

• Larval Damage & Node-Injury Scale: Newly hatched larvae migrate through soil pores to feed on nodal root rings. Root feeding destroys plant anchoring, causing severe "goose-necking" (lodging) that interferes with combine harvesting and disrupts water and nutrient uptake. UNL Extension utilizes the Iowa State 0–3 Node-Injury Scale (NIS) to evaluate root damage:
  • 0.00: No root injury or minor scarring.
  • 1.00: One complete node (circle) of roots pruned to within 1.5 inches of the stalk base.
  • 2.00: Two complete root nodes pruned away.
  • 3.00: Three or more root nodes completely destroyed.
  • Economic Injury Threshold: An NIS rating between 0.25 and 0.75 represents economic loss depending on grain value, soil moisture, and lodging severity. Severe lodging typically occurs when ratings exceed 1.00.
• Adult Beetle Feeding: Adult beetles emerge in July and August. Adults feed voraciously on corn silks, green ear tips, and pollen. If beetle feeding clips silks to less than 1/2 inch prior to 50% pollen shed, kernel fertilization fails, resulting in poorly filled, barren ears.
• Bt Resistance & Multi-Tactic Management: Field-evolved resistance to single-trait transgenic Bacillus thuringiensis (Bt) crystalline proteins—including Cry3Bb1, mCry3A, eCry3.1Ab, and Cry34/35Ab1, renamed Gpp34Ab1/Tpp35Ab1—has been documented across Nebraska continuous corn acreage. Managing rootworms requires integrated strategies: (1) rotating out of corn into non-host soybeans or sorghum, which starves hatching larvae; (2) planting pyramided Bt corn hybrids expressing multiple subterranean modes of action plus a non-Bt refuge; (3) utilizing at-planting granular or liquid soil insecticides (T-band or in-furrow); and (4) monitoring late-summer adult populations with Pherocon AM yellow sticky traps (threshold of 0.5 to 1.0 beetle per trap per day) to forecast larval pressure in subsequent years.

2. European Corn Borer (Ostrinia nubilalis)

In Nebraska, the European corn borer completes two distinct generations per growing season:

• First Generation (Whorl Stage): Moths emerge in late spring and deposit flattened, overlapping egg masses resembling fish scales on the undersides of corn leaves. Early larvae feed inside the vegetative whorl, producing characteristic "shot-hole" foliar feeding patterns. Treatment thresholds for non-Bt corn require scouting when 50% of plants display whorl feeding and live larvae remain in the whorls prior to stalk tunneling.
• Second Generation (Reproductive Stage): Mid-summer moths lay eggs near ear leaves. Larvae feed on pollen accumulations in leaf axils before boring into stalks, ear shanks, and kernels, leading to stalk breakage and severe ear drop. Bt corn expressing Cry1F or Cry1Ab provides excellent control. For conventional corn, chemical insecticides must be applied after egg hatch but before larvae bore into stalks, as insecticides cannot penetrate stalk tissue once larvae enter.

3. Soybean Aphid (Aphis glycines)

The soybean aphid is an invasive, piercing-sucking insect capable of explosive population growth under moderate summer temperatures (70°F to 85°F). Aphids feed on phloem sap, excreting sticky honeydew that fosters black sooty mold growth, retards photosynthesis, and vectors plant viruses (e.g., soybean mosaic virus).

+-------------------------------------------------------------------------+
|                    SOYBEAN APHID ECONOMIC THRESHOLD                     |
| - Economic Threshold: 250 aphids per plant on at least 80% of plants   |
| - Population Status: Actively increasing and predators insufficient     |
| - Crop Growth Window: Vegetative stages through R5 (beginning seed)     |
| - Economic Injury Level (EIL): ~674 aphids/plant (cost equals loss)    |
+-------------------------------------------------------------------------+

Scouting involves examining 20 to 30 plants across a representative field pattern. Treatment is triggered when counts average 250 aphids per plant on 80% of plants with increasing populations from vegetative growth through the R5 (beginning seed) stage. The 250-aphid threshold provides a 7- to 10-day lead time before populations reach the Economic Injury Level (EIL) of 674 aphids per plant. Once soybeans reach R6 (full seed), insecticide applications rarely produce an economic return unless plants are suffering from severe moisture stress.

4. Spider Mites: Twospotted (Tetranychus urticae) & Banks Grass Mite (Oligonychus pratensis)

Spider mites are minute arachnids that proliferate in western and central Nebraska during hot, dry, drought conditions. Mites feed by puncturing plant epidermal cells and sucking out chlorophyll, producing visible yellow-white stippling, leaf bronzing, heavy webbing, and premature leaf drop.

• The Chemical Flare Hazard: Broad-spectrum pyrethroid insecticides applied to manage corn rootworm beetles, earworms, or grasshoppers frequently decimate beneficial predatory mites (Neoseiulus spp.) and minute pirate bugs (Orius spp.). This predator elimination, combined with pyrethroid hormoligosis (sub-lethal reproductive stimulation), triggers catastrophic mite flare-ups. Applicators must select targeted miticides or translaminar chemistries (such as bifenazate, etoxazole, or spiromesifen) when mites threaten fields.

---

Major Nebraska Agronomic Weeds & Resistance Management

Herbicide-resistant weeds represent the foremost agronomic challenge facing Nebraska producers. Heavy reliance on single herbicide sites of action has selected for resilient biotypes exhibiting multiple resistance mechanisms.

1. Palmer Amaranth (Amaranthus palmeri) and Waterhemp (Amaranthus tuberculatus)

Both Palmer amaranth and waterhemp belong to the pigweed family (Amaranthaceae) and are dioecious (having separate male and female plants). This obligate outcrossing facilitates rapid genetic recombination and the swift dissemination of herbicide-resistance genes via wind-borne pollen.

• Biological Traits: A single female Palmer amaranth plant can produce 300,000 to over 500,000 seeds and grow at rates exceeding 2 inches per day in warm weather. Unlike traditional summer annuals that emerge primarily in spring, Palmer amaranth and waterhemp exhibit an extended germination window spanning from May through August.
• Multi-Herbicide Resistance: Nebraska populations have confirmed resistance across multiple Herbicide Resistance Action Committee (HRAC) modes of action:
  • Group 2: ALS inhibitors (imazethapyr, chlorimuron-ethyl)
  • Group 4: Synthetic auxins (2,4-D, dicamba)
  • Group 5: Photosystem II inhibitors (atrazine)
  • Group 9: EPSPS inhibitors (glyphosate)
  • Group 14: PPO inhibitors (fomesafen, lactofen)
  • Group 27: HPPD inhibitors (mesotrione, tembotrione)
• Anatomical Differentiation: Palmer amaranth possesses a petiole that is longer than the leaf blade (demonstrated by bending the petiole back over the blade), hairless stems, and female plants with long, spiny seedhead bracts. Waterhemp features petioles shorter than the leaf blade, hairless stems, and narrower lanceolate leaves without sharp bracts.

2. Giant Ragweed (Ambrosia trifida) and Kochia (Bassia scoparia)

• Giant Ragweed: Emerges extremely early in spring (March through April). Rapid early growth outcompetes emerging crops. Documented widespread resistance to Group 2 (ALS) and Group 9 (glyphosate) chemistries across eastern Nebraska river valleys.
• Kochia: Highly adapted to arid, saline conditions of western Nebraska. Matures into a tumbleweed that rolls across landscapes, dispersing tens of thousands of seeds. Widespread resistance to ALS inhibitors (Group 2) and glyphosate (Group 9), with emerging resistance to dicamba and fluroxypyr (Group 4).

Pre-Emergence vs. Post-Emergence & Overlapping Residuals

Relying exclusively on post-emergence (POST) contact herbicides guarantees control failure against resistant pigweeds. Sustainable programs mandate overlapping (layered) residual herbicides:

1. Pre-Plant or Pre-Emergence (PRE): Apply a broad-spectrum residual herbicide mixture combining multiple modes of action at planting (e.g., Group 15 chloroacetamide + Group 14 PPO inhibitor + Group 5 atrazine or metribuzin). This establishes a critical weed-free period during early crop establishment.
2. In-Season Layered Residual (POST Tank-Mix): Before the soil activity of the PRE application dissipates (typically 21 to 30 days after planting), apply an in-season POST foliar herbicide tank-mixed with an additional soil residual herbicide (such as S-metolachlor, acetochlor, pyroxasulfone, or dimethenamid-P).
3. Critical Concept: Residual Group 15 herbicides have zero activity on emerged weeds; they only inhibit seedling shoot emergence. Therefore, any weeds emerged at application must be controlled by an effective, non-resistant foliar mixing partner.

---

Corn and Soybean Pathogens: Fungal vs. Bacterial Etiology

Accurate diagnosis of foliar diseases is vital because chemical fungicides provide zero protection against bacterial pathogens.

1. Tar Spot (Phyllachora maydis)

Tar spot is an aggressive fungal pathogen of corn that has spread across eastern and central Nebraska. It produces distinctive raised, circular-to-oval, shiny black fungal structures (stromata) on both upper and lower leaf surfaces, leaf sheaths, and husks. Unlike insect frass or secondary saprophytes, tar spot stromata are physically embedded in leaf tissue and cannot be rubbed off with water or fingers. Under prolonged moderate temperatures (60°F to 72°F), high relative humidity (>75%), and extended leaf wetness (7+ hours), tar spot rapidly causes complete canopy blighting, premature plant death, and grain yield reductions exceeding 50 bushels per acre. Effective management requires tolerant hybrids and prophylactic two- or three-way fungicide applications (combining FRAC 3 DMI, FRAC 7 SDHI, and FRAC 11 QoI) applied between tasseling/silking (VT/R1) and early dough (R3).

2. Goss's Bacterial Wilt and Blight (Clavibacter nebraskensis)

Goss's wilt is a destructive corn disease caused by a systemic bacterium. It presents in two phases: systemic seedling wilt and foliar blight. Foliar lesions appear as long, gray-to-light-green water-soaked streaks with wavy margins. Critical diagnostic indicators include:

• "Freckles": Dark green-to-black water-soaked spots scattered inside the lesion margins.
• Bacterial Exudate: Bacterial ooze that dries into a shiny, crystalline film on the leaf surface in sunlight.

[!WARNING] Diagnostic Reality: Because Goss's wilt is caused by a bacterium, foliar fungicides have zero efficacy. Fungicides target fungal biochemical pathways and cannot prevent, suppress, or cure bacterial infections. Managing Goss's wilt relies entirely on selecting resistant corn hybrids, rotating away from corn to non-host crops, and controlling alternative grassy weed hosts (such as shattercane, foxtails, and barnyardgrass).

3. Other Significant Fungal Pathogens

• Gray Leaf Spot (Cercospora zeae-maydis): Affects corn; characterized by distinct, rectangular, tan-to-gray lesions strictly bounded by parallel leaf veins. Favored by warm (75°F to 85°F), humid weather and continuous no-till corn.
• Frogeye Leaf Spot (Cercospora sojina): Affects soybeans; circular-to-angular lesions with dark reddish-brown borders and light tan/gray centers. Widespread resistance to strobilurin (FRAC Group 11 QoI) fungicides has been confirmed in Nebraska; applicators must utilize multi-site or premix fungicides containing FRAC 3 (triazoles) or FRAC 7 (SDHIs).

---

Category 01A: Soil Fumigation Standards & Safety

Soil fumigation is a specialized agricultural sub-category involving volatile chemicals that convert into toxic gases within soil pores. Fumigants are applied to control devastating soil-borne plant-parasitic nematodes, soil-borne fungal pathogens (Verticillium, Fusarium, Rhizoctonia), and persistent weed seeds prior to planting high-value seed crops, potatoes, sugar beets, or orchard nursery stock.

Core Fumigant Chemistries

1. Metam Sodium and Metam Potassium: Dithiocarbamate salts that break down rapidly in moist soil to generate methyl isothiocyanate (MITC), a potent non-selective biocide.
2. 1,3-Dichloropropene (1,3-D / Telone): Highly volatile halogenated hydrocarbon acting as a broad-spectrum nematicide and soil insecticide.
3. Chloropicrin: Broad-spectrum soil biocide and warning agent (strong lachrymator/tear gas), frequently co-formulated with 1,3-D to provide fungal control alongside nematode suppression.

Soil Physical and Environmental Application Parameters

To ensure efficacy and prevent catastrophic atmospheric venting, applicators must verify three mandatory soil physical parameters:

+-------------------------------------------------------------------------+
|                 MANDATORY SOIL FUMIGATION PARAMETERS                    |
| - Soil Temperature: 50°F to 80°F at 3- to 8-inch depth                 |
| - Soil Moisture: 50% to 85% of field capacity                          |
| - Soil Tilth: Granular, clod-free, with plant residues decomposed       |
| - Soil Sealing: Immediate roller packer, water seal, or TIF/VIF tarp   |
+-------------------------------------------------------------------------+

• Soil Temperature Window (50°F to 80°F): Soil temperature must be measured at the injection depth (3 to 8 inches). If soil temperatures are below 50°F, the chemical fails to volatilize adequately and moves sluggishly through soil pores, resulting in inadequate pest control. If soil temperatures exceed 80°F, the fumigant volatilizes violently, diffusing too rapidly and escaping into the atmosphere before maintaining adequate lethal contact time with target pests.
• Soil Moisture Window (50% to 85% of Field Capacity): Soil moisture acts as a regulator for gas diffusion. In dry soils (<50% field capacity), gas channels directly to the surface and escapes. In waterlogged soils (>85% field capacity), soil pore spaces are flooded with water, preventing gas diffusion and suffocating fumigant movement. Soil moisture is evaluated using the USDA feel and appearance method (soil forms a stable ball that leaves light moisture on fingers without extruding water).
• Soil Sealing: Immediately following shank injection, the soil surface must be sealed using a heavy cultipacker, roller, water seal (overhead irrigation), or high-barrier plastic tarp (Totally Impermeable Film [TIF] or Virtually Impermeable Film [VIF]) to trap vapors underground.

Mandatory Fumigant Management Plans (FMPs)

Under federal EPA mandates and Nebraska Department of Agriculture enforcement, a certified applicator must author a comprehensive, site-specific Fumigant Management Plan (FMP) before application begins. The FMP must document:

• Certified applicator and handler credentials, training verification, and licensing numbers.
• General site information, soil texture, organic matter, soil temperature, and moisture logs.
• Accurate application dosage calculations, injection depth, tractor speed, and sealing methods.
• Buffer zone determinations, including EPA buffer calculation tables, application credits (e.g., TIF tarp credits), and buffer zone maps.
• Emergency response plans, notification of local first responders/fire departments, nearest hospital route, and 24-hour emergency contacts.
• Air monitoring protocols and direct-read detection equipment (colorimetric detector tubes) for handler respiratory protection.
• Post-application verification: The certified applicator must monitor the site, verify the soil seal, inspect posting, and archive the completed FMP for at least 2 years under federal rule (3 years under Title 25 NAC Ch 2).

Buffer Zones and Field Warning Postings

A buffer zone is an established perimeter surrounding the application block where bystanders, agricultural workers, and unprotected individuals are strictly prohibited from entering while fumigant vapors dissipate (typically 48 hours post-application). Buffer zones cannot overlap residential housing, schools, nursing homes, or public roads unless official transit agreements or permissions are established.

• Posting Mandates: Fumigant warning signs must be posted at all usual points of field entry, along all likely access routes, and along the perimeter of the buffer zone. Signs must feature the skull and crossbones symbol, the signal word DANGER/PELIGRO, the words "Area under fumigation, DO NOT ENTER/NO ENTRE," the fumigant active ingredient, and the certified applicator's 24-hour emergency contact number.