And then there's something else they're doing - they're mating. Because our temperatures are falling, we know the butterflies will go into diapause soon, so perhaps this is a final push to inseminate the females before diapause happens. I have looked into how White Butterflies mate and it's remarkable.
Below I share information from several researchers - please know this is not my work, it is entirely theirs. Nathan Morehouse and his colleagues, including Nathan Clark who is also at the University of Pittsburgh, have shown that the sex life of this seemingly unremarkable butterfly is utterly remarkable.
"A cabbage white’s ejaculate is very different from a human’s. Rather than a blob of white gunk, it’s a complex solid package called a spermatophore, which consists of a hard outer shell, soft nutritious innards, and a ball of sperm at the base. The male deposits this into a pouch within the female reproductive tract called the bursa copulatrix. Once inside, the sperm swim off into a second pouch—the female will later use these to fertilize her eggs. Meanwhile, she starts to break down the outer shell of the spermatophore to absorb the nutrients within. So, the spermatophore acts as a nuptial gift—a way for the male to nourish the mother of his future offspring, long after he flies away.
As gifts go, the spermatophore is a substantial one. On average, each packet makes up an astonishing 13 percent of the male’s body weight. “Scientists who work on ejaculates will often show up to meetings with props,” says Morehouse. “I’ve never had it in me to bring a five-gallon bucket along. But that’s what we’re talking about [if you scale it up to human size]. It’s a water-cooler-sized ejaculate.”
The female only mates again when she has drained her existing spermatophore of nutrients, which is why males have evolved to coat these packets in layers of extremely tough proteins. To analyze these layers, Morehouse and Clark’s team first had to break them down. And they could only do that by boiling them in concentrated sulphuric acid. It’s as though the cabbage white’s nuptial gift is sheathed in nearly-indestructible wrapping paper.
A female butterfly has no access to sulfuric acid. Instead, she chews her way into the spermatophore using an organ called the signum, which sits inside her bursa. It looks exactly like a pair of toothed jaws, with a hinge in the middle. “I like to tell people that we found the vagina dentata, but it’s in butterflies,” says Morehouse. When a female takes up a spermatophore, her bursa clenches and contracts, and the signum flexes around its hinge. It takes between 24 and 36 hours of constant chewing to break into the spermatophore.
To help the signum along, the bursa also secretes a cocktail of proteases—the same enzymes that stomachs use to digest food. But Morehouse’s colleague Melissa Plakke showed that the bursa produces 20 times the concentration of proteases as cabbage white caterpillars have in their guts. These insects have a reproductive tract that’s more stomach-like than their own stomachs. And while all this is happening, the actual sperm get out of dodge. Within minutes, they swim out of the bursa and into a separate pouch, so they’re not degraded by the flood of digestive enzymes. "
....And there's more, which you can read in their article....
"This Common Butterfly has an amazing Sex Life"