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Monarch Butterfly Larvae Are Cannabalistic

The very first meal that a Monarch Butterfly caterpillar eats is its own eggshell.  In order to hatch, it eats its way out of the egg, and then polishes off the remainder of the eggshell.  It then starts to wander around the leaf and if it finds another Monarch egg, it will start to eat it.

Female Monarch Butterflies lay 300-500 eggs over two to five weeks of egg-laying. Normally, a Monarch only lays one egg at a time (on the underside of a tender, young milkweed leaf).  It is fairly rare to find more than one egg on a leaf, or even on the same plant.  After a female lays an egg, several seconds up to a minute goes by before she lays another egg (referred to as a refractory period). During this time she usually moves on and finds another milkweed plant on which to lay the next egg.  This lapse of time between the laying of each egg probably evolved to discourage the laying of multiple eggs on one leaf and to encourage the dispersal of a female’s eggs on different milkweed plants so as to decrease the chances of cannibalism occurring.

According to Dr. Lincoln Brower, renowned Monarch entomologist, a cluster of Monarch eggs on any given milkweed leaf indicates that either milkweed is in short supply, or the female that laid the eggs is either sick, very old or she has been flying for a very long time and several eggs have matured.

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11 responses

  1. Alice Pratt

    What a beautiful photo! I only saw a Monarch here, twice, a few weeks ago, then Wed., often & yesterday I saw two! At last took some photos, the female flies around so much. The male was here for hours and hours during the afternoon. So pretty to see them flying around. I must have 200 Milkweeds, so plenty of nourishment for caterpillars…now I just need to find gnawed leaves. Only saw one grown up caterpillar, last year, the next day it must have made a chrysalis.

    July 12, 2019 at 9:10 am

  2. Susan Greenberg

    Considering the small size of the monarch egg – this is an amazing photo, Mary. Do you know about Montreal’s Insectarium’s “Mission Monarch” – It is a citizen science project to count monarch eggs, chrysalis, and butterflies any time with a blitz week in late July. Check our the web site –

    July 12, 2019 at 9:30 am

  3. Alice

    (My comment didn’t get posted, so I’ll try, again) monarch here this morning. I’m truly wondering how the Monarch can possibly remember on which Milkweed plant she laid an egg, especially if she lays it on the bottom of a leaf.

    July 12, 2019 at 3:34 pm

    • Great question. I’m guessing pheromones or some chemical communication tells her, but I don’t know for sure.

      July 12, 2019 at 4:08 pm

  4. Alice

    Your wonderful posts certainly make me think more about everything I encounter, Mary, thank you!

    July 12, 2019 at 5:03 pm

  5. Sally

    Hi Mary, I have long been a lover and keeper of monarchs. I preserve many milkweed plants but when they infringe on my cold frame, I remove them. After I’d pulled a 10” plant I remembered this mornings photo. So I checked under the leaves. Sure enough there were 2, each on different leaves. Now the milkweed sits in a vase of water. What do you think – will they survive if I supply new leaves as they grow?

    July 13, 2019 at 1:23 pm

    • They certainly should, Sally. Good for you for looking!

      July 13, 2019 at 6:31 pm

  6. Bill On The Hill

    Very nice macro shot Mary. DOF is practically non existent…
    Bill… :~)

    July 13, 2019 at 1:38 pm

  7. Mairead

    This apparent adaptation to reduce cannibalism is fascinating. Having seen the level of cannibalism when hatching praying mantids, I wonder why more insects don’t do this?

    July 21, 2019 at 1:18 pm

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