How do the larvae get onboard the bees? Well, I looked high and low and didn't see much except this small aggregation of Oil Beetle larvae waiting in ambush on the tip of some young willow leaves.
Here is a photo of some Oil Beetle larvae waiting in ambush in a Coltsfoot flower.
Hey, look! Here come a little green bee! Let's see what happens!
In the photo below that same larva has disappeared because it has scrambled up onto the unsuspecting bee.
I looked at many flowers and only saw two with larvae in waiting. As I mentioned, I also looked an the tips of many branches and stems and only saw one small aggregation of the larvae. Obviously the little green bees are picking up their passengers at scattered locations or I haven't learned to look at the right spots.
I think these larvae and their life style to be interesting. However, they go through hypermetamorphosis. The first instars, the ones that hitch rides, are very active and go scrambling around. Once they reach the a bees nest they soon molt and turn into more of a grub-like larvae before turning into an adult Oil Beetle.
I'm intrigued with the claws of the bee-riding larvae. In the photo below notice the position of the claws.
Think about it! Nearby those roadside willows, in quite a few Halictid bee nests there will soon be a bunch of plump, grub-like Oil Beetle larvae living large on the pollen/nectar supplied by the diligent work of some unlucky little green bees.
Judging by the amount of little green bees working the willows and Coltsfoot flowers, I'd have to say, "Not every flower nor every branch tip has little clingy larvae waiting in ambush... and plenty of these busy green bees never do pick up hitchhikers."
Do you have a small patch of willows to visit? I'll bet there's much to be seen there amongst the willow shoots.