Missing Larvae Found!

Isolation cage in shade house

In the last few days I have begun to fill a major gap in our knowledge of the Passiflora-feeding flea beetles.  I found the missing larvae!  Or at least some of them.  And they are different!

After returning from our holiday trip to the USA, I saw that one of the isolation cages (cage C) had more flea beetle adults than expected:  I had put 9 Parchicola DF1 adults (Black-legged Yellow or YBL for short) in the cage but now had 10.  Other cages had good survival over the 6-8 week interval, but even 100% survival could not account for this result.  I reasoned that the beetles had successfully reproduced in that cage.  The cages are kept in a "shadehouse," the equivalent of a greenhouse here at La Selva.   Here, the only danger to potted plants outdoors is too much heat from the sun and herbivores such as leafcutter ants.  In a shade house, porous cloth is used to exclude herbivores and about 70% of sunlight.  This creates a good balance between sunlight for growth and reducing heat build up.  Rain goes right through the cloth so watering is usually not an issue unless there is a dry period.

Black-legged Yellow flea beetle egg, <1mm long

To see what might have happened I carefully opened a slit in the cage C door and shook the foliage, driving the 10 beetles to sit on the cage wall.  I then removed the entire potted plant (actually there were two potted plants in this cage, both Passiflora oerstedii "OER"), and started a close examination using magnifying glasses and a bright headlamp. The first thing I found were five loose clusters of 6-12 tiny spine-like eggs, on the underside of the oldest leaves of the plant.  Newer leaves had no eggs, and I could not find any larvae.  I took the leaves into the lab and photographed them using the stereo microscope.

Black-legged Yellow flea beetle larva. 

Next I looked at the soil near the base of the plant where the stem enters the ground.  There, near the surface, was a dead flea beetle larva, but one I had never seen before.  It looked like a smaller version of the Ptocadica larvae that I often find on the Passiflora (see previous blogs for photos) except that the dorsal and lateral protuberances (bumps on the top and sides of the body) had short spines tipped with tiny spheres!  I have not seen this form of larva before, and it is safe to say that the YBL larva does not feed upon Passiflora foliage!  I would have seen them in my thousands of hours of looking at plants in the field if they were there.  What is the function of the spheres?  They are composed of cuticle and are patterned.  Perhaps they have a defensive function, dispensing chemicals to potential predators such as ants and spiders that might be found at the base of the plants.  The tiny ants in the isolation cages I have not identified, but they seem to completely ignore the larvae, walking over and around them as if they were not there.

Larvae feeding on Passiflora stem tissue

I then looked closer at the base of the plant, and saw where the larvae were chewing on the stem epidermis.  This behavior is very much like that of Pedilia larvae, which also eat the stem-skin of the their host Passiflora pittieri.  However, looking lower down, about 10 cm into the soil, I saw where the larvae were chewing well into the woody part of the stem, perhaps 30% of the way through.  These stems are tough and woody, but also flexible, and it is not clear that there is a distinct layer of live "sapwood" as opposed to dead "heartwood."  Judging from the recruitment of new adults, and the amount of feeding required, it seems clear that the bulk of nutrition for these larvae comes from this woody tissue.  I also found a pupa in the loose soil near the base o the plant.  I suspect it pupated in an earthen cell, which I unknowingly cracked open in examining the plant.

  

Black-legged Yellow flea beetle pupa

These new findings are from only one species, but three other Passiflora-feeding flea beetles are closely related, Parchicola DF2, Monomacra violacea, and M. chontalensis.  I have not yet seen larvae of these species.  My guess is that they also have basal stem (or root) feeding larvae, and that with luck my other cages will revel their presence.  I set up cage C with a different Passiflora species (P. auriculata; AUR) to see if they will reproduce on that plant.  In nature, I find 5-10% of adult YBL on AUR, but who knows about the larvae?   I will try to find out.  Back to work!

Black-legged Yellow flea beetle adult