Bring a Gorishi Scent Gland and 5 Un'Goro Soil samples to Alchemist Pestlezugg in Gadgetzan.
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.