Friday, May 23, 2008

This Tree Needs Caterpillars

Caterpillars are pests aren't they? Not always.

It turns out the native coast live oak (Quercas agrifolia) that grows around my house co-evolved with a tiny caterpillar, appropriately named the oak-leaf caterpillar. Without the caterpillar, the oak tree might die during years of stress.

A nursery in Los Altos, CA has a gorgeous coastal live oak about 75 feet in diameter. During the drought of the early ‘70s a lot of trees were very stressed. A neighbor near the nursery sprayed their oak, “to get rid of the messy droppings and fallen leaves”. The nursery carefully watched their tree lose many leaves and be riddled with leaves partially eaten by the oak-leaf caterpillar.

Guess which tree died?

The neighbor’s. Because it had 100% of it’s green leaves to transpire moisture. Even an organic spray, such as Bacillus thuringiensis var. kurstaki, would have lead the tree to an early death. At the nursery, the tree survived as much of it’s leaf area was consumed by the caterpillar, offering less area for transpiration.

Judging by the moths at my screened windows and the partially-eaten leaves, the first round of oak-leaf caterpillars is here. There are two to three groups of moths each season depending on how dry the year is.

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