Are the dainty sulphurs in San Diego county an irruption?

All of a sudden (this past week) there are Dainty Sulphur butterflies (Nathalis iole) all over the west part of San Diego county, after not being seen much prior. Does this constitute an irruption? I’m not sure about the appropriate usage of the term.

Also does anyone have knowledge or a theory on why there are so many right now, this year, this week, etc. I had never seen one before, and have seen about 40 in the last week across multiple locations.


It certainly sounds like one. Here’s a bit more information - A Butterfly Boom | Your Great Outdoors ( My knowledge of butterflies is small, but I saw an irruption of a different species a few years ago in the fall. I don’t know if the causes are fully understood.

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I don’t believe it necessarily means there’s an irruption. They do tend to be a lot more common this time of year in general

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I’ll just add that there have also been large numbers in the deserts of San Bernardino Co. I was out about 1.5 weeks ago and didn’t see any, but then visiting the same areas one week later, I saw at least 50-100 Dainty Sulphurs.

I think part of it is that the deserts actually got some decent summer rain. I’ve also heard that Arizona has huge numbers of butterflies right now. This may result in more strays than typical.

There have been quite a few American Snouts so far, and I know of a handful of Variegated Fritillary reports.


Rain, that make sense. =) Thanks for sharing.

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Dainty sulphur is irruptive. It’s usually pretty uncommon in California, but this year is a boost in them or sure. And yes, Arizona is crazy right now. There’s tens of thousands of snouts and sleepy oranges. Thousands of thousands. The grasses and shrubs are covered in them, and so is the sky.


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