To my knowledge aphids are not harmful to monarch butterfly eggs. Ants, however, are another matter. There are usually a few ants on our milkweed, and I have to remove monarch eggs if I want them to have a chance to develop.

That's good news. Thanks. I brought some of the milkweed stems in, rinsed them off very well. I hope they dont get invaded by ants in the future.

That's good news. Thanks. I brought some of the milkweed stems in, rinsed them off very well. I hope they dont get invaded by ants in the future.

Can you remove the egg from the mw s damaging it? I just pick the whole leaf/stem that the egg is on and painstakingly transfer the newly hatched larvae onto fresh stock. Sometimes, I'll wait a few days before harvest to let the eggs darken so they'll have fresh stock immediately upon hatching. Of course, this method opens up possibilities to all kinds of predators. Occasionally, I'll find a larger one that I missed earlier.

They arrived today and already hatching. I went outside and grabbed some swamp mw and put them on there. I rinsed it off very well. I have to go back outside and get more as 6of 11 have already hatched! Im so glad I have over 18 different varieties now growing... thanks for the feedback.

About how many fresh stems shall I need? One per cat?

I have read that a Monarch caterpillar can eat an 18 inch stem of milkweed. I know they can eat a lot, especially as they get close to pupating. Washing the milkweed and drying it is a good idea. Have fun with your Monarchs. I like your son's birthday present request. A budding lepidopterist!

I see they are eating the swamp mw .. I see the tiny holes on the leaves. The leaves are about 6 inches long. How fast will the consume one of those. Today is day 2 ...there's still plenty of food on the stem for them n I have it in one of those floral things. 1 of of them keeps going 3 leaves over n up where another cat is. I keep moving him. Is that ok? So ive had this mw in here for 2 days now. Is that ok.

I think I'm more excited about the cats than him. :)

I change my mw every other day. I stick a fresh sprig in the floral block next to the older sprig. As soon as all cats are on the new sprig, I discard the old c any frass that may have accumulated.
Be careful when moving a cat. Never move a cat that is still and not feeding for more than an hour or so. It is either ready to molt or just has recently molted. These should never be disturbed until they begin to feed again.
Only move cats that are actively feeding.

I put some tropical mw in there today but they haven't taken to that yet. I cleaned out the cage today too. Just with water. These guys are none stop moving. So tiny too. I can see the size difference in just 2 days!

What are they doing when thy are still? I only moved him bc I read some where that they can eat each other if they share the same leaf.

Larger cats may eat smaller cats, but I've never had any problems c two or more cats in the same instar.
Caterpillars will generally outgrow their skins in 2 days or so, and it takes about a day or so to form a new one under the old.
A larger new head forms just behind the smaller head as well.
During this time, the larva neither eats nor produces frass. When the cat finally does molt, you'll notice an immediate difference in head size, and appetite. The first thing they eat is their old skin. They will generally start eating an hour or so after they molt, as it takes time for their heads and jaws to set and harden.
This is the best time to move them, if at all necessary.

I guess they did that this morning while we were at baseball practice. I can see the difference today from yesterday. Only 2 haven't changed so I moved those leaves to a different cage.
Thanks for all the interesting info.