Ok, I think I understand Willy, upon reflecting on your comment. You don't have to press the turn button. You place the agent, and invade. If it is with information warfare, the agent is unharmed (but with planetary bombardment, the agent is destroyed?).
Some additional noob thoughts: Information warfare costs 800 clams and doesn't seem like it will work against high loyalty races, like the Drengin and Yor. What is weird to me is that both Drengin and Yor rely heavily on morale improvements, leading me to think that morale serves some other purpose for their races than it does for Terrans/Arceans/Altarians.
Another thought is that you don't need to sabotage a farm for the purpose of invasion, but for the purpose of cutting his income. Suppose he has a cash-cow planet with five stock markets or equivalent improvement, two morale structures, and a farm built on a double leaf that keeps the population soaring. If I want to kill his income from that planet, it seems like it would be better to sabotage the farm than one of the stock markets. If his pop is 16B and he has Xeno Farm III keeping it propped from standard 8B, than if he can't generate the agents in time, after two turns his pop will drop to 8B and it will take many, many turns to recover. For example, if his planet is doing very well, then he is generating population of 250M per turn, the it will take 4*8=32 turns to get back to 16B. Now, according to this page, the equation for taxation is:
constant * sqrt(population_in_billions) * tax_rate * (1 + (sum from buildings on planet)) * (1 + (sum from racial bonuses/maluses) )
So his multiple just went from sqrt(16) = 4 to sqrt(8) = 2.8, that is, you just reduced the income from the planet by 1.2/4 = 30%. and it will take him many turns to get it back, whereas the stock exchange only provides a 25% bonus and if he has factories can nullify the agent in a few turns.
Of course, the income loss doesn't remain at 30%, but grows as his population rebounds, whereas hitting the stockmarket kills the 25% until he can nullify the agent which may be very soon. You can buy agents, you can't buy population. I'm curious how the math works out in the end and what experience people have with this.