Andreas,
As far as I know the pitchers only actively produce the attractant for
about a month. This is actually a pretty neat system, since at the same
time, the pitchers also secrete a different attractant which brings in the
female pitcher plant mosquito. She lays her eggs and the mosquito larvae
develop during the peak trapping period for the pitcher while there's
plenty of microbial food available for the little guys.
Since the really smelly pitchers are pretty much anaerobic, and S. purpurea
relies on microbial breakdown of prey instead of producing its own enzymes,
it might be that the additional smell might bring in a few curious flies
etc. In my experience these really stinko pitchers are pretty rare, and
usually are the result of "pitcher overload" such as when a large slug or
caterpillar falls in (in some areas I have seen pitchers full of ants).
Otherwise, it seems that the pitchers are working aerobically most of the
time, and don't have too much smell. Although no one knows much about the
benefits of the mosquito larvae to the plant, there is some evidence that
they function as members of a nutrient "processing chain" involving a midge
larva, the mosquito larvae, and the microbes that helps to get the
nutrients from the prey to the plant. These insect larvae die off in
anaerobic pitchers and the processing chain is disrupted slowing the decay
process and the uptake of nutrients by the pitcher.
Although I have seen incidental catches (crawling insects may accidentally
fall in) in pitchers that have successfully overwintered, I don't think the
plant is putting any additional resources into producing nectar in these
old pitchers.
Liane Cochran-Stafira