I have seen S. purpurea in the wild and grown it for a long time, and it
is my experience that aside from a few drops that may be in an unopened
pitcher, (which evaporate as soon as the pitcher opens), they really
don't secrete enough water to be of any use in trapping; this comes from
rainwater. During the several summers I spent in Minnesota, I never
found any water in S. purpurea pitchers unless it had actually rained
within the last few days. For that matter, I rarely even found much prey
in the pitchers, but what there was was usually dried up. Since the
water is an important part of the trapping mechanism in this species, I
would guess that in particularly dry summers, the plants just don't get
much in the way of insect prey. However, here in Seattle, I keep my
plants about 3/4 full all the time, and they are very efficient at
catching insects, especially yellowjackets, which have no fear or
hesitancy about walking right in. The water is a foul-smelling brown
sludge now. Yumm. Lately I have been getting a lot of white-faced
hornets as well, because they are being attracted by an Angelica gigas
plant nearby in the garden. It is sort of a 'butterfly bush from hell,'
attracting lots of hornets and yellowjackets.