black swallowtail caterpillars dying

[St. Louis gardener]

Why do some caterpillars go through the stages to chrysalis fast, and others take a longer time? And why do some just stop growing and die, even though there is plenty to eat? I have been raising black swallowtails for the last few years. This year we had an overabundance after the Midwest drought and heat finally broke the end of August. We only have a handful left that haven't yet made their chrysalis, but in the last week or so, several smaller ones and one large one died. The small ones I find hanging from a parsley stalk, usually upside down, with a drop of liquid on their head/face. When I touch them, they put out their orange antenna as a reflex, but they still die. Is there something innately wrong with them (I.e., Mother Nature's way of "aborting" the weak)? Is it the lateness of the season? Am I doing something wrong? Even though we still have about 45 caterpillars (in other years we've never had more than 3-4 at a time), I'm still heartbroken every time one dies. Does anyone out there know why this is happening? Is it just the law of averages? I read that a female black swallowtail butterfly can lay up to 100 eggs at a time. What percentage would survive in the wild if someone like me wasn't around to make sure they had enough parsley/carrot tops, didn't drown in a rainstorm or get eaten by predators, etc.

[applestar]

My kids and I raise Monarch butterflies every year, and as an adjunct also raise any other caterpillars we come across including Black Swallowtails.

In my experience, small percentage do die. Typically, they have bacterial disease such as Bt which is naturally occurring, but they may have been infected by garden or agricultural application. For this reason, I only use it as a last resort in my own garden. Wind or insect, contaminated pollen transference are other sources of infection. The caterpillars can die at any stage but most often at very young instars -- they just stop feeding, turn brown and turn into mush.

(Note that GM Bt corn pollen has been shown to kill Monarch caterpillars ingesting wind-blown pollen covered milkweed)

If an older -- at least 3rd instar -- caterpillar (up to and including a new chrysalis) hangs limp and dies, be sure to examine the remains for trailing mucus strand(s) and inspect container closely for maggot(s) or a brown pupae of Tachnid fly parasite.

Monarch butterflies are also sometimes infected with an organism commonly abbreviated as OE. The adults carry the spores on their wings and pass the infection to their offspring. OE infected butterflies can only be discovered by examining the wing scales under the microscope, and carriers wow no sign of disease. But the caterpillars often have black splotches on them and infected chrysalis show black shadows inside. -- I don't know if Black Swallowtails are also subject to infection.

Then, of course, some caterpillars are collected from plants that have been chemically sprayed or have been caught in herbicidal or insecticidal drift. Often the chemicals are neurotoxins and they deteriorate quickly.

Despite these heartbreaks, I think it's incredibly rewarding to raise the healthy caterpillars and watch them metamorphose and eclose as butterflies, don't you?

[St. Louis gardener]

Absolutely, it is very rewarding. I know there are no pesticides on the parsley and carrots I grow from seed, so anything like that would have to be airborne. I'll certainly check the butterfly cage for maggots or pupae that shouldn't be there, but it was the younger caterpillars that I noticed hung lifeless and had clear mucous on their faces, not the larger one. The larger one looked and acted fine until one day he was just on the bottom of the cage, dead. He was the last of a group of caterpillars that was about the same age, and he just sat around for a week to 10 days, not eating or looking for a place to "stick" to, so I knew something was wrong. Mostly once they stop eating they start wandering around the cage, and attach either to a stick, the side of the pot, or, more often this summer, the top of the cage, just under the seam where the zipper is. My guess is they like the mesh. I even had one stick to the bottom of the cage at the bottom seam. Strange little creatures, but I love them!

[biwa]

I wonder if they would do better with more food choices? I could be mistaken, but I was under the impression swallowtail larva liked cherry tree leaves, tulip poplar leaves, and paw-paw tree leaves.

[St. Louis gardener]

I thought they only liked plants in the carrot family, including fennel and parsley. I raise them on parsley, and when that's gone they get the carrot tops. But the choice of food can't be what's killing them, because the ones that are dying were outside the longest (the last batch of eggs to be laid, I suppose), and they enjoyed the parsley the longest. That's why I thought it had something to do with how late in the season they were coming to maturity. I only lost one earlier in the season that was nearly fully grown. All the others (8) were in the first or second stages of life. Since I never lost any before this (of course I never had more than 3 or 4 at a time before this summer), I'm wondering if 8 out of 55-60 cats is a normal percentage to lose.

[lily51]

We had lots on our fennel last year. Thought I read somewhere that those caterpillars that developed too late in the season died.

[St. Louis gardener]

I would be anxious to read about that, because that's what seems to be happening here. Can you remember where you read it? Many thanks.

[lily51]

I've been looking, but no success yet. I'm sure it was an Internet site, though. When il do, I'll let u know.

[lily51]

Here's some information. Some butterflies enter diapause , the insect version of hibernation, to survive winter. To do this they lower water levels in their bodies and produce anti-freeze type compounds.
Each species that survives by diapause is programmed to do so at a specific stage...some at the egg, others at larvae, etc. it is triggered by daylight hours.
Black swallowtails enter diapause in the chrysalis stage. So I'm thinking if it is time for that but the caterpillar is not ready to form cryssalis, they die.
Or if for sOme other reason they are prevented from doing so.
Hope this helps.

[applestar]

I don,t know if that could be the reason for them dying. From what I have seen, caterpillars are more liables to prematurely form chrysalis (making tiny chrysalises and enclosing as tiny butterflies if they survive.

As for food/host plants -- there are different species of swallowtails -- Blackswallowtails eat carrot family and rue plants, there are also Zebra swallowtail that specialize in PawPaw, Tiger swallowtail that eat tulip tree and sweetgum, Spicebush swallowtail that eat Spicebush and sassafras, pipeline swallowtail that eat pipevine/Aristolochia, Giant swallowtail that eat citrus, etc. Can't remember what eats cherry... I'm thinking Tiger swallowtail eats wild black cherry, but I have to look it up.

[biwa]

We have Tiger Swallowtails around here, not Black Swallowtails, thus my confusion about the host plants. I do wonder though, if larval food plants for swallowtails can cross species boundaries. My paw paw trees always have holes eaten in the leaves, and yet I've never seen a Zebra Swallowtail around here.

Are you keeping these caterpillars inside or outside? If they're inside, how do you get them to go dormant for the winter? Maybe wait until they've changed into whatever their over-wintering state is (chrysalis I guess?) and then move them outside or into a refrigerator?

I wasn't aware that Tiger Swallowtails liked sweetgum trees. Maybe I can hate my sweetgum tree a little less now. It's a very pretty tree, but the spiky pinecone things it drops are annoying.

[applestar]

We keep them inside. Some people say this isn't true, but in my experience, black swallowtail caterpillars make tan/brown crystals when they go into hibernation mode (I was intrigued, because these are much better camouflaged in frost killed vegetation than their summer green chrysalis). Once they make those brown chrysalis, they won't come out until spring unless unseasonably warm temperature confuses them. Normally we put the butterfly chrysalises and moth cocoons in the unheated garage.

My pawpaw trees are only knee high and my spicebush is currently being overwhelmed by the summer sweet which fell over because it was overwhelmed by an errant wild blackberry. No zebra or Spicebush caterpillars so far, though I'm hopeful and keep looking for them. Currently looking for a place to plant aristolochia.... Tiger swallowtails come to my butterfly garden all through the summer but not because of any of my plants. Thinking they use the sweetgums and other trees in the woods behind my house. I do have a tulip tree in the backyard now though.

Oh, BTW, the milkweed vines from those seeds ou sent me are doing very well in both locations I planted them. They fill the arbor trellis along with the trumpet honey suckle growing on the opposide side of the arbor at the entry gate to the butterfly garden, and compete with blackberries in a shadier spot on the other side of the garden.

I planted turtleheads in hopes of attracting Baltimore checkered butterflies (not swallowtail) -- a long shot around here but I'm still hoping.

[St. Louis gardener]

I've always only have black swallowtails, though I have seen one or two tigers from time to time. That's because BS cats love the parsley and carrots I plant every year. I bring mine inside until they form their chrysalis, then move them to the shed during the late fall/winter. I had only 7 of about 55 or so emerge last month. All the rest I believe will overwinter with us. The last batch are the ones that have been dying. Those in the 1st or 2nd stage. There are only 5 now that haven't made their chrysalis. One is 1st stage, one is 2nd stage, one is 3rd stage, and two are, I think, ready to stick. I'm concerned because none of the 5 seems to be eating, they don't seem to be getting any bigger, though all are still alive. I just wonder if there's anything I can do to speed them along toward forming their chrysalises.
I have some "stick" on sticks, and their chrysalises are always brown. I have MANY stick to the mesh at the top of the cage (and even one at the bottom), and they're usually brown, but those that stick to the parsley stalks as always a beautiful bluish green. It amazes me how they know what color to turn to blend into their surroundings and increase their chances of making it to adult (butterfly) stage. I could tell them that they need not worry, since I always take good care of them, but instinct is stronger, obviously!

[lily51]

Being hatched too late in season is what I found out caused some of our swallowtail caterpillars to die last year. St Loius, for the reason I explained.
If that's the case, there's not much you can do for them.

[biwa]

If the problem is that they were hatched to late, it seems like there ought to be something you can do. Maybe simulate earlier in the season with a light or a heater? I dunno. Or are we thinking they are just defective caterpillars because it was cold out while they were developing and they turned out wrong or something?

@applestar: That milkweed vine is cool. I've been trying to spread it throughout my yard more. I've got 3 patches of it so far. I haven't noticed any monarch larva since that one time though.

[St. Louis gardener]

It could be either (developed too late in the season or were defective to begin with). I wouldn't be surprised if it was the former, since we had such a hot, dry summer. As I mentioned, we didn't see the first batch of caterpillars on our parsley until mid-August this year. Those were the ones that already emerged from their chrysalises last month. Normally we begin seeing them much, much earlier in the summer, so having caterpillars this late in the season is very unusual, and moreso since they seem to be taking so much longer to make it to larvae stage. We had another 1st stage cat die this morning. He just wasn't making any progress at all, so I knew it was just a matter of time. I noticed one of the 3rd stage fellows took his big green "dump" today (down the side of the cage, thank you very much!) so I'm hoping he sticks soon. We're supposed to get some warmer weather this weekend, so I'm keeping them outside as much as possible. I thought the cooler weather would speed their internal clocks along toward getting into their warm cozy chrysalises for the winter, but maybe warmth is what they need. I'll let you know how the last four do. Thanks for all your encouragement.