Monarch butterflies lay eggs on Common Milkweed near our patio as early as April, when the shoots are barely 2 inches tall. This year, until today, no caterpillars. Why is any one's guess.
It was reported in March, 2010 that the overwintering Mexican population, devastated by storms, had decreased by as much as 60%. That summer, frustrated by my inability to find a Monarch chrysalis in the yard, despite seeing Monarchs everywhere, I purchased a butterfly house (basically a 2 1/2' by 1 1/2' net bag) so I could see the transformation from caterpillar to butterfly. I was soon overwhelmed. The milkweed I brought in had so many eggs attached I was unable to keep up with the emerging caterpillars (I ended up harvesting milkweed from the abandoned railroad track nearby). Friends with milkweed gardens reported similar experiences, both last year and this.
A quick search of the "monarch tracker" on www.learner.org/jnorth/monarch shows nothing amiss for this year. I'm not taking any chances - this little guy is safe in the butterfly house!