Mine used to die in the summer heat every year, along with my dill and parsley, no matter how much I watered it, even when I grew it in mostly shady spots. Now I only grow rue for black swallowtails, and that needs to be in part shade/sun to keep it from dying in the heat. The only time my dill, fennel or parsley ever looked good was in the winter and early spring, so I assume cool temps are what they need to thrive.
Is it hot or what?! Whew!
Sherry

Are you letting your fennel flower? It takes a lot of the energy away from it. If you cut off those flowering stems, it will improve the appearance, and grow more foliage. That works for me in Oklahoma, and we get pretty darned hot and humid here, too!

Susan

The one dying is trying to flower. One is still healthy. It has 23 eggs on it. My bronze is not doing good either. My dill is basically dead. My parsley is dying, also. I haven't had much luck with rue. I have a little in pots. I am worried about the large number of eggs on the one fennel. This is the first time I have had that much.

You can always feed them the Rue. They will "trade" foods easily, IMHO, and experience.

Dill and parsley are cooler weather plants. I imagine it's a bit warm where you're located for them. They will work in the cooler months, but when temps get in the 90s and 100s, they really poop out.

You can trim your fennel and put the eggs in a container with a tiny sprig of fennel. Keep changing the sprig until the eggs hatch so they have a bit of food to get them started.

Susan

I kept trying to grow rue from seeds, but always killed it. They are still in pots. I have not been able to find rue here. I called every nursery in the phone book. Several did not know what it was. I finally ordered 2 plants. Now I have a new batch of cats. If I run out of food I will go to the grocery store and buy parsley. I still have one healthy fennel.

I think that Shady Oaks Butterfly Farm carries Rue plants as well as fennel (Maybe). You might check them out.

Here is a link that might be useful: Shady Oak Butterfly Farm

Carrots also are a host plant for the BST and I plant a huge patch of them every spring and fall. I don't care if I get any carrots or not, just the tops for the cats. It has been a life saver in the food department for those cats!

My fennel is looking horrible as well. Much to my surprise, I have found about 20 caterpillars (all 1st instars) since yesterday! All on that dead looking fennel! And never saw the eggs! I have seen one female BST fluttering around, but she didn't lay any eggs while I was watching. They are so sneaky laying their eggs!

Mechelle

4 BST cats ate all fennel fronds from 6 plants that weren't long out of their little growing pots. Eggs were laid on 3 plants that were still IN pots on patio.

Was told they'd switch to parsley which they did from 6 plants in garden.

The fennel were getting a little "long in the frond" and yellowing/drying out/flopping with the heat. After cats went into chrysalis I kept watering all six (3 in ground, 3 now in larger pots) and they're sprouting new fronds from joints. I cut off stems just above joints or anyplace I saw new growth.

Just had yellow swallowtail nectaring in garden but didn't go near host plants as far as I could see. Maybe I'll find some eggs/cats anyway.

The fennel really wants to flower badly, and sends up numerous stalks of flowers. I just cut them off as far down as I can. The butterflies really don't nectar on the flowers, just the WASPS! EEEEK! Sometimes you can find carrots at the market that still have the feathery foliage on top. You can also purchase these, cut toward the top of the carrot and plant the shortened carrot in the ground for foliage.

Zizia aptera is another plant that the BSTs will eat. It is not their preferred host, but the zizia doesn't die back like other apiaceae family members. I have had eggs on my zizia and also fed them to BSTs when running out of other food for them.

Susan