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Hi. I have 8 Swallowtail caterpillars on my fennel (NW Brittany). They are small - first and second instars. I’m wondering about their chances of making it to pupating as the temperature/weather becomes less good? Is there anything I can feasibly do to help them along? Thanks for any experiences/advice. The photo shows the most advanced.

Swallowtail larvae grow very quickly about 3 weeks from hatching to pupating so you should be OK - you could always dig up the foodplant and pot it up in a large cage or sleeved pot and bring it inside. Once the pupae harden you can store them in a cool dark place for the winter.
Best of Luck!