Pruning or tissue surgery can sometimes head off one other class of parasite—the canker-forming fungi. Whenever Curiously, both mistletoe and the other Christmas evergreen, holly, yield viscous exudate called "lime" (from the Latin "limere," to smear), which was used immemorially by men to snare birds.

Such mechanical aids are attempted they should be followed up by feeding, usually with a high-nitrogen, to help the tree quickly seal off its canker lesions with healthy new cells before remnant fungoid mycelia (thread-roots) can spread, as in animals' fibroid tumors.

But chemical rather than physical warfare is necessary to combat the vast majority of tree pests and parasites. It is not within the scope of this handbook to describe all the thousand-odd kinds, symptoms, and treatments of such troubles. Some standard works on the subject are listed at the end of this chapter for readers who, grasping here the strategic outlines, may wish to arm themselves in depth to defend their trees.

"Chemical warfare" is meant literally. With ever-increasing success, men have learned to poison their trees' foes, at least in those years when the counterattacks are properly timed. How important timing is can be seen in two cases of some prevalence.

One is the poisoning, through its stomach, of an adroit one-inch herbivore called the bagworm, which spins and carries around with it a conical sack of silk and chewed-up plant material. After only a few days of foliar feeding, this creature attaches its bag to a twig and sacks in, to sleep until emerging as a moth. The only time you can hope to make it eat poison is during its brief browsing period. Otherwise it is sheathed against any attack you may make short of picking off all the bags and destroying them, which is no small task in an arborvitae hedge or a grove of maples.