With the last boulder fired, the Crab-Mecha (or was it Mecha-Crab?) immediately starts to sputter and cough, before finally shrinking to about the size of a lemon by ejecting spare parts (mechanical and biological, by Celestia the mess and the smell!). Recognising that it's been cornered, it chooses to scuttle away to safety, with you and Applejack in hot pursuit.
The pair of you manage to chase the crab to a section of the farm where there are five mini-orchards in the western orchard lined up in a row. On Applejack's instructions, the rest of the Apple Family have completely surrounded the area, with the crab having seemingly no hope of escaping. However, knowing how wily the foe truly is, Applejack instructs her family not to leave their designated guarding post, lest the crab somehow escape.
Unfortunately, the sun isn't going to remain high in the sky forever, and you estimate it will be another six hours before Princess Luna raises the moon in the sky to mark the start of the night. By then, Granny Smith must turn in for the day, Big Macintosh has chores to complete, and Applebloom needs to finish up on some homework. On top of that, Applejack's night vision absolutely stinks, largely because of her shunning of carrots in favour of apples, the one and only answer to all of life's problems. Except this one.
You don't have a torch, and only you and Applejack are free to move around. However, you do have a slight advantage in your favour: judging from the crab's behaviour, you reason that as soon as you start to search the five mini-orchards, the crab will move from one adjacent mini-orchard to the other in an attempt to escape, and will never remain still for very long. You and Applejack working together however are free to search any of them in any order you like, but it will take an hour to comb that orchard from head to tail for any suspicious scuttling or acidic bubbles.
Better hurry it up though: you only have six hours to catch the crab, and you really need to be getting back to Canterlot shortly to report on the roof you were meant to be fixing today. So where should you start searching?