H'mmmm ...... Slack writing, Chino could be using the term "beetles" to mean the girls, but from context he could equally mean the whole group. Given that the girls are first described as being dressed so as to be indistinguishable from the men ( while riding ) and a short while later, as being dressed in the same fashion as the BRMC, although the second group are specifically described as being quite different in appearance, I'd say it's just lax editing of what was, after all, a fairly low budget picture, with the writers using genetic "hip" slang without worrying too much about what it actually meant
Via that link there are a few references to the beetles and sometimes it isn’t exactly clear whether it means Chino’s group in its entirety or just the girls.
However, also via that link you’ll read the following which was going to be said by Johnny: ‘Hey, where's my gold? Where'd that beetle go Chino gave my gold to?’
In that sentence Johnny is talking about Kathie because Chino had given her his (Johnny’s) gold (the trophy) to hold onto.
Earlier on via that link, attached to Chino's front fender is Johnny's trophy. After some pushing/shoving/whatever, Chino waves the trophy aloft and shouts over the noise. Here’s part of what Chino says: ‘The lovely young lady here will hold the golden stakes! The timid maiden of the hills clutches the gold ...’
The guys race/scramble/whatever, following the line of white flour. After the race, Chino lifts the trophy from Kathie's arm and kisses her on the forehead. But Johnny and Chino fight over the trophy. Chino shakes himself and thrusts the trophy on Kathie again. The fight between the two men continues. Eventually Chino gets taken away by the town cop and just after that is when Johnny asks where his gold (the trophy) is and where’d that beetle go who Chino gave his gold to.
In this instance with Johnny referring to a beetle, he means Kathie because she’s a girl. And that is consistent with Chino referring to the girls in his group as beetles, and I don’t think Chino was referring to his group in its entirety.
In 2007 on the old Harley Tech Talk forum a few things about the movie were mentioned, including the beetles. Wanting to find proof of the word’s meaning, I went to the State Library here in Perth and I found the word beetle in four books and I can name them and their authors/editors if required. All four publications, two of which included the words American slang
as part of their titles, listed a beetle as a girl and/or young woman.