Tuesday, December 18, 2012

Time for larvae

A bluish-grey larva - maybe Monomacra violacea?
6.2mm long
Larva, larvae, funny words.  Now that I have a picture (at least an idea) of how the adult flea beetles spread themselves across the Passionflower plants, it's time to tackle a harder question: what are the flea beetles' larvae eating?  There is a possibility that they are more specialized than the adults, since they not only have to survive out there, but also need to grow from egg to pupa.  Heliconius larvae grow extremely rapidly, quadrupling in size in just a few hours and then moulting, five times in rapid succession.  They grow 1000 times as large in 5-6 days!  They are true "growing machines."  But what about the beetle larvae?  How fast do they grow?  Which plants can they survive on?  How do they behave when feeding?

A yellowish larva - maybe the Red-white Ptocadica?
5.9 mm long
I began a couple of months ago to bring a few larvae from the forest into the laboratory, putting them on leaves similar to the ones they were found on.  The ones raised in containers in the lab did poorly, not feeding well and many dying in a few days.  I then decided to try to raise them on live potted plants in the greenhouse.  Actually we call it a shadehouse here at La Selva, a frame covered with shady mesh allowing the interior to stay cool by allowing free entry of air while creating shade to partially block the rays of the sun.  Just last week some of the Passiflora lobata plants in the shadehouse became large enough to use, and I put two flea beetle larvae on a new leaf.  They soon began feeding and seem to be thriving.

The Red-white Ptocadica adult.
The adults may stay on the shadehouse plants
for a few days.
I want to measure how fast the larvae grow, but they are delicate and should not be handled any more than necessary.  I found out a way to use my camera as a measuring device.  I put the camera on a single, consistent manual focus, and take a sharply focused picture.  All such pictures should have the same field of view (measured in millimeters) from side to side.  By comparing the length of the larvae in the photo to the full width I can then calculate low long the larva is in millimeters.  In this way I can see how fast a larva is growing by taking its picture every day.   I don't need to touch the larva at all.

A shield bug just happened to walk by,
with the brightest orange spots!

This dragonfly has amazing eyes. 
Are the brown parts sunglsses?
Of course I have to take photos of other creatures, such as the cute Red-white Ptocadica beetle, perhaps the parent of one of the larvae shown above.  Not to mention this spectacular brown-eyed dragonfly, er, brown on the top 1/3 of the eye dragonfly.  and the orange-spotted shield bug that happened to walk by.  The riches of nature never cease to amaze!

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!).