Millipedes, like many other soil dwellers, are beneficial in the garden helping digest dead plant tissue. If the population is quite large you may find that they will eat living, not dead and decaying, plants and then they may become pests. Like other soil dwellers these arthopods need cool, moist conditions to live in and well mulched gardens provide that environment. Perhaps this article might be of some interest.

http://ipm.ucanr.edu/PMG/PESTNOTES/pn7472.html

kimmq is kimmsr

imo.. bugs in the garden are never a problem .. unless a population explodes .. as they all do.. every decade or so ....

and im not talking 'bunches' .... im talking hundred .. or more ... [per square foot, or something like that] .....what do you mean by bunches???

as kim alludes to.. its all about what damage you observe .. rather than just seeing a bug .... action is taken when damage is severe enough to take such ... and a few holes in a leaf.. or some such is rarely that ...

the problem with trying to eradicate these .. is what you end up eradicating in the process .... as in.. beneficials .... of which.. this one probably is ... short of a horde ...

the general process is integrated pest management ... if you care to search on ...

all that said ... learning what it is.. it does.. etc.. is always good

ken

Thanks guys. Good to hear there's no immediate danger. I was just worried because they're in an area that I planted onions in last month, so I won't really be able to tell if they're eating the onions until it's too late. But my soil is pretty healthy, with a lot of compost material, so they should have plenty of other things to munch on. I guess I can pull an onion every once in a while and see if it's becoming an issue. I'll just keep an eye on it. Thanks again