Christmas tree pests and weeds required restrictions and requirements to be placed upon Christmas tree growers with plantations in "gypsy moth regulated areas". The restrictions and requirements included ongoing site monitoring, active moth eradication by the grower, accurate record keeping, and government directed moth eradication programs. Failure to abide by the rules could result in revocation of the "approved grower" status from the Canadian Food Inspection Agency. The major types of conifers cultivated, pines, Douglas-firs, true firs, and spruce trees, are all affected by some types of insect pests. Douglas-firs are most vulnerable to the Cooley spruce gall adelgid. The true firs are most susceptible
Tree walking automaton Tree walking automaton A tree walking automaton (TWA) is a type of finite automaton that deals with tree structures rather than strings. The concept was originally proposed by Aho and Ullman. The following article deals with tree walking automata. For a different notion of tree automaton, closely related to regular tree languages, see branching automaton. All trees are assumed to be binary, with labels from a fixed alphabet Σ. Informally, a tree walking automaton "A" (TWA) is a finite state device which walks over the tree in a sequential manner. At each moment "A" visits a node "v" in state