If you think about it, the two aspects of that make a lot of sense:
First we have to consider the "survival instinct" of all living creatures and the drive to reproduce and extend the gene pool, but then we might want to bring in a bit of "Maslow" thinking - hierarchy of needs.
If a plant is not growing under optimal conditions (thinking back to Andy Golley's comments), it is having to work hard just to survive, so all of its energies are focused on that. Once it is in a "comfortable" spot culturally, it can afford to "waste" some of its energy supply (the sugary sap) in order to attract some external defenders.
The only "gotcha" on that is that those exudates attract parasitic insects as well. In fact, ants have been known to "farm" them on orchids, as insects like aphids exude their own "honeydew" that is apparently more to the ants' liking.
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