Lifecycle of the Monarch – Female monarch butterflies lay three to four hundred of their eggs on the leaves of the milkweed plants. The female will spread her eggs over many milkweed plants. The subsequent larvae will feed on the milkweed leaves during their larval stage and increase their body mass more than two thousand times. The caterpillar will then form a cocoon or chrysalis. The metamorphosis occurs inside the cocoon and after about 9-15 days, the butterfly will emerge.