I'm pretty sure it's the extracted and concentrated oils that repel things in most cases. For example, citronella oil is proven to repel mosquitoes. But that doesn't necessarily mean that actual plants of lemongrass have any effect. You'd have to be constantly bruising the leaves to get the oils released into the air, and it's probably not going to hang around in sufficient concentrations to overpower the insects' needs.

There are some survival videos on YouTube of
guys sitting in swarms of mosquitoes, rubbing mugwort leaves onto their skin. It does work for a bit: they don't land on the skin, but they still hang around it, checking it out, and then they start biting again after a few minutes.

Nematodes might be different though; I don't know. What are pink elephants? All Google gave me were some cute pictures of coloured elephants.