This is wonderful news, 13,000 sightings in one location is 6x more than ALL western US locations last year.
Western US populations have collapsed: 4.5m in 80s, 1.2m in 1997, to 100k in 2002, to 2k in 2020.[1][2]
One issue is Monarch larva can only eat two plants: both milkweeds, toxic to their predators. Birds will vomit and then avoid them.[3] However, in recent years not one milkweed plant could be identified without heavy insecticide contamination. Even when no pesticides were used by the landowners or nearby.[4]
Random anecdote: I keep a couple of milkweeds casually on my 4th story apartment balcony in SoCal and I get many monarchs coming thru, sometimes a handful of cocoons at a time. I got a cheap butterfly enclosure and I put the entire plant in it to keep spiders and the like from getting to them either in the cocoon or shortly after. I purchased the plant only incidentally as it looked nice and the monarchs were a pleasant addition.
I'm not sure about the western monarch, but the eastern monarchs out here in the midwest grow as caterpillars here and pupate in to a butterfly. I don't think they lay eggs down in mexico, I think they only have the butterfly form down there.
Additionally, there are three or four generations of them as they migrate. They don't migrate in one huge push. So the ones in mexico will migrate back north a bit, lay eggs, and die. Then that new generation will become adults, migrate north, and lay eggs, and die. Eventually they will reverse and start migrating south, laying eggs, and dying. Finally the last generation makes it to mexico to hang out for a while and avoid the cold, before starting it all over again!
> Which brings us to the reason why tropical milkweed is such a problem in Southern California. See, tropical milkweed works fine as caterpillar food in colder parts of the United States, when it dies back during the winter, killing any parasites that live on the plants. But in Southern California tropical milkweed stays green and blooming year round. Xerces Society researchers believe this evergreen milkweed confuses normal monarch migration and allows harmful microscopic parasites — Ophryocystis elektroscirrha, or OE — to multiply on the plants. Monarch caterpillars end up eating a lot of this nasty parasite as they devour the leaves and researchers believe OE is sickening the adults, messing up their lifespans, migration patterns and ability to reproduce.
Yeah, I haven't been seeing Monarchs for some time now, though just in the past few weeks I saw both a Queen (Danaus gilippus, a relative of the Monarch) and two Snout-Nosed butterflies (Libytheana bachmanii, another migratory species).
Not sure why, but this season does seem to have more butterflies than usual, at least locally. So maybe it's not just Monarchs.
One mutation, and an a species can become resistant to certain toxins. For example, the plant itself is toxic to birds, yet the monarch eats it.
Did some mutation happen, and now the progeny of that lineage are multiplying? If other monarchs die, they'd have lots of food available, so would exponentially grow.
