I know that in one of his webinars, @john.kempf stated that if leaf surface temperatures rise above a certain temperature, I think its somewhat around 25 degree celsius, plants switch from photosynthesis to photorespiration.
In order to cool them themselves they start metabolizing compounds that they synthesized prior, eg. protein > amino acid or polysaccharides > monosaccharides,
if that happened in your case those breakdown products could have offered a food source to the beetles.
So cooling the plants down during hot periods would be my best bet.