A Question of Timing

Leaf exchange in holly

We know that trees usually lose their leaves in autumn, which is why we sometimes call the season fall. A few trees do things differently, though. One of those is American holly, Ilex opaca.

Holly is a tree that we usually call evergreen because it stays green year-round. But if you look carefully at this time of year, you will see yellow leaves on holly just as the new leaves are emerging. Have a look at the pictures to the right. You will see the young developing shoot of this year’s new growth, but you will also see yellow fading leaves on the tree and on the ground.

This unusual pattern is called leaf exchanging. The new leaves are emerging as the old leaves die. The tree is evergreen in the sense that it is always green, but the leaves are replaced each spring.

For the next couple of weeks, you should be able to see this pattern in our American holly and some of the ornamental hollies.  [Scroll down for the story of a fly with good timing]

American holly

Leaf Exchange in American holly. The yellow and dark green leaves are from last year, the pale green shoots are this year’s growth

A Fly With Good Timing

The timing of leaf exchange in holly is interesting to us, but for a one insect, it is a matter of life and death.  The native holly leaf miner, Phytomyza ilicicola, is a tiny fly. The fly larvae live inside the holly leaf, protected from enemies by the tough, leathery holly leaf.  The problem for the female fly is how to get her eggs into the leaf.  It is not possible for the fly to penetrate the mature leaf with her ovipositor, as it is too tough. She needs to deposit her eggs in the young leaves as they develop. Perfect timing is essential. The fly spends the winter inside the leaf, in the mine made the previous year. As soon as spring comes, the mature fly needs to emerge from the leaf, mate, and deposit eggs in the young leaves. If she emerges too soon, there will be no young leaves. If she emerges too late, the leaves will be too tough. Like the porridge in Goldilocks, the leaf must be just right.

Reference: Kimmerer and Potter 1986 Oecologia 71:548-551

 

 

Holly leaf

The track of a native holly leaf miner larva in a mature holly leaf

Holly leaf with a leaf miner track

Holly leaf with a leaf miner track