Question and Answer

I've heard of people bleaching their milkweed and monarch eggs while raising monarchs. Should I do this?

Dr. Sonia Altizer, director of Project Monarch Health based at the University of Georgia, contributed the answer to this question.

When raising monarchs to study parasite infection at the University of Georgia, we often bleach milkweed stalks that might have been contaminated with OE spores or other harmful microbes before feeding to monarchs.  We especially do this when caterpillars in our lab die of unknown causes, or if we suspect that OE is present in wild monarchs in the area. Milkweed fed to captive monarchs can be bleached by soaking 15-20 min in ~5% chlorox solution (regular strength chlorox in tap water), followed by a thorough tap water rinse. Be sure to submerge stalks completely and gently agitate under water. Stalks can be partially air-dryed or blotted with a clean towel.

We view bleaching eggs to be an extreme measure that is generally not necessary under otherwise healthy growing conditions, but that can be useful to limit outbreaks of disease. Healthy growing conditions include rearing monarchs singly or at low density, adequate ventilation in containers, preventing the buildup of frass, limiting excess moisture, providing fresh milkweed stalks daily, surface sterilizing work areas and rearing containers, and quickly removing monarchs that show signs of disease. To learn more about healthy monarch rearing conditions, read the Rearing Monarchs Responsibly MJV Handout.

Monarch eggs can be bleached by submerging them in 2% chlorox* for 2 minutes, followed by a double soak in fresh tap water for 5 min each.  This can be done using eggs that remain on the leaf or that have been detached gently. Eggs must be submerged completely and can be gently agitated.  A stronger bleach solution or longer soak time will destroy the egg chorion and result in eggs that appear gelatin-like and lose their structure.

Note that bleaching eggs can help prevent the transmission of viruses and bacteria to monarchs, and can lower the risk of OE transmission, but will not completely kill all of the OE spores.  This is because the OE spores have a thick spore wall, and can be lodged in the grooves of the egg chorion where they are protected from bleaching.  A strong enough bleach solution to kill all of the OE spores would also destroy the monarch eggs.

*To mix 2% chlorox, add 1 part chlorox bleach (regular strength, not 2x) to 49 parts tap water (i.e., a part can be a mililiter or an ounce).

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