Although sizes of both females and males showed the 

 same trend, only female size was analyzed. Average 

 female length at 0.3 m of egg gallery per 930 cm^ ranged 

 from 5.49 mm for thin phloem to 5.81 mm for thick 

 phloem (fig. 3). At 2.1 m of gallery per 930 cm^, female 

 length ranged from 5.14 mm in thin phloem to 5.46 mm 

 in thick phloem. Similar relations were observed in 

 lodgepole pine (Amman and Pace 1976). However, beetle 

 size is greater in ponderosa than in lodgepole for any 

 specified combination of phloem thickness and egg gal- 

 lery density, which suggests that ponderosa phloem is 

 quaUtatively better food than lodgepole phloem for 

 mountain pine beetles. Even when MPB of a single 

 population from lodgepole pine were reared in ponderosa 

 and lodgepole pines, those from ponderosa were signifi- 

 cantly larger than those from lodgepole (Amman 1982). 

 Two effects associated with small MPB are oviposition 

 of fewer eggs and smaller eggs than occurs with large 

 beetles (Amman 1972b; McGhehey 1971; Reid 1962). 



Sex Ratio 



The percentages of new adult beetles that were female 

 were not significantly correlated (P > 0.05) with attack 

 density in any of the three phloem thicknesses. There- 

 fore, comparisons were made between percentages of 

 beetles that were female from all attack densities. The 

 percentage of new adult beetles that were female did not 

 differ significantly (P > 0.05) between thick (66.8 per- 

 cent) and medium (67.0 percent) phloem, but was signifi- 

 cantly less (P < 0.005) in thin phloem (62.9 percent). 

 Factors that adversely affect developing MPB generally 

 result in lower survival of males than females. For exam- 

 ple: drying (Amman and Rasmussen 1974; Cole and 

 others 1976), cold storage (Safranyik 1976; Watson 

 1971), and thin phloem (Amman and Pace 1976) in lodge- 

 pole pine. The reduced percentage of females coming 

 from thin rather than thick ponderosa pine phloem is 

 just the opposite from the trend observed in lodgepole 

 pine, where thin phloem yielded 72 percent female and 

 thick phloem 66 percent female (Amman and Pace 1976). 

 No explanation is offered for this apparent reversal. 

 Additional observations are planned to determine if this 

 difference in sex ratio between beetles from thick and 

 thin ponderosa phloem is consistent. 



Emergence Time 



Emergence time of new adults was unrelated to attack 

 density except in thick phloem, where a significant nega- 

 tive correlation occurred (P < 0.05). Therefore, emer- 

 gence times for aU beetles, regardless of attack density, 



were combined for each phloem thickness. These data 

 showed that emergence of beetles from thin phloem was 

 delayed from that of beetles in medium and thick 

 phloem. The time interval for 50 percent of the beetles, 

 starting from introduction of parent adults into the bark 

 areas, was approximately 82 days in thin phloem com- 

 pared to 71 days in medium, and 75 in thick phloem. 

 The delay in thin phloem is even more apparent later in 

 the emergence cycle (fig. 4). 



130 



DAYS AFTER BARK INFESTED 



Figure 4— Cumulative percentage of mountain pine beetle 

 emergence from three thicknesses of ponderosa pine 

 phloem (hand-fitted curves). 



The differences in time of emergence may be due to 

 nutrition, with medium to thick phloem being best for 

 beetle development. Larvae feeding in thin phloem, par- 

 ticularly those in later instars, probably are chewing 

 outer bark and sapwood in addition to phloem as they 

 feed, thus delaying development. These tissues probably 

 are not as nutritious as phloem, which is readily availa- 

 ble to larvae feeding in medium to thick phloem. The 

 rate of beetle emergence was also slower in thin than in 

 thick lodgepole pine phloem (Amman and Cole 1983). 



CONCLUSION 



The significant association of MPB brood production, 

 beetle size, sex ratio, and time of emergence to phloem 

 thickness in ponderosa pine indicates phloem thickness 

 is an important factor in MPB dynamics in ponderosa 

 pine forests. 



6 



