Monarch Caterpillar Diet & Size/Vibrancy

I live in south-central Pennsylvania. Each year we begin only seeing monarchs in late-July, sometimes early August and as a result, our milkweed has already been decimated by aphids or, is actively in the process of being decimated. Many years we’ve witnessed monarch caterpillars struggling to consume what little plant material remains; they often consume aphids in addition to plant matter. They also move from instar to instar at such a slow rate they are sometimes attempting to consume seed pods that are beginning to dry/crack (in late August/early September).

This summer the adult monarchs visiting our yard are all very small and are also faded in their coloration. I did a quick google and see that it has been proven when larvae struggle to find adequate nourishment they tend to be smaller and also less vibrant as adults.

I am curious: is it possible our smaller/faded returning adult monarchs are the same ones who eclosed in our backyard last summer - thus, they are the caterpillars we observed struggling to find nourishment?

Or…is my theory goofy…are butterflies not like herps (who - I think - return to their birth ponds to mate)?

In NZ we don’t have migrating monarchs as such, they tend to move around but it’s not migratory like in the US. As far as I know, the migratory US ones go through a number of generations during the migration, so it won’t be the same individuals returning that left at the start


I was pretty sure that is the case…but I thought maybe these guys were the ones we saw struggling last summer in our own backyard.

I suppose this is an indication the monarchs are still struggling here in the US. I’m curious why monarchs would travel so far north to mate/deposit their eggs - risking a dwindling food supply for their larvae - why not make a shorter journey and settle farther south?

I wonder if the milkweed supply is still (despite efforts to educate the public) lacking in the entire US? Are they literally traveling to where the food is?

I am completely speculating here, but I think it is a “numbers” thing… they emerge in such high numbers that the larvae rapidly consume available food supplies, and not finding a large amount of food plant available for oviposition the adults tend to travel in a certain direction looking for more larval food plants, perhaps following the direction in the sky that the sun generally sits, so tending south in your winter and north in your summer. Here in NZ we don’t get a large amount of them, maybe a tree overwintering a few hundred, but certainly not the large numbers in one place like is seen in North America, so larval food plants recover at a rate that can keep them happy…

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Well, sadly… when I was a child, and even just a couple decades back, there were hundreds of ~thousands~ of monarchs in their winter perches at the Santa Cruz/Monterey coast. On a chill morning visiting the butterfly groves, we could not see the actual (huge) trees as they were completely covered with sleepy butterflies. When the fog burned off and the temperatures warmed into the 70s F, the air would literally FILL with fluttering insects. Now, there are, perhaps,1200 hundred individuals in a favored grove during the winter season. Seeing 10 to 20 butterflies when the air warms is terribly sad to me, though the younger visitors like it.


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