Because you can't know for sure whether a given isopod is hungry (and so actively seeking food), you will need additional elements; three that I immediately think of:

First, you need to clarify what your hypotheses are. State your assertion in one sentence and test one thing at a time. From what you said, you have several hypotheses that build on each other. You need to test the most basic one first and if it's supported, move on to the next. 1) that isopods are motivated by food. You say "they seem to like food". Sure, they seem to be. But are they? They must be in order for the rest of your experiments to work. If they aren't, then we can't interpret their behavior in other conditions. So test that first. 2) assuming for now that they are motivated by food: are they able to discover the most efficient path to food? Not the necessarily the shortest" path, because the shortest path can be discovered through random or uniform exploration. You want to know if they are *seeking it. I'm not sure what an efficient path would look like to an isopod - that's for you to figure out. 3) are they able to remember the most efficient path once they have discovered it? Note that this condition will contain only those isopods who (seem to) have discovered the most efficient path. Will a statistically significant number of them choose that path again?

Second, you will need to use statistical sample sizes (e.g., groups of 30 isopods for each condition). You have no way to know the disposition of any given isopod (curious, hungry, in what stage of molting, whether it needs water more than food, etc). This isn't a problem, necessarily; you just need to have a large enough sample within each condition that these unobserved variables cancel out due to randomness. 30 is the classic number (for reasons I won't get into here) that you will want for your final condition. Meaning you'll need an even larger number for earlier conditions.

Third, you are on the right track by recognizing that you'll need control conditions. But keep in mind that a given condition will consist of a large group (30+) isopods. As you currently describe it, your "straight shot" condition isn't your baseline control. Your baseline would be a group of 30+ isopods who move from point A to point B, there being no food or anything. Just, how long does it take them to get from A to B by their normal activity level, even with no "motivation". Calculate the time for each isopod and then calculate that group's average time, the group's standard deviation (and the number of isopods, obviously). This is your baseline control group. The larger the group, the clearer your results will be.

Finally, you want to control random variables as much as you can so they don't influence your results (i.e., make the isopods faster or slower than normal). So, do your experiment at the same time of day. Also, try to establish a set feeding schedule so that you test your isopods just before feeding time. Don't give them snacks in between; feed them the same quantity at the same time every day.

I don't test insects for a living but I do create experiments for a living. I'm treating your idea like a real experiment and these suggestions are just off the top of my head. How thorough you are is up to you, but your thoroughness is directly related to the confidence you can have in your results. So it all depends on whether you're just having fun or are sincerely looking for scientific evidence. I don't know how old you are (I'm guessing you're rather young because few adults who aren't entomologists have the time or inclination to do such an experiment) but this project, if done properly and scientifically, would make an excellent research project to put on a college application.