Question
I have a garden of giant yucca - they've been in place for at least
50 years and are 20 - 30 feet tall.

I have recently noticed that the trunks of the plant have small ( 1/2
inch - 3 inch) dark holes, some of which are seeping a gooey
substance.

The leaves on these plants are looking dull and have more dead
leaves than the healthy yucca's on the property.

Any help you can provide is greatly appreciated!

Answer
Dear Val,

Your Yucca is under attack by a boring insect, either a moth larva or a weevil of some kind. Both, depending on the infestation, can cause enough damage to kill the plant. There are systemic pesticides that may work, Lindane and chlorpyrifos (Dursban) are for borer control. Both are very toxic, so be very careful using it if decide to try and save your Yucca.

It may be too late for your infected plant, but you may want to get some of this to protect the rest of your Yuccas. Also, you will want to remove the Yucca (if it dies) ASAP and get rid of it, also to protect your living Yuccas.

Hope this helps.

Sincerely,

Greg