For technical information on the subject, perhaps start with "vertical transmission insect symbionts" on google, which will lead to you to a variety of info's. Studies for symbionts of lice, fruit flies, mosquitos, termites, isopods, stinkbugs, butterflies and planthoppers.
Insect Symbiosis Vol 3 page 145 states:
Insects have been shown to use transovarian, egg smearing, and coprophagy, transmission, else entirely from the environment at every generation. There is a list of summaries of different insects that use different methods.
this also has masses of information:
Samples for large insects are taken with tweezers, small insects can be dunked in alcohol and then microbes are extracted.
Microbia RNA can be checked for evidence if the symbionts come from the parents. You can probably figure out a variety of methods for transmitting your insects symbionts, simply applying the wanted gut bacteria to the substrate of new cultures. I expect it can work in between species. you can try smearing eggs too.