Hardiness Zone: 6a
jt_luves_dn from Syracuse, NY
Sounds like you have a classic case of some codling moth or winter moth caterpillars eating your crab apple leaves. There are several preventative tactics to employ in the control of these pests. For now you'll need to pick them off or spray them with a garden hose to dislodge them. Then try a few of these preventative measures to reduce the next generation (there can be up to 4 in one summer) of worms.
You can try wrapping the trunk of your tree with a band containing Vaseline (don't apply it directly to the bark). Do this to prevent the female caterpillars from making their way back up the truck. You can also buy similar sticky bands at garden centers, which will trap the caterpillars right on the band. When you see the caterpillars cocooning in your neighbor's trees, it's okay to remove the band. You can also leave it on longer in an effort to prevent any larvae from migrating up the tree. Check the band every day or two to make sure it remains sticky.
When your tree is flowering, pheromone traps (available at garden centers) can also help reduce worm populations.
Always remove any infested apples (hanging or dropped) as soon as you see them.
Encourage birds to the area by erecting a bird bath and feeding station near your tree.
There are chemical controls as well, but these need to be timed appropriately and are only partly effective.
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By (Guest Post)05/31/2006
I have a similar problem with my zuchini , bell pepper and tomato plants. I read different advise for getting rid of the snails.. Since my plants are in containers, I bought copper tape and stuck that around the container. It sort of gives the snails a 'shock' so they won't want to come there.
Also crumbling egg shells around the tree may work for you.
Hope this helps.
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