Mountain Pine Beetle in 

 Ponderosa Pine: Effects of 

 Phloem Thickness and Egg 

 Gallery Density 



Gene D. Amman 

 Judith E. Pasek 



INTRODUCTION 



The mountain pine beetle (MPB) (Dendroctonus 

 ponderosae Hopkins [Coleoptera: Scolytidae]) kills more 

 pine trees in the Western United States than any other 

 insect. During epidemics, the beetle frequently kills over 

 a milUon trees a year in a single National Forest (Klein 

 and others 1979) or National Park (McGregor and others 

 1978). 



Hopkins (1909) observed during an outbreak that as a 

 rule the largest and best trees are attacked first— 

 lodgepole (Pinus nontorta Dougl.) and ponderosa 

 (P. ponderosa Dougl. ex Laws.) pines. He further states 

 that "it is only in the thicker bark on the lower portion 

 of the trunk of the medium to larger trees [lodgepole 

 pine] that beetle broods will reach their best develop- 

 ment." Craighead and others (1931) state: "In contrast 

 to the trees [lodgepole pine] selected under endemic con- 

 ditions, it is the larger thick-barked trees that are first 

 attacked during epidemics." The reason for this has been 

 related to thick phloem, the food of beetle larvae, in 

 such trees (Amman 1969). Beetle production, measured 

 as beetle emergence holes, in lodgepole was directly 

 related to crevice thickness of bark on the dead trees. 

 Ammgm (1969) also showed phloem thickness on living 

 lodgepole pine was highly correlated with bark crevice 

 thickness and suggested that beetle production is depen- 

 dent on phloem thickness of the infested tree. Subse- 

 quently, in a laboratory study, positive correlations 

 between beetle production and phloem thickness were 

 demonstrated (Amman 1972a; Amman and Pace 1976). 

 Phloem thickness of the host tree may also be important 

 in dynamics of other bark beetles. Haack and others 

 (1984) found significant correlations between egg gallery 

 construction and oviposition of Ips calligraphus (Germar) 

 and thickness of slash pine (P. elliottii Engelm.) phloem. 



Because of the significant role phloem thickness plays 

 in the dynamics of MPB in lodgepole pine forests, simi- 

 lar studies were initiated in ponderosa pine, another 

 important host of the beetle and one in which losses con- 

 tinue to be high (McGregor 1985). Blackman (1931) 

 related MPB brood production in ponderosa pine to 

 crown characteristics and rainfall. Under conditions of 

 normal rainfall, trees having heavy crowns produced 42 

 percent more beetles than trees of medium crowns, and 

 92 percent more beetles than trees of light crowns. This 



relationship to crown characteristics during normal rain- 

 fall suggests that phloem thickness may be involved 

 because trees having leirge crowns are probably the most 

 vigorous and would have the thickest phloem. Cole 

 (1973) found lodgepole phloem thickness to be signifi- 

 cantly and positively correlated with cheiracteristics of 

 good tree vigor. Therefore, a study of MPB brood 

 production in ponderosa pine of different phloem thick- 

 ness was conducted. The objectives of the study were (1) 

 to determine beetle brood production in relationship to 

 phloem thickness, beetle attack density, and egg gallery 

 density, and (2) to determine effects of phloem thickness 

 and attack and gaUery densities on beetle size, sex ratio, 

 and rate of emergence. 



MATERIALS AND METHODS 



Three uninfested ponderosa pine trees (one with thin 

 phloem, one with medium, and one with thick) were 

 felled on the Ashley National Forest in northeastern 

 Utah in November 1979. In addition, three infested trees 

 were felled at the same place and time. Billets 50 cm 

 long were cut from each tree and taken to our laboratory 

 in Ogden, UT, where the ends of the billets were waxed 

 to slow moisture loss. Uninfested billets were stored at 

 2 °C. Infested billets were kept at room temperatures of 

 20 to 27 °C so immature beetles could complete develop- 

 ment and emerge. 



In Jemuary 1980, we delineated 25 areas (15.2 cm wide 

 by 30.5 cm tall) on uninfested billets taken from each of 

 the three trees. Phloem thickness measurements were 

 made in the center of the four sides of each of the 25 

 areas from each tree. Mean phloem thickness for each 

 tree was thin (x = 1.53 mm; sd = 0.23), medium (x = 

 2.88 mm; sd = 0.43), and thick (x = 3.49 mm; sd = 

 0.44). A 2.5-cm-wide strip of bark was removed from the 

 perimeter of each area to confine developing larvae. 

 Exposed sapwood and bark edges were waxed to slow 

 moisture loss. 



To provide a start for egg galleries, vertical holes 

 (5 mm in diameter and 2.5 cm long) were drilled with the 

 long axis of the billet in the bark along the lower edge of 

 each area, and one pair of beetles was introduced into 

 each hole. Sex of beetles was determined by characteris- 

 tics of the seventh abdominal tergum (Lyon 1958). Holes 

 and beetle introductions were evenly spaced and vairied 



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