From よつばと！vol. 1 ch. 6 p. 15:
ジャンボ： 近｛ちか｝づく途中｛とちゅう｝で 鳴｛な｝きやんだら 警戒｛けいかい｝してるんだ その時｛とき｝はじっと 鳴｛な｝き出｛だ｝すのを待｛ま｝つ
This is translated in the English edition as such:
If it stops buzzing as you approach, that means it's looking out for you. So wait until it's buzzing a lot before you try and catch it.
I understand 鳴｛な｝きやんだら is 鳴く (sing), but how do you get from the dictionary form to that?
By the way, I would appreciate if you could also give a more literal translation of the sentence.