My own theory:

This snippet juxtaposes a similar construction “Tickell treasure,” and this turns out to be part of a fairly well-known poem, Brittle Beauty (16th century):

Brittle beauty, that Nature made so frail,
Whereof the gift is small, and short the season;
Flowering to-day, to-morrow apt to fail
Tickle treasure, abhorred of reason :
Dangerous to deal with, vain, of none avail
Costly in keeping, past not worth two peason
Slipper in sliding, as is an eel's tail
Hard to obtain, once gotten, not geason
Jewel of jeopardy, that peril doth assail
False and untrue, enticed oft to treason
Enemy to youth, that most may I bewail
Ah ! bitter sweet, infecting as the poison
Thou farest as fruit that with the frost is taken ;
To-day ready ripe, tomorrow all to-shaken.

This dictionary from 1887 that records South Cheshire folk speak says that ‘tickle’ used to mean “sensitive, used of balances.” It quotes Gascoigne, 1577:

Vayne is the rest, and that most vayne of all,
A smouldring smoke which flieth with every winde,
A tickell treasure, like a trendlyng ball,
A passing pleasure mocking but the minde,
A fickle fee as fansie well can finde.
A sommers fruite whiche long can never last,
But ripeneth soone, and rottes againe as fast.

The line "slipper in sliding, as is an eel's tail" in the first poem, and the proximity to "fickle" in the second one both seem to imply a sense of treasure having a volatile nature, but I can't be sure that this is indeed the authors' intention.

So: Perhaps treasure is tickle means 'treasure/fortune is capricious and unpredictable'.

It's the best guess that I've got.

My own theory:

This snippet juxtaposes a similar construction “Tickell treasure,” and this turns out to be part of a fairly well-known poem, Brittle Beauty (16th century):

Brittle beauty, that Nature made so frail,
Whereof the gift is small, and short the season;
Flowering to-day, to-morrow apt to fail
Tickle treasure, abhorred of reason :
Dangerous to deal with, vain, of none avail
Costly in keeping, past not worth two peason
Slipper in sliding, as is an eel's tail
Hard to obtain, once gotten, not geason
Jewel of jeopardy, that peril doth assail
False and untrue, enticed oft to treason
Enemy to youth, that most may I bewail
Ah ! bitter sweet, infecting as the poison
Thou farest as fruit that with the frost is taken ;
To-day ready ripe, tomorrow all to-shaken.

The Folk-Speak of South Cheshire, 1887, contains a dictionary compiled by Thomas Darlington, which says that ‘tickle’ used to mean “sensitive, used of balances.” It quotes Gascoigne, 1577:

Vayne is the rest, and that most vayne of all,
A smouldring smoke which flieth with every winde,
A tickell treasure, like a trendlyng ball,
A passing pleasure mocking but the minde,
A fickle fee as fansie well can finde.
A sommers fruite whiche long can never last,
But ripeneth soone, and rottes againe as fast.

The line "slipper in sliding, as is an eel's tail" in the first poem, and the proximity to "fickle" in the second one both seem to imply a sense of treasure having a volatile nature, but I can't be sure that this is indeed the authors' intention.

So: Perhaps treasure is tickle means 'treasure/fortune is capricious and unpredictable'.

It's the best guess that I've got.