5.5 Recognizing Pest Damage: Weeds, Insects, Diseases, and Vertebrates

Diagnosis Precedes Treatment

ATCP 29.30(5) is the shortest route to an obvious point: you cannot select a pesticide correctly without identifying the pest correctly. ATCP 29.30(9)(d) then makes "unnecessary use and misuse of pesticides" its own competency, which is the regulatory way of saying that spraying the wrong thing is not a neutral mistake.

Field diagnosis works backward from a damage signature to a short list of causes, then confirms with direct observation of the organism or its signs.

Step 1: Biotic or Abiotic?

Before naming any pest, settle whether a living organism is responsible at all.

The last row is the strongest single discriminator. A disease that damages corn, an adjacent oak, and the lawn simultaneously is not a disease. It is drift, carryover, a soil problem, or weather.

Common abiotic causes to rule out: nutrient deficiency, drought or waterlogging, compaction, winter injury and frost, salt or de-icer damage, herbicide drift, herbicide carryover from last season's residual, girdling roots, mechanical injury, and planting depth errors.

Step 2: Which Pest Group?

+-----------------------------------------------------------------------------+
|                    DAMAGE SIGNATURE DECISION TREE                           |
|                                                                             |
|   Tissue physically REMOVED (holes, notches, cut stems, frass present)      |
|        --> CHEWING INSECT, slug, or VERTEBRATE                              |
|                                                                             |
|   Tissue present but DISCOLORED (stippling, yellowing, distortion,          |
|   honeydew, sooty mold) with NO tissue removed                              |
|        --> PIERCING-SUCKING INSECT or MITE                                  |
|                                                                             |
|   Lesions with defined margins, spots, cankers, wilts, rots, mold,          |
|   often spreading from a focus                                              |
|        --> PLANT PATHOGEN (fungus, bacterium, virus)                        |
|                                                                             |
|   Unwanted PLANTS competing for light, water, nutrients                     |
|        --> WEEDS                                                            |
|                                                                             |
|   Stunting, root galls, patchy poor growth with no aboveground organism     |
|        --> NEMATODES or root-feeding larvae -- DIG AND LOOK                 |
+-----------------------------------------------------------------------------+

Weeds: Grass vs. Broadleaf vs. Sedge

The first split determines herbicide selection.

The growing point row explains a great deal of field experience. Because a young grass keeps its growing point protected at or below the soil line, mowing and contact herbicides injure grasses far less than broadleaves, whose exposed apical meristems are destroyed by the same treatment.

The second split is life cycle, which determines timing:

• Annuals complete a life cycle in one year — summer annuals germinate in spring, winter annuals in late summer or fall. Control is easiest at the seedling stage.
• Biennials need two seasons, forming a rosette the first year and bolting the second. Target the rosette.
• Perennials live more than two years and regenerate from rhizomes, stolons, tubers, or creeping roots. They usually require a systemic, phloem-mobile herbicide, often applied in fall when the plant is moving sugars downward.

Insects and Mites: Mouthparts Determine the Signature

Two operational consequences. Piercing-sucking pests are often the ones that vector plant viruses, so their economic threshold may be far below the level that causes visible feeding injury. And spider mites are arachnids, not insects, which is why they need a miticide and why some broad-spectrum insecticides make mite outbreaks worse by removing predatory mites.

Life-cycle knowledge sets timing:

• Complete metamorphosis — egg, larva, pupa, adult. Larva and adult often look and feed completely differently. The vulnerable stage is usually the young larva.
• Gradual metamorphosis — egg, nymph, adult. Nymphs resemble small adults and feed the same way, so both stages damage the crop.

Plant Diseases: The Disease Triangle

A plant disease requires three things at once: a susceptible host, a virulent pathogen, and a favorable environment. Remove any one and disease does not develop — which is why cultural controls that change the environment (irrigation timing, canopy airflow, residue management) work at all.

The virus row is a frequent exam item: spraying a fungicide on a virus accomplishes nothing. The only chemical lever is controlling the vector.

Distinguish signs from symptoms: a sign is the pathogen itself or its structures (mycelium, spores, ooze); a symptom is the plant's reaction (spot, wilt, stunting). Signs confirm a diagnosis; symptoms only suggest one.

Vertebrate Pests

Deer, voles, mice, rats, pocket gophers, moles, and birds leave signatures too. Deer tear rather than cut, leaving ragged browse; rabbits and voles cut cleanly at a sharp angle; voles girdle bark at or below the snow line, which in Wisconsin means damage discovered at snowmelt; moles push soil in ridges while feeding on invertebrates. Distinguishing mole from vole matters, because a rodenticide bait aimed at a mole targets the wrong animal entirely.

Any rodenticide bait station placed by a licensed or certified person must be marked under ATCP 29.58 with the name of the person responsible for its maintenance and the EPA registration numbers of all pesticides currently in use.

Step 3: Confirm Before You Spray

1. Sample and inspect directly. Dig roots, beat foliage over a tray, turn leaves over, cut a stem lengthwise.
2. Look at the pattern from a distance, then at the individual plant up close. The two views answer different questions.
3. Use a diagnostic resource. UW-Madison Extension and the UW Plant Disease Diagnostics Clinic exist for the calls you cannot close in the field.
4. Compare to the threshold, not to your discomfort. Section 5.1 covers the economic injury level and economic threshold.
5. Only then choose a product, and confirm the label covers your site and does not prohibit control of that pest.