YourGardenShow.com "Ask Ian"
Video reply: You are quite right: what's happening is the caterpillars are able to get a jump start on all the new growth in spring, they decimate the foliage, and the plant reacts by trying to put out a second set of foliage.
By this time the plant is in photosynthetic imbalance - it can't produce enough energy from the sun to then store enough nutrient in the winter to get through to the spring for its push of new growth. So, in short - you've got to interrupt the lifecycle of those caterpillars.
First step is to identify the type of caterpillar and find out when they are at their most active in your area. If a tree is large, even if you were to spray it with a soapy compound, it would be difficult to cover the whole tree.
My recommendation is to set your parameters for care and maintenance of your trees: how in love are you with these trees? How much time do you have, are you good at pruning, do you want to climb a ladder? You might need to climb in the tree with a sprayer and get in there and spray the undersides of the foliage, to prevent the caterpillars from getting started.
I personally would be more interested in pruning out the crown and thinning out the head, so that the roots that are there are not trying to feed so much of a branch system. Less branches mean less foliage, which means less for the caterpillars to feed on, and less area for you to prune and less area for you to spray.
You can see successful results, but it's all down to your tolerance for pain - how much time and energy you can invest.
Regarding fertilizer - you don't want to heavily fertilize these trees. And organic fertilizer for fruit trees, in early or mid spring would work - but you've got to be observant enough to spot the onset of caterpillars.This video reply was first broadcast online during our live plant and gardening Q&A show; the video you see here has been edited from a recording of that show. Got another question? Go to the Q&A page