24 BULLETIN 1112, U. S. DEPARTMENT OF AGRICULTURE. 
of the total number of black jacks on the Tusayan plot in 1909 wer 
killed by mistletoe alone or in conjunction with other agents, mainly 
insects. While this figure itself is not high, it is alarming because 
the indications are that the death rate will increase ee year to) 
year, and although an effort was made to have all heavily y infected — 
trees removed when this area was marked for cutting, it is evident — 
that mistletoe did not receive the attention which it deserved. 
TaBLeE 17.—Classification of dead trets on permanent sample plots of 456 acres on the — 
Tusayan and 135 acres on the Coconino National Forest. ; 
Per cent of dead trees, 1909-1914 (based — 
on total number of trees which died — 
during 5-year period). 
456-acre plot, Tu- | 185-acre plot, Coco- 
Cause of death. 
Forest. Forest. 
Black Yellow Black Yellow 
jack. pine. jack. pine. 
— |_| |__|... 
Per cent. | Per cent.| Per cent.| Per cent. . 
26. 4 At 7.9 
Mistiethe, andunsetts . 0. Ji 25) Gots... edad tober Sh cil 35. 0 AVS. Ot. dee ae 2052 
Mistletoe: ant Porcupine: 55 - tue Ss beepers ease ee eae OOo Ao tes aes Silt A ae 
Mistletoe and SUpPPressiON | - a. =. 2. no on ot dew epee ms nial = 14 asco c wins cl] folk ae tn) Cee eee 
Mistletoe, porcupine, and Insects...) S...5. ss. ws 5 Sen cee Poe oe Pe Fe Sree ye ee elie mae oa 
ATT OUNET: CAUSES? 5/3. wc Bac Sede Posaa de < asee c sass ee uee 31.3 95. 2 92.5 71.9 
PGA Se LS SSRI teas 2 va Joe bw Rae SEES 100. 0 100.0 100.0 100.0 
Weir * has noted the same direct relation between mistletoe infec- 
tion and insect infestation in the Northwest, especially with Dendroc- 
tonus valens. The origin of numerous insect infestations has been 
traced to stands of western yellow pine heavily infected with mis- 
tletoe. , 
The main difference between mistletoe and insect injury is in the 
length of time required for the injury to become manifest. A 
sporadic insect infestation may develop rapidly and may kill a large 
number of trees in a relatively short time, and it may even continue 
to increase until it is overcome and reduced to normal proportions 
through some natural agency. Mistletoe, on the other hand, de- 
velops slowly but continuously and persistently. The bark beetles, 
when attracted by the resinuous exudations of the mistletoe-infected 
trees, may materially hasten the death of the host which has already 
been weakened by the parasitic mistletoe. Trees dead or dying from 
mistletoe infection have been found infected with species of secondary 
bark beetles and borers, such as Ips, Pityophthorus, Chrysobothris, 
and Melanophila. | 
‘Three black-jack saplings under 4 inches in diameter breast high on 
one of the intensive mistletoe-study plots died during the five-year 
4 Weir, James R., Mistletoe Injury to Conifers in the Northwest. U.S. Dept. Agri. Bulletin 360, 39 pp., 
1916. 
sayan National nino National if 
