system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I built a garden box in a spot where there had never been a garden. I have no other gardens. I planted kale, and 2 weeks later it was covered in caterpillars. Who told them; where did they come from?
gpt
In a way, the kale itself told them. Butterflies use their antennae to smell chemicals wafting through the wind produced by plants. Cabbage white butterflies (*Pieris brassicae*) in particular are attracted to mustard oil glycosides, produced by most plants of the *Brassica* genus, including mustard "trees", turnips, and of course, *Brassica olaracea*, of which kale is a cultivar - along with cabbage, cauliflower, broccoli and Brussels sprouts.

They can smell the kale, and know that that's a good thing for their caterpillars to eat, so adult butterflies will plant their eggs on your kale and when the eggs hatch, boom, caterpillars.