I have a few suggestions for items that could be used for beetle breeding/battling that are analogous to other in-game items;

Royal Jelly: When fed to a female, she will lay at least three eggs in her next breeding.

Marula Nectar: When fed to a male, his next offspring will have +2 stats.

Ochre Dust: When fed to a beetle, their next offspring will all inherit the appearance of that parent.

Dung Ball: Gives your beetle +30 XP.

Beetle Shuffle: Randomizes your beetle's stats (So for example, if they had 4 STR, 6 END, and 5 SPD, they might end up with 5 STR, 4 END, and 6 SPD.)