EFFECT OF GRAZING ON WESTERN YELLOW PINE. 23 



Certain needle diseases exclusive of rusts are believed to attack dwarfed 

 and unhealthy needles more readily than vigorous needles. Since the effect of 

 moderate grazing wounds is to stimulate the growth of plant tissue, these needle 

 diseases are not apt to secure a foothold more readily because of them. How- 

 ever, when persistent and severe wounds reduce the vitality of the trees until 

 the needles reflect the weakened condition, certain needle diseases are apt to 

 find favorable hosts. The total amount of damage caused by needle diseases or 

 the increase in this character of damage which may be attributed to grazing 

 is not known, although it is believed to be relatively unimportant. 



During the last two years studies of the extent of damage by in- 

 sects on the Forests of Arizona and New Mexico have been made a 

 special study by A. J. Jaenicke, forest assistant in charge of insect 

 studies. Mr. Jaenicke has expressed the opinion that grazing in- 

 juries are not responsible for a serious amount of insect damage to 

 western yellow-pine reproduction. He states that — 



Western yellow-pine reproduction in Arizona and New Mexico suffers particu- 

 larly from the following insects: 



1. Tip moths of the genus Retinia. These kill the terminal shoots of the 

 reproduction, causing it finally to assume a bushy form, but rarely kill it. 



2. Engraver beetles of the genus Ips, particularly Ips confusus Leo. These 

 insects are responsible for the death of considerable reproduction in various 

 parts of the district, particularly on cutting areas. 



3. Small scolytid beetles, which work in the thinner bark of the young trees 

 and occasionally cause their death. 



4. Flat-headed borer of the family of Buprestidce. These were found doing 

 serious damage on only one Forest in the district. 



During the past field season careful attention was given the relation of graz- 

 ing injuries to attacks by the above-named insects. It was found that only in 

 localities where serious overgrazing has taken place for a number of years is 

 there ever any material increase in insect damage. 



The tip moth kills the terminal shoots of healthy and unhealthy reproduction 

 with equal ease. Grazing injuries, therefore, are an unimportant factor in 

 infestations by this insect. 



Wherever the vitality and health of reproduction has been seriously lowered 

 by overgrazing or any other cause, particularly on areas recently cut over, en- 

 graver beetles may become responsible for the rapid death of the injured seed- 

 lings. The same is true of attacks by various small scolytlds. Both the en- 

 graver beetles and small scolytids breed in slash, and so it is on the areas where 

 there has been recent cutting that reproduction injured by grazing suffers most 

 from these insects. It is not believed that attacks by flat-headed borers are 

 increased by grazing injuries. 



The minor injuries caused by regulated grazing, therefore, are of little 

 importance in augmenting insect attack. Even serious injuries are rarely 

 responsible for insect damage of any consequence, except on recently cut- 

 over areas. 



SUMMARY. 



(1) Of 8,945 trees of a size subject to grazing, observed over a 

 period of three years, 1,493, or 1G.7 per cent, were severely damaged 

 each year and 1,442, or 16.1 per cent, were moderately damaged. 



