Friday, November 29, 2013

Back at La Selva (it's nice to be back!)

Passiflora auriculata in cage.  Soil prepared for beetle pupae.
A few weeks ago Kim and I returned to La Selva for another extended stay (until the end of March).  Kim is already adding animals to her "accidental museum" and I hope to find the larvae of the flea beetles, and see which Passiflora they eat.  I also plan to continue testing the plants and insects for production of cyanide gas when crushed or damaged.

I brought down 6 cages, each large enough to hold a potted plant.  My plan is to put one species of adult beetles in each cage and see what the resulting larvae look like.  Right now I have 5 caged species: Red-brown-white (Ptocadica bifasciata that feeds on Passiflora subgenus Decaloba), Red-white (Ptocadica sp. RWh that feeds on P. lobata), Black-legged Yellow (Parchicola DF 1 that feeds on subgenus Passiflora), Yellow-legged Yellow (Parchicola DSF 2 that feeds on subgenus Decaloba) and Blue (Monomacra violacea that feeds on all species of Passiflora).  I have seen mating pairs in the cages, so I know there are females present in reproductive condition.

I don't need to isolate the red Pedilia flea beetle that eats Passiflora pittieri, since we have a large population already in residence in the lab clearing and I have tested its larvae.  Two other species are simply too rare to work with: Yellow Ptocadica that feeds on subgenus Passiflora, and Monomacra chontalensis  that feeds on subgenus Decaloba.  Perhaps these species are more common at higher altitudes or in other habitats than I can currently sample at La Selva. 

Passiflora garden in La Selva lab clearing.
Before leaving La Selva last April we established 12 species of Passiflora in an outdoor garden, with assistance from botanist Orlando Vargas and the La Selva staff.  The vines thrived in my absence and now, in spite of attacks by Atta leafcutter ants and wild peccaries, have grown into medium-sized vines.  Five species have flowered and set fruit: oerstedii, lobata, auriculata, biflora and megacoriacea.  Two species of flea beetle have been found on the garden plants: Parchicola DF 2 and Monomacra violacea.  I also have a Psiguria vine in the mix, in the hopes of attracting Heliconius butterflies seeking pollen.

Galerucine beetle on P. lobata
I have been using these vines to measure cyanogenesis in Passiflora foliage and for cuttings to use in the shade houses.  I also have found some unusual herbivores on the leaves including Juditha molpe, a metalmark caterpillar (Riodenidae) tended by the Ectatomma ants on P. auriculata.  I also have found a brown-striped species of galerucine Chrysomelidae eating the leaves of P. lobata.  Galerucines are the subfamily of chrysomelid leaf beetles that include the flea beetles (Alticini), and this brown striped beetle resembles a large flea beetle except that it lacks the jumping mechanism in the hind legs. The pattern of eating holes in the leaves also resembles the feeding damage made by a flea beetle. 

An example of looking at patterns of cyanogenesis in Passiflora ambigua can be seen in the graph below.  First, look at the red diamonds in the graph.  They show the amount of cyanide gas produced by the tips of leaves 1,3,5,7,9,and 11, (counting backwards from the tip of the branch).  The branch was fresh and undamaged prior to the leaf tips being removed for analysis.  The next day (day 1), I removed a strip from the cut end of the same six leaves, and measured that cyanide output (see the blue squares).   As you can see, the amount of cyanide increased in the newer leaves!  Also on day 1 I cut and measured HCN from the tips from the undamaged alternate leaves 2,4, and 6.  These amounts (green triangles) were comparable to the original amounts measured for non-damaged leaves.  Thus, the increase seen in leaves 1,3 and 5 (a phenomenon known as "induction") did not extend to the neighboring leaves.  Finally, still on day 1, I cut strips off the ends of the even-numbered leaves to see if there was somehow more cyanide in that penultimate part of the leaf as opposed to the tip.   There wasn't.  The next day (day 2) I sampled the twice-cut ends of the odd-numbered leaves (red circles). Here the HCN content seemed to even out between leaves, increasing in leaves with little HCN and decreasing in those with more.

Taken together, these results tell us that cyanide production roughly doubled 24 hours after a new leaf was damaged and that this response was localized to the leaf in question.  Adjacent leaves did not increase, and neither did older damaged leaves.  A more complex response seemed to occur in the next 24 hours.   One goal of this coming year at La Selva is to conduct studies like these in order to explore how Passiflora may be adapting to their complex herbivore community.  Ater all, they have simultanously to deal with herbivores that thrive on cyanogenic glycosides (Heliconius butterflies) and others that thrive when glycosides are absent (flea beetles).  These balancing agents of selection could seemingly lead to complex interactions within the whole community, including, perhaps, great variability in HCN production within and between species (something I have ample evidence for!).  More about this in my next blog posting.

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