My research revealed to me that the silithid are like bees; destroy the queen of the hive, and the rest of should be thrown into disarray. I'll brew a lure that we'll use to bring out the queen; once summoned, you'll take her down.
First though, we need some reagents; we don't want you bungling into the jungle unprepared. Go west to find the silithid hive in Un'Goro Crater and obtain a scent gland from one of the bugs. I'll also need some samples of the native Un'Goro soil.
The scent gland, ah yes. The membrane appears intact - excellent. The soil, yes, this should work out perfectly. With my special additions, I should be able to grind all this up into the perfect lure.
Give me a moment to make the lure, <name>. Once it's ready, we'll be able to enact the next step that may hopefully stop the silithid before they overrun Gadgetzan... and the rest of Kalimdor.