Grubs in my Yucca!

[Charlotte Sometimes]

The other day when I was out sowing some annuals, I noticed one of my Yuccas was looking a bit paggard.  I pulled some of the unhealthy looking leaves out of the top (the rest looked OK) only to find some little grubs/maggots about 0.4 cm, white in colour, in what I suppose is the crown of the plant.  :P

I'm an organic gardener, so I'm wondering what's for the best???  I am hoping that the improved ventilation will mean its not the ideal home for the little blighters anymore...  but I have two more Yuccas nearby and I don't want them to spread.  Anyone know what they are and what to do??

They are the tree-type yuccas, not the squat succulent-type ones.

[Charlotte Sometimes]