Green Fly???????

Hi, I have on my parsnips what appears to be Green Fly grubs, that's to say that they are green and they are grubs. They are approx 0.5mm long and there are lots of them, (well, I think that there are lots of them). I found them by noticing that one parsnip's leaves were beginning to "curl up", similar to the way that rose leaves curl up when attacked by "wee grubbies", so I had a close look and found these wee green grubs. I doubt that I could photograph them as they are so small. Obviously I inspected a few more parsnips and found that they too had these wee green grubs, but the leaves weren't curled up yet.
The question is "what type of insecticide do I use on them??". I have a very old book that I usually follow for gardening advice, but it mentions "nicotine" and "pyrethrum" amongst other remedies. I need a relatively modern insecticide, such as "Rose Clear" or similar. Thank you in advance for any assistance. arh.
ps The Book is Charles Boff's "The Big Book Of Gardening".
update!!, they may be "little green eggs" as I haven't seen any move, (yet).

Thank you for that JayG, mine are a lot lot smaller than those, and bright green, I'm going to have a read on the bottle of Rose Clear and see what it gets rid off and test it on one plant.