It's been nearly 2 months here at La Selva, working on the flea beetle project. Seems like a good time to sum up what I have found out. I hope the amount of detail isn't too boring!
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| Ptocadica sp. ("fat yellow") prefers to feed on Passiflora ambigua |
The basic idea is that flea beetles parallel
Heliconius butterflies in the interactions with
Passiflora species. I have found 9 species of
Passiflora here that are common enough to work with, four in the subgenus
Passiflora and five in the subgenus
Decaloba/Astrophea. I have seen five species of
Heliconius feeding on these plants, with three more species that are hard to find but that I know are here (total = eight species). I also have five species of adult flea beetles I can work with, with a sixth species found only in the lab clearing (
Pedilia sp "red".) and two more that are rare but findable (total = eight species). Four of the
Heliconius and three of the flea beetle species feed on subgenus
Passiflora. Of these, two species of
Heliconius and two species of flea beetles also feed on subgenus
Decaloba (I call them "generalist" species). The other four
Heliconius and the other five flea beetles are restricted to feeding on subgenus
Decaloba.
The correspondences are as follows:
Monomacra violacea (Blue),
Parchicola d.f.1 (Black-legged Yellow),
Heliconius cydno, and
H. hecale are generalists, feeding on most or all
Passiflora species.
Passiflora ambigua (in subgenus
Passiflora) hosts the specialist butterfly
Heliconius doris and is the preferred host for
Ptocadica sp. "yellow".
P. oerstedii and
P. menispemifolia host the butterfly
H. melpomene, but don't host a correspondingly specialist flea beetle. In Decaloba/Astrophea,
P. pittieri hosts the specialist butterfly
H. sappho and the specialist flea beetle
Pedilia sp "red".
P. lobata hosts
Heliconius charitonia (which I have not seen yet at La Selva this trip) and is the preferred host for
Ptocadica sp. "red-white".
P. auriculata hosts the specialist butterfly
H. sara and is the preferred host for
Ptocadica bifasciata, the red-brown-white flea beetle, and
Parchicola d.f. 2, the Yellow-legged Yellow flea beetle.
P. biflora hosts
H. erato and is the preferred host for
Monomacra chontalensis.
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| Crematogaster ants sharing Passiflora auriculata nectary with flea beetle larva |
The above list shows a strong correspondence between butterfly and flea beetle use of
Passiflora species, but with some exceptions.
P. oerstedii and P. menispermifolia host the specialist
H. melpomene, with no corresponding specialist flea beetle. My earlier work suggests that the specialization of
H. melpomene on
P. oerstedii and
P. menispermifolia is a consequence of the unusual petiolar nectaries on these plants, attracting parasitic hymenoptera rather than ants.
H. melpomene responds by seeking out and specializing on these ant-free plants, laying their eggs in shoot tips where the parasitic wasps will have difficulty finding them. Flea beetles, in contrast to
Heliconius caterpillars, are seemingly not affected by the presence or absence of ants or parasitic wasps. A second difference is that there are seemingly two species of flea beetles which prefer
P. auriculata, but only one species of
Heliconius (
H. sara) specializes on that plant.
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| Pedilia sp "red" larvae eating stem epidermis on P. pittieri |
One of the biggest differences I have seen between between flea beetles and Heliconiine butterflies is that the beetles tend to avoid plants or plant parts which produce high amounts of cyanide gas when crushed. Even
Pedilia sp. that feeds on the highly cyanogenic
Passiflora pittieri seems to avoid some of the cyanide (perhaps 90%) by feeding on the epidermis of the stems and leaves (see photos of feeding damage). The butterflies seem completely unaffected by the presence or absence of HCN, with caterpillars feeding and growing rapidly on plants with the highest HCN content such as
P. arbelaezii and
P. costaricensis. Also, preliminary measurements indicate that
Pedilia sp. "red" are not themselves cyanogenic, in either the larvae or the adults. This is a strong contrast to Heliconiine butterflies and larvae, which are strongly cyanogenic. Another big difference I mentioned above: flea beetles seem unaffected by the presence or absence of ants (see photo of flea beetle larva with ants).
Heliconius, by contrast, seldom survive to pupation on plants tended by the "wrong" species of ants (such as
Ectatomma tuberculatum).
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| P. pittieri leaf vein that has had its epidermis removed by feeding Pedilia flea beetles. |
The biggest mystery remaining may be what the larvae are doing. Which plants do they feed on? I'm starting to observe mating behavior as the weather dries out a bit, so maybe we'll get a pulse of larvae to observe.
Overall summary of project: Progress! We're having fun! Remarkably stable communities of plants and insects over the 40 year interval (thank you, La Selva!).
