Let's break down the information given step by step to find the number of ladybirds on the tree trunks:
1. There were initially 24 ladybirds.
2. One-sixth of the ladybirds flew away, so 24 * (1/6) = 4 ladybirds flew away. This leaves us with 20 ladybirds.
3. Half of the remaining ladybirds sat on a yellow sunflower, so 20 / 2 = 10 ladybirds on the sunflower.
4. Half of those on the sunflower flew onto a fence post, so 10 / 2 = 5 ladybirds on the fence post.
5. One-fifth of the ladybirds on the sunflower flew onto the lavender bush, so 10 * (1/5) = 2 ladybirds on the lavender bush.
6. There were no ladybirds on the hedge, so none on the tree trunks either.
7. Therefore, the sum of ladybirds sitting on various places is 4 (flew away) + 10 (on the sunflower) + 5 (on the fence post) + 2 (on the lavender bush) + 1 (on the grass) = 22 ladybirds.
8. So, the number of ladybirds on the tree trunks is 24 (initial total) - 22 (in other places) = 2 ladybirds.
Therefore, there are 2 ladybirds on the tree trunks.