Sunday, December 9, 2012

Sum up first weeks

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!

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.

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.   
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). 

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!).

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