11.2 Economic Crop Pests Identification & Threshold-Based Control

Effective pest management requires more than just identifying the insect; it demands an understanding of population dynamics and the economic impact of the pest. Integrated Pest Management (IPM) relies heavily on the concepts of Economic Injury Level (EIL) and Economic Threshold (ET).

The Concept of Economic Thresholds

• Economic Injury Level (EIL): The lowest population density of a pest that will cause economic damage. It is the point where the cost of pest control equals the value of the crop yield saved.
• Economic Threshold (ET) / Action Threshold: The pest population density at which control measures should be initiated to prevent an increasing pest population from reaching the EIL. The ET is always set lower than the EIL to allow time for the control measure to take effect.

Applying insecticides only when the ET is reached prevents unnecessary chemical applications, saves money, preserves beneficial insects, and delays the development of insecticide resistance.

Key Pests of Corn

Corn Rootworm Complex (Western and Northern)

• Identification: Western corn rootworm (WCR) adults are yellow with black stripes; Northern corn rootworm (NCR) adults are solid green to yellowish-green. Larvae of both are white with brown heads.
• Life Cycle & Damage: They have one generation per year. Adults lay eggs in cornfields in late summer. Eggs overwinter and hatch in late spring. The larvae feed on corn roots, causing severe pruning. This reduces water/nutrient uptake and leads to 'goosenecking' or lodging, making harvest difficult. Adults feed on corn silks, which can interfere with pollination if populations are high.
• Management: Crop rotation to a non-host crop (like soybeans) was traditionally highly effective. However, the WCR variant lays eggs in soybean fields, and the NCR exhibits extended diapause (eggs remain dormant for two winters). Current management relies on crop rotation, transgenic Bt corn hybrids targeting rootworm (though resistance is a growing issue), and soil-applied insecticides.

European Corn Borer (ECB)

• Identification: Adults are yellowish-brown moths with wavy bands. Larvae are flesh-colored with small brown spots.
• Life Cycle & Damage: Usually 1-2 generations per year depending on latitude. First-generation larvae feed on leaves in the whorl, causing "shot-hole" damage. Second-generation larvae bore into stalks and ear shanks. Stalk tunneling weakens the plant, leading to stalk breakage and dropped ears.
• Management: The introduction of Bt corn revolutionized ECB control, rendering it a minor pest in many areas. For non-Bt corn, scouting and timing insecticide applications based on degree days (targeting larvae before they bore into the stalk) are critical.

Fall Armyworm

• Identification: Larvae vary from light tan to green to nearly black, with a distinct inverted 'Y' on the head capsule and four spots arranged in a square on the last abdominal segment.
• Life Cycle & Damage: They do not overwinter in the Corn Belt but migrate north annually. Larvae feed deep in the whorl of late-planted corn, producing ragged, severely defoliated leaves and large amounts of frass (excrement). They can also feed directly on the ear.
• Management: Early planting helps avoid peak populations. Insecticide applications must be timed to reach larvae deep in the whorl before they grow too large.

Key Pests of Soybeans

Soybean Aphid

• Identification: Very small, light green to yellow insects with black cornicles (tailpipes) on the abdomen.
• Life Cycle & Damage: Overwinters as eggs on buckthorn. They migrate to soybeans in early summer and reproduce rapidly (parthenogenesis - females give birth to live clones without mating). They suck sap from stems and leaves, causing stunting, reduced pod set, and smaller seeds. They excrete honeydew, promoting sooty mold.
• Management: Regular scouting is essential. The universally accepted Economic Threshold is 250 aphids per plant, with 80% of the plants infested, and populations actively increasing (checked from the R1 to R5 growth stages). If natural enemies (lady beetles, lacewings) are abundant, they may keep populations below the threshold.

Bean Leaf Beetle

• Identification: Adults vary in color (yellow, tan, red) but usually have four black spots on their back and a distinct black triangle behind the head.
• Life Cycle & Damage: Adults overwinter in leaf litter and emerge early, feeding on emerging soybean seedlings (cotyledons and unifoliate leaves). Later generations feed on leaves and, more importantly, scar the pods in late summer, which allows moisture and pathogens to enter, reducing seed quality. They transmit Bean Pod Mottle Virus.
• Management: Early season defoliation rarely causes yield loss unless it is severe. Pod feeding thresholds dictate treatment if defoliation is significant or pod scarring exceeds 10-15% with active beetles present.

Broad-Spectrum Pests

Two-Spotted Spider Mite

• Identification: Not insects, but arachnids (8 legs as adults). Tiny, yellowish to green with two dark spots on the abdomen.
• Life Cycle & Damage: They thrive in hot, dry weather. They pierce plant cells and suck out contents, primarily on the underside of leaves. Damage appears as yellow stippling, progressing to bronzing and leaf drop. Severe infestations produce visible webbing.
• Management: Drought stress exacerbates mite damage. Rain and high humidity promote fungal pathogens that naturally control mites. If chemical control is needed, specific miticides must be used, as broad-spectrum insecticides (like pyrethroids) can flare mite populations by killing their natural predators while leaving the mites unharmed.

Black Cutworm

• Identification: Greasy-looking, dark gray to black caterpillars that curl into a 'C' shape when disturbed.
• Damage: Larvae cut young seedlings (corn, soybeans, cotton) off at or just below the soil surface. A single cutworm can destroy multiple plants.
• Management: Weed management is crucial, as winter annual weeds attract egg-laying moths in the spring. Rescue treatments with insecticides are effective if applied when cutting is first noticed and larvae are small.