Geranium Budworm (Caterpillar) Infestation: Identification & Control

The Culprit: Geranium Budworm (Helicoverpa virescens)

Your description—hollow stems, black frass (droppings), caterpillars with stripes—matches geranium budworm (tobacco budworm). They bore into stems and buds, causing collapse. The unscented geranium (likely a zonal type) is less preferred.

Immediate Action

1. Isolate infested plants.
2. Prune out all damaged stems to healthy tissue.
3. Hand-pick caterpillars at night (flashlight helps). Drop in soapy water.

Effective Treatments

• Bacillus thuringiensis (Bt) kurstaki: Spray thoroughly (especially bud undersides) every 5-7 days. Caterpillars must ingest it. Safe for beneficials.
• Spinosad: Organic, translaminar—more rainfast than Bt. Follow label.
• Horticultural oil (summer weight): Smothers eggs/young larvae. Good coverage essential.

Prevention

• Fine mesh row cover over plants at night (when moths lay eggs).
• Daily scouting for early signs (frass, entry holes).
• Healthy plants resist better: Full sun, well-drained soil, balanced fertilizer.
• Consider unscented/zonal types if pressure is high yearly.

Saving Remaining Plants

If stems are hollow/blackened to base, they won't recover. Salvage only plants with healthy crowns and roots. Repot in fresh mix after treatment.