How to Test Monarch Butterflies for OE Spores: Clear Tape Method, 400x Microscope Diagnosis & Tropical Milkweed Management

What is OE? (The Parasite Life Cycle)

Obligate protozoan replication inside the monarch body cavity

Ophryocystis elektroscirrha (OE) is a single-celled protozoan parasite that exclusively infects Monarch (Danaus plexippus) and Queen (Danaus gilippus) butterflies. The parasite was first discovered in Florida in the late 1960s and is now present across all North American monarch breeding populations.

The transmission cycle occurs through ingestion: when an adult female butterfly infected with OE lands on milkweed to lay eggs, microscopic dormant spores shake off her abdomen onto the egg surface and surrounding leaf foliage. When the tiny first-instar caterpillar hatches and eats its eggshell and initial milkweed leaf, it ingests the protozoan spores.

Once inside the larval gut, gastric fluids dissolve the spore coat, releasing sporozoites that penetrate the gut wall into the caterpillar’s hypodermis and fat body. During pupation, the parasite multiplies exponentially. As the adult monarch forms its scales inside the chrysalis, the parasite produces millions of resistant, lemon-shaped dormant spores on the outside of the butterfly’s abdominal cuticle.

The Clear Scotch Tape Sampling Method (Step-by-Step)

A 100% non-invasive, non-lethal citizen science diagnostic

Developed by Dr. Sonia Altizer and researchers at the University of Georgia’s Project Monarch Health, the tape sampling method samples surface abdominal scales without causing pain, harm, or wing damage to the adult butterfly.

Equipment: standard ultra-clear cellophane tape — frosted or matte tape obscures the view — clean glass microscope slides or clear index cards, clean fine-point forceps, and 70% isopropyl alcohol for sanitising tools between specimens.

Timing: handling comes 4 to 6 hours after adult emergence, or overnight, by which point the wings have hardened and dried.

400x Microscope Diagnosis & Spore Grading Scale

Distinguishing OE spores from butterfly scales under transmitted light

The slide goes under a compound light microscope. 100x magnification locates the cluster of scales; 400x is where spores are counted and told apart from scales.

Under 400x, normal butterfly scales appear as large, translucent rectangular or triangular shingles (100 to 200 micrometers long) with visible longitudinal striations and ridges.

OE spores appear as distinctly small, dark brown or amber lemon/football-shaped ovals (approximately 13 x 8 micrometers). They have smooth edges and pointed tips, looking like tiny miniature brown watermelon seeds adhering to or floating between the scales.

Project Monarch Health uses a 4-tier standardized scale (Score 0 to Score 3) to quantify spore density and assess infection severity.

Interactive OE Spore Density & Reticle Simulator

Calibrate your optical diagnosis against standardized laboratory scores

The interactive simulator below runs through OE spores against normal butterfly scales, across the 0 to 3 scale.

The Tropical Milkweed (Asclepias curassavica) Epidemic

Why non-native evergreen milkweed creates year-round parasite hotspots

The single biggest driver of runaway OE parasite loads in southern and coastal North America is the widespread planting of non-native Tropical Milkweed (Asclepias curassavica).

In native ecosystems, North American milkweeds (such as Asclepias syriaca, A. incarnata, and A. tuberosa) naturally die back to the ground every autumn with the first frost. This natural winter dieback completely cleanses the ecosystem, killing all protozoan spores deposited on old foliage.

Tropical milkweed, however, is a frost-sensitive tropical perennial that remains green and blooming throughout winter in USDA Zones 8–11 (Florida, Texas, Gulf Coast, and Coastal California). Because it never dies back:

1. Spores accumulate exponentially on the same leaves over months, turning backyard gardens into contagious disease vectors.

2. Continuous nectar and foliage trick migrating monarchs into breaking diapause and breeding in winter instead of traveling to central Mexico or coastal California overwintering sanctuaries.

3. Monarchs that breed through the winter on tropical milkweed carry far heavier OE infections than migratory monarchs do.

Ethical Husbandry Protocol: Release vs. Humane Freezing

Balancing compassion for individual insects with wild population survival

One of the hardest ethical decisions in Lepidoptera husbandry is determining what to do when a reared butterfly tests positive for Score 2 or Score 3 OE. Releasing heavily infected butterflies into the wild spreads millions of lethal spores onto wild milkweed, directly causing the death of hundreds of wild caterpillars.

Entomological societies and wildlife conservation bodies hold the same position: a heavily infected monarch is not released into the wild.

Humane Euthanasia Protocol (Rapid Freezing):

1. Gently place the butterfly inside a small glassine envelope or folded paper triangle with its wings folded closed.

2. The envelope goes into an airtight freezer container or ziplock bag.

3. The container sits in a standard home freezer (-18°C / 0°F) for at least 24 hours. Because butterflies are ectothermic invertebrates, cold exposure induces immediate dormant numbness followed by painless cessation of biological functions.

Bleach Sterilization & Enclosure Biosecurity

Eliminating persistent protozoan spores from mesh and rearing tools

OE spores possess a tough outer proteinaceous coat that resists soap, water, vinegar, and standard alcohol wipes. The only verified household chemical capable of dissolving the spore wall is sodium hypochlorite (chlorine bleach).

Biosecurity Protocol for Cages and Containers:

• Dilution: Prepare a fresh 10% bleach solution (1 part standard 5.25%–6% household bleach to 9 parts cool tap water).

• Soak Time: empty mesh pop-up enclosures, plastic bins and floral tubes go completely under the solution for a full 20 minutes.

• Rinse: clean running water until no bleach odour remains.

• Sun Drying: Allow enclosures to dry completely in direct natural sunlight (UV light provides secondary germicidal protection) for at least 24 hours before housing new caterpillars.

Safety: bleach solution and chlorine fumes kill living eggs, larvae, chrysalises and adult butterflies.