How to Sanitize Milkweed for Monarch Caterpillars (The 19:1 Bleach & Water Wash)

What the Wash Is For

OE spores are microscopic and cannot be washed away with water alone

Ophryocystis elektroscirrha (OE) is an obligate neogregarine protozoan parasite that infects monarchs (Danaus plexippus) and queens (Danaus gilippus). Dormant OE spores adhere to the external abdominal scales of adult female butterflies. When an infected female deposits an egg on an Asclepias leaf, she inadvertently scatters thousands of microscopic spores around the chorion.

A neonate (first instar) caterpillar eats its own nutrient-rich eggshell on hatching, and takes in whatever spores are on it. A water rinse alone does not remove them: OE spores have hydrophobic lipid coats that shed plain tap water. The 1:19 bleach dilution dissolves the spore protein envelope, and leaves no toxic residue once rinsed.

The Exact 19:1 Bleach Sanitization Formula

Plain unscented bleach only — not scented, not splashless gel

The wash is plain unscented household bleach (regular 5.25%–6.0% sodium hypochlorite) in cool filtered water. Scented and splashless formulations are thickened and carry additives beyond the hypochlorite, so they are not what this dilution is built on. Hot water is not used either: heat accelerates chlorine gas breakdown and wilts the foliage.

Step-by-Step Sanitization Protocol

The four-basin sequence

1. Physical Pre-Inspection: both sides of every leaf go under bright light, and any beneficial insects or wild eggs worth relocating come off first — the bleach bath kills them too.

2. The Bleach Submersion: stems and leaves go completely under the 19:1 dilution bath for 90 seconds, with gentle agitation so the solution reaches the microscopic leaf hairs and crevices where spores sit.

3. Triple Clean Water Rinse: Transfer foliage into Basin 1 of pure water, swirl for 30 seconds, then Basin 2 for 30 seconds, and Basin 3 for a final rinse. This removes all lingering chlorine smell and surface ions.

4. Gentle Centrifugal or Towel Dry: Pat thoroughly between fresh unbleached paper towels. Wet leaves inside closed rearing enclosures foster Pseudomonas bacterial rot.

Safe Long-Term Leaf Storage (Up to 10 Days)

How to keep sanitized leaves crisp and nutritious

Once leaves are sanitized and dry to the touch, they go into an airtight glass container or heavy-duty food-grade ziplock bag lined with a slightly damp (not wet) unbleached paper towel, laid in flat single layers.

Storage is 38°F–42°F (3°C–5°C) in the crisper drawer, which holds moisture and sugar content while inhibiting mould. Refrigerated leaves come back to room temperature over 10–15 minutes before going into the caterpillar enclosure, since cold leaves can trigger digestive stasis.

Where the Wash Goes Wrong

• Using dish soap or castile soap: Soaps contain surfactant surfactants that coat caterpillar spiracles (breathing holes) and cause asphyxiation.

• Skipping the rinse step: Chlorine residue burns larval mouthparts and induces regurgitation.

• Harvesting from nurseries with systemic neonicotinoids: Bleach eliminates biological pathogens but CANNOT remove chemical systemic insecticides that travel through plant vascular tissue.