Sunday, January 27, 2013

Making real progress with larvae, but so many mysteries!

First instar larva, crawling up stem from lower leaf.
In my last posting I introduced the subject of flea beetle larvae, and how I hoped to observe them and measure how well they do on different Passiflora.  This past month I have been doing just that.  I have measured their growth rates, placed them on different Passiflora species, and taken hundreds of photographs.  I still don't know for sure which species of larva I am working with (but have narrowed it down to two).  Nor have I seen any of their eggs (although I think I should have since I was looking for them), and have not yet seen a pupa.  Even with all those unknowns, I now have a much better knowledge of what life is like as a flea beetle larva.

The first picture is of a first instar larva, crawling up a stem of P. lobata.  I believe it recently hatched from an egg down lower on the plant, and is now walking up to a leaf where it can feed.  Based on its size, I estimate this larva to weigh about 0.5 milligrams.  This is probably close to the weight of the egg that it hatched from.  Given that I can't find the empty egg shells, I suspect it eats the egg shell as its first meal.

Second instar larva, shown at the same scale as the one above.
After about 2 days the larva moults, shedding its first instar skin and head capsule.  The larva digs the hard parts of its legs and jaws into the leaf, and then moults by lifting its body away from the old parts.  These can usually be found later at the site of moulting.  Moulting seems to take at least a day, and maybe up to 2 days to complete.  The new hard parts are about 1.5 or 1.6 times as large (in linear dimension) as the shed parts.  The flea beetle moulting process takes substantially longer than moulting in Heliconius butterflies, which complete their moult in about 12 hours. 

Second instar larva actively walking across Passiflora vitifolia petiole.
The unusual shape of the flea beetle larva has probably evolved to protect the larva from predators such as ants.  The larva has large, strong legs and an enlarged anal clasper, both of which can grab the plant strongly.  The body is protected by heavy, thickened ridges on the back, and is flattened, covering the head and jaws from view.  The body shape is similar to larvae of lycaenid butterflies, many of which are adapted to live in and among ant colonies.  Ant protection is a very useful trait for a small, slow-growing insect feeding on Passiflora, plants well-protected by ants.

Third instar larva, same scale as the above two photos.
After a day or two in the second instar, the larva repeats the moulting process to the third instar.  This larva has even larger ridges and hard parts that are, again, about 1.6 times as large as the second instar parts.  After 1-2 days of feeding and growth, the larva reaches its full size of about 12-15mg.  Once the larva grows this much it begins crawling around the plant and neighboring plants.  Usually it disappears after a few days of moving and feeding, but what they are looking for and what cues are used to find a pupation site are still a mystery.  Larvae penned in containers usually die and are consumed by fungus.

Red-white Ptocadica, likely the same species as the larvae above.
The larvae shown in these pictures belong to either the Blue flea beetle Monomacra violacea, or, more likely, the Red-white species of Ptocadica.  Two weeks before the larvae appeared, I had placed adults of these two species on the leaves of Passiflora lobata., plant #1.  Most of the adults flew away and disappeared, but one of each beetle species remained on plant for several days.  They sat under leaves and moved around very little.  The Red-white beetle mainly stayed on the lower leaves, while the Blue beetle was more often on the upper leaves.  Two weeks later, when I found larvae on plant #1, they were mainly on the lower leaves, with a few up higher.  Also, after a few days the Red-white beetle moved across the shade house to the lower leaves of a second P. lobata, #2.  A similar number of larvae appeared 2 weeks later on this plant.  Based on the timing and location of the larvae I am fairly sure that most, if not all, of the larvae I found belong to the Red-white species.

The life-cycle drawing summarizes the findings to date.  Obviously there is a long way to go before being able to complete the drawing!

Comparing these larvae to the larvae of Heliconius, I find that the beetle larvae are slower-growing, with a relative growth rate of about 0.35 instead of 0.6.  The egg stage is longer, 14 days instead of 10 days.  The female beetle sits on the host plant for days in order to lay eggs, rather than the brief visit by the butterfly, and she eats the plant while waiting.    My previously published work shows that Heliconius larvae are extremely vulnerable to predation by ants whereas my observations here at La Selva suggest that most Passiflora-attending ants pay no attention to flea beetle larvae.  They seem effectively ant-proof, although experimentation is needed to determine this for sure.

I also began to test the shade house P. lobata larvae on a range of Passiflora species.   If Red-white Ptocadica is the correct species for these larvae, I predict best performance (survival and growth) on P lobata, since this species seems to specialize on P. lobata as an adult food plant.  More about his in the next blog posting!