Saturday, March 2, 2013

More similarity/differences and a sum-up statement!

Here's another similarity/difference between flea beetles and Heliconius:

Red-white Ptocadica on Passiflora lobata
I have been putting larvae of RWh Ptocadica on different species of Passiflora to see if they can survive and/or grow.  On their normal host plant the larvae grow and survive quite well, as summarized a few blog posts ago.  Although the results are sketchy, being based on only 2-3 larvae for each plant tested (alas, that was all I had!), there seems to be a correspondence between adult and larval food preference.    Adult RWh Ptocadica clearly prefer P. lobata as a host plant, with P. auriculata as a second choice (less than 10% occurrance).  No other Passiflora are acceptable.  RWh Ptocadica larvae clearly grow fastest and feed the most on P. lobata, but will eat and grow slowly only on P. auriculata of the other species tested.  Species which release high quantities of poisonous cyanide are not fed upon at all, while Passiflora with zero or low amounts may be eaten slightly but with no observable growth.

How does this result compare with Heliconius charithonia, the Heliconius that specializes on P. lobata at La Selva?  H. charithonia larvae feed and grow well on lobata along with many other species of subgenus Decaloba, including auriculata and biflora.  They grow slowly or not at all on members of subgenus Passiflora, including ambigua and oerstedii.  This is similar to RWh Ptocadica, with the exception that species of Decaloba other than the low-cyanide auriculata can't be eaten by the flea beetle.   All the other Decaloba are high in cyanide output.  As stated above, this flea beetle larvae cannot handle strongly cyanogenic food plants.
H. charithonia, only Heliconius that can eat P. lobata

It makes sense that the RWh Ptocadica might be very sensitive to cyanide poisoning, given that the normal, preferred host plant generates no measureable amounts of HCN when crushed.  In contrast, the Red Pedilia seem more tolerant of cyanide.  Although their feeding behavior seems designed to avoid cyanide release whenever possible, selecting thin leaf layers and stem epidermis, this species' host plant P. pittieri is highly cyanogenic.  The feeding trials on alternate Passiflora species indicate increased tolerance, with measureable feeding and growth on several Decaloba species.  This parallels the pittieri specialist butterflies H. sappho and sister species H. hewitsonii, which also show a limited feeding ability on Decaloba species, particularly arbelaezii and biflora.  Molecular evolution data tells us the connection between pittieri and these Decaloba is probably not phylogenetic, but the pattern of insect food choice suggests a similar chemistry or other nutritional factor.  The fact that flea beetles and butterflies share a similar response further suggests that both types of herbivore respond in the same way to these plant nutritional/chemical characteristics.

Flower of P. lobata.  Pretty!
To summarize, I have been able to work with two species of flea beetle larvae, the Red Pedilia and the RWh Ptocadica, both of which specialize on different species of Passiflora subgenus Decaloba.  Both species' larvae show feeding preferences and behaviors that reflect the preferences and behaviors of the adult flea beetles, with complete specialization of Red Pedilia on P. pittieri, and a weaker specialization of RWh Ptocadica on P. lobata and P. auriculata.  This suggests the hypothesis that flea beetle larval feeding preferences and tolerances may be accurately mirrored by those of the adult beetles, which are far easier to count and observe.  In addition, both species have a counterpart butterfly species, Heliconius sappho and charithonia, respectively, that have strong similarities in food tolerance and preference.

Taken together, these findings suggest that  unique chemical and physical defenses in Passiflora foliage create opportunities for herbivores to specialize if they can counteract the defenses.   The findings also suggest that one Passiflora species can host at least two specialist foliavores as long as their is substantial intraspecies variation in plant size, ant presence and/or foliage cyanogenesis.  If these results hold up as more Passiflora and herbivores are investigated, it will make a strong case that host plant foliage characteristics determine species diversity here at La Selva.  Obviously there is a lot more to be learned from these beautiful creatures.  Pura vida! as they say in Costa Rica.  Pure life!