Why are parasites not affected by the plant poisons (alkaloids) the caterpillar has sequestered?
Many caterpillars munch on plants which are highly toxic. For instance, the Tobacco Hornworm Manduca sexta can be found lunching on practically anything in the Nightshade family Solanaceae. Which includes: tobacco, petunia, tomato, pepper, potato, nightshade, etc. M. sexta can sequester these poisons and be unaffected itself. However, these plant poisons in the caterpillar's body don't stop the Microgastrinae wasp Cotesia congregata from turning the hapless caterpillar into an incubation chamber. Why? Are Microgastrinae unaffected by alkaloids, period? Or are they affected by some and not others? There are many different species of sphinx most of which are named after their different plant hosts. Does knowing the caterpillar's preferred food plant tell us anything about which wasps or even which tachinids will use it as a host?
Is the Catalpa Sphinx Ceratomia catalpae is really parasitized by the Cotesia congregata wasp? Or do plant alkaloids matter? The fact that the wasp cocoons are larger and fewer in number than those seen on M. sexta makes me wonder if it is indeed a different species of wasp. There is vast difference in width between these two varieties of Sphinx caterpillars. Is this difference in mass a superficial difference only? Or does it affect the choice of parasite? For that matter C. catalpae. is much skinnier than most of the other Sphinx caterpillars which makes me wonder if it shares less genetic material in common with the other members of the Sphinx family.
Take the challenge and raise the wasp cocoons from C. catalpae or some other variety of sphinx caterpillar to see if they do indeed turn out to be Cotesia congregata or some entirely different species or genus of parasitic wasp.
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Are the plant poisons sequestered by the caterpillar localised in certain parts of the caterpillar's strictures i.e. tubercles and are there primarily to deter larger potential predators like birds? Could the internal parasite have evolved to avoid eating the parts of its host that has the poisons or is the parasite able to consume and localise the poisons in the same way as its host does?
I'm not an expert just speculating on your interesting article :)
That is an excellent question: Where does the Sphinx keep its toxins? Or does the parasite also sequester the poison as you suggest? Or is it a case that the plant toxins have no effect on the braconids or tachinids because their biology has no connection to it? As a citizen scientist, I have more questions than answers right now. Fascinating to think about, however.
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