Tuesday, February 26, 2013

Differences and Similarities!

I have found that flea beetle larvae are tricky and difficult to work with, at least as compared with butterfly caterpillars.  The biggest difference is that seem very willing to walk off the host plant; something butterflies such as Heliconius rarely do until they are ready to pupate.  That poses a mystery.  If they are willing to leave the host plant, then how do they relocate it later when they are ready to feed?  Do they walk at random until re-acquiring the plant?  Is there an odor trail or way to orient?  They also seem content to go for long periods without eating; again, very different from Heliconius larvae.  Perhaps their walking and waiting ability enables them to relocate the host plant before starving to death.  There is also the possibility that they may feed (sparingly?) on other plants besides their host Passiflora.  Something I need to test.

Red Pedilia eating vein of P. megacoriacea leaf
In the feeding trials I have conducted with flea beetle larvae, I often see signs of feeding from the epidermis of the stems and petioles, and from one layer of the leaf.  These larvae are experts at selectively taking tiny bites from very thin layers of plant tissue.  Even very thin membranous leaves such as those of Passiflora biflora may be selectively skeletonized, with one layer intact and the other removed.  As I mentioned in earlier blogs, this may allow the larvae to feed on cyanogenic Passiflora without getting poisoned.

Red Pedilia larvae turn pink-orange as does their frass.
Red Pedilia is related to Pedilia sirenae found on the Osa Peninsula in Costa Rica.  I have been told by colleagues that the La Selva Pedilia is very similar to P. sirenae except for being bright red rather than yellow-orange.  It is interesting to note that a few of the La Selva Pedilia are yellow-orange instead of the usual red.  Red Pedilia larvae range from pale white to orange or pink, getting pinker as they age.   Their diet changes from young leaves eaten by the tiny first and second instar larvae, to stem and petiole epidermis, eaten by the larger second and third instar larvae.  These latter plant tissues contain a lot of reddish pigment that is very apparent in the brightly colored frass left behind as the larvae feed.  For some unknown reason the larvae often chew leaf petioles, causing leaves to fall from the plant.

Red Pedilia larvae have a pronounced widening of the body over the front segments, effectively covering the head and front legs.  Since the host plant P. pittieri is often well-patrolled by ants visiting the nectar glands at the base of the leaves, this body shape probably protects the larvae from ant predation.  The P. pittieri plant in the La Selva lab garden is well tended by Ectatomma tuberculatum.  This large predatory ant seems to ignore the presence of Red Pedilia entirely.
3rd instar Ptocadica larva on leaf of P. lobata

Larvae of the Red-white Ptocadica ("RWh") and the Red-brown-white Ptocadica (Ptocadica bifasciata a.k.a. "RBrWh") are similar to Red Pedilia, but with recognizable differences.  These larvae have longer legs, and their body is somewhat more narrow, so that the head and legs are more visible from the side.  The lobes of the dorsal surface (the "back" of the larva) are rounded rather than pointed, and are very stiff and "armored" looking.  The legs are strong.  These larvae are more difficult to pick up than Pedilia larvae. 

I should note here that I am only 90% sure that these larvae belong to the genus Ptocadica.  It is possible though unlikely that they are Monomacra violacea, the Blue flea beetle.  For now, to keep things simple, I will assume that Ptocadica is correct.  I hope to make a definitive identification in the near future using genetic "bar-coding."

Tiny RWh Ptocadica larva feeding and growing on P. lobata
The is one very interesting difference between RWh Ptocadica and Red Pedilia.   RWh's preferred Passiflora is P. lobata, a species with hooked hairs covering the leaves and stems, like velcro.  These hooks are very sharp and are capable of killing caterpillars of most species of Heliconius butterflies.  The one exception: Heliconius charitonius.  Apparently charitonius has a thicker cuticle on legs and underside that is resistent to the hooks.  The RWh Ptocadica is also resistent to the hooks.  However, when larvae of Red Pedilia are placed on P. lobata they become hooked and die.  In future I hope to examine both species of flea beetle larvae with a scanning electron microscope to see if differences are visible than might explain these results.  I also would like to test larvae of the RBrWh Ptocadica (Pt. bifasciata) to see if they can survive on Passiflora lobata.  Since this species does not normally feed upon P. lobata I predict that, as in non-adapted Heliconius, the hooks will prove to be deadly.

Red Pedilia larvae hooked to death on P. lobata
I still have the goal of understanding the causes of species diversity and specialization in these two groups.  Comparisons like those above, between the flea beetle and the butterfly larvae, are all I have to go on so far.  In theory, there must be a balance between differences that allow the flea beetles and butterflies to coexist on the same plant species, and similarities that force both kinds of herbivore to split up the Passiflora species in the same way.

I have found major differences in sensitivity to ant predation (unlike butterflies, beetles are not very vulnerable to ants) and presence of cyanide (beetles avoid cyanide in their diet, butterflies do not).  These differences may be enough to prevent flea beetles and butterflies from competing face to face on the same host plant, at least most of the time.  Heliconius are generally more successful on the subset of plants which have few predatory ants, including the smaller, more isolated plants that can be searched out by the super-mobile highly visual females.  Flea beetles appear to be most successful on larger plants that can sustain "colonization" by less-mobile tiny-eyed beetles (are larger plants more likely to be colonized by ants?), also with reduced amounts of HCN-releasing chemicals.

The similarities have proven more difficult to figure out, but so far they are of the kind discussed above: specific adaptations to overcome specific plant defenses such as hooked trichomes, or, conceivably, special chemical defenses.  More about this topic later...

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