Friday, November 30, 2012

A new idea to test

Ptocadica bifasciata on Passiflora auriculata
 As I began measuring cyanide gas in more and more Passiflora, I began to notice a correlation: flea beetles seem more abundant on Passiflora with a reduced amount of cyanide.  The two species of beetles shown here are among the most abundant, and you can see the holes in the leaves left by the feeding beetles below.  The beetle on the right feeds on Passiflora auriculata, a plant with variable, but usually reduced, amounts of cyanide, and the P. oerstedii shown below makes none (that I can measure).  Other heavily attacked Passiflora include P. lobata and P. vitifolia, two other species with reduced or no cyanide.  The species with a lot of cyanide (P. ambigua, P. menispermifolia, P. biflora, P. costaricensis, P. pittieri) usually have fewer flea beetles.  (Sorry to throw so many latin names out there, but these plants don't have common English names).
Parchicola D.F. 1 on P. oerstedii, ate holes in leaves, and now seems to be drinking from the extraflora nectar.

Most, if not all, of the flea beetles species have the ability to eat cyanogenic Passiflora, but it remains possible that they survive better (or prefer) food with reduced amounts of cyanide.  One way to test this idea in a more controlled way is to choose a single, variable species such as P. auriculata, and then compare cyanide production in plants with and without flea beetle feeding holes.  I would predict that plants with many holes have low levels of cyanide, and that plants with few holes average more.  To carry out this plan I found 20 auriculata plants in the successional plots at La Selva, a site where I can easily see the plants.  Of these, 7 plants were extensively holed by flea beetles and 13 had few holes or none.  I then measured HCN production by grinding one leaf plucked from each plant.  Result: all seven of the flea-beetle-infested plants had cyanide levels below 0.1 μM/g (micromoles per gram of leaf weight), while the plants without flea beetles ranged up to 0.6 μM/g, six times as much!  Although these results tend in the direction of my prediction, they are not conclusive because the results depend on only four plants with high levels of cyanide.  I therefore went to another site where I could sample auriculata, and did a similar test on 10 plants.  In this set I found one flea-beetle-infested plant with high levels of cyanide (0.6 μM/g), and an even more cyanogenic plant (1.1 μM/g)with no beetles.  These results tell me that things are more complicated than a simple correlation between cyanide and presence of flea beetles, and that I need to dig in a little deeper to understand what is going on.  For example, the beetle holes in the two sites may have been caused by different beetle species, with Parchicola being more common at the second site.
Cebus capucinus at La Selva

Fortunately all my work is being supervised by troops of monkeys, including the White-faced Capuchin seen here!

No comments:

Post a Comment