I think they are eating my Neptunia! The caterpillars can be found with every step in the dried lagoon. I wonder when and where they pupate. Do they pupate before the lagoon floods, and, if so, do they do so in the lagoon? Are they able to survive the flooded lagoon either as pupa or larva? How can I answer these questions?
Simple. Through the mature catepillars into the lagoon and watch the fish eat them or wait, this could your method of euphanizing them after the study is completed.
Could you simulate this in the laboratory to find an answer? I think that would be easier since there are many more variables you can control and manipulate.