Characteristics of Mountain Pine Beetles Reared in Four Pine Hostsl GENE D. AMMAN Intermountain Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Ogden, Utah 84401 ABSTRACT Environ. Entomo!. ]]: 590-593 (1982) Mountain pine beetles, Dendroctonus ponderosae Hopkins(Coleoptera: Scolytidae),obtained from naturally infested lodgepole pine, Pinus contorta var. latifo/ia Engelmann, were reared in four common hosts: ponderosa pine, P. ponderosa Lawson; western white pine, P. monticola Douglas; whitebark pine, P. albicaulis Engelmann; and lodgepole pine. Emerging beetles were collected daily, counted, and sexed, and pronotal,width was measured. Significantdifferencesin brood production, sizeoffemale beetles, and developmentalrate, but not sex ratio, occurred among hosts. Differenceswere not all associatedwith the same speciesof tree. However, the results indicate that, overall, lodgepole pine is the poorest, and ponderosa pine is the best, of the four hosts for mountain pine beetles. The mountain pine beetle, Dendroctonus pon- ditions, any acceptable host and sometimes any derosae Hopkins (Coleoptera: Sco]ytidae), infests conifer may be attacked. 13 species of pine native to North America (Wood Another interesting aspect of host switching by 1963) and several exotics (Furniss and Schenk 1969, the mountain pine beetle, and one which this study McCambridge 1975). In addition, it infests severa] addresses, is the effect on the first-generation pro- nonpine hosts from which little or no brood is pro- geny when parents from lodgepole pine are intro- duced (BeaI1939, Evenden et at. 1943, Furniss and duced into other pine hosts. Host effects were Schenk 1969). The phenomenon of mountain pine evaluated by using four mountain pine beetle char- beetles infesting one host species and their brood acteristics-brood production, female size, sex ratio, then infesting a different host species is of interest to and developmental rate. The most suitable host for entomologists and of great concern to land mana- the mountain pine beetle was hypothesized to yield gers, the most beetles per unit area of bark, the largest Hopkins' host selection principle states that in- beetles, the most even sex ratio, and the most rapid sects tend to infest the same species of plant as the beetle development. It also was hypothesized that one on which they completed development (Allee et the beetles would do somewhat better in lodgepole at. 1949). Richmond (1933) observed mountain pine pine than in the other host trees because parent beetles believed to have come from lodgepole pine, beetles were obtained from lodgepole pine. Pinus contorta Douglas, infest ponderosa pine, P. ponderosa Lawson, Field observations and experi- Materials and Methods ments by Evenden2 suggest that, when a shortage of One green tree with phloem exceeding 2.5 mm the preferred host exists as a result of epidemic con- thick from each of four pine hosts of the mountain ditions, other hosts may be attacked successfully. pine beetle was selected. Thick phloem was Baker et at. (1971) thought that Hopkins' principle specified, because thin phloem in lodgepole pine applied to low mountain pine beetle populations yielded low brood production, small brood size, and both in mixed stands of whitebark pine, P. albicaulis sex ratios strongly in favor of females (Amman and Engelmann, and lodgepole pine and in adjacent Pace 1976). Trees used in this study were ]odgepole pure stands of these pines. The study by Baker et at. pine and whitebark pine from northwest Wyoming, (1971) showed considerable differential tree mortal- ponderosa pine from southwest Idaho, and western ity in the two host types, thus demonstrating beetle white pine, P. monticola Douglas, from northern preference. Stock and Guenther (1979) and Stock et Idaho. These trees were felled in late June and early at. (1978) found little genetic difference between July, and billets 36 cm long were cut from the lower mountain pine beetle populations in pure stands of trunks. The billets then were transported to Ogden, adjacent but different hosts, and suggested that con- Utah. Twenty slabs 15 by 36 cm were cut from the siderable gene flow occurs between beetle popula- billets of each species, thus providing a total of 80 tions in different hosts, probably during epidemic slabs for the study. Thickness of exposed phloem phases. Wood (1963) summed up present thinking was measured in the center of each cut edge of the on the subject. He stated that, under endemic condi- slabs. Exposed edges of bark and all exposed wood tions, populations may exhibit a strong preference surfaces were then coated with hot paraffin to slow for one host species even when intermixed with the rate of drying. other acceptable species. However, during epidemic Mountain pine beetles were obtained by cutting infested lodgepole pine in northwest Wyoming in ~ved for publication 15 May 1981. June and transporting infested billets to Ogden. 'James C. Evenden. 1933. Host selection in relation to the control of bark These billets were held at 22°C for beetles to com- beetles. USDA. Bureau of Entomology. Coeur d·Alene. Idaho. Unpublished report. 13 pp. plete development and emerge. Emerged beetles 590 June 19H2 AMMAN: MOUNTAIN PINE BEETLE CHARACTERISTICS 591 were collected daily and sexed, using characteristics thinner than whitebark, but brood production did of the seventh abdominal tergum (Hopkins 1909, not differ significantly among these three hosts. Egg Lyon 1958). gallery density (m/930 cm2) did not account for the Six pairs of beetles selected at random were intro- difference in beetle production between lodgepole duced into 4,5-mm-diameter holes spaced 2.5 cm and western white either, since a significant differ- apart within the phloem across one end of each slab. ence in total length of egg galleries was not demon- Not all of the slabs could be infested in a single day strated between these two pines. A significant dif- because of time constraints. Therefore, equal num- ference in egg gallery density occurred only between bers of slabs from each host were infested each day. whitebark and western white. These results suggest The order of slab infestation within each host was differences in nutritional quality among hosts for determined randomly. Beetles were placed in the mountain pine beetle production, with lodgepole holes, and a piece of aluminum window screen was apparently being the poorest and western white stapled over the holes to prevent beetles from escap- being the best of the group, although beetle produc- ing. Slabs were placed in a vertical position with tion from neither of these was significantly different beetles at the base, thus simulating field conditions from the intermediate beetle production in pon- where beetles bore vertically, parallel with the grain derosa and whitebark pines. of the wood. The infested slabs were kept uncaged until the mature adults started to expel boring frass F66Tlale Size from the bark during maturation feeding before Pronotal width of female beetles differed signifi- emergence. Slabs then were placed in individual cantly among hosts (0.025 > P> 0.01). However, cages. Emerging beetles were collected daily from only the means of lodgepole and ponderosa pines the cages, their pronotal width was measured, and differed significantly, with ponderosa yielding the their sex was determined. After emergence was largest beetles (Table 1)_The larger size of field-col- complete, bark was pe~led from each slab, and the lected beetles from ponderosa pine was noted pre- length of egg galleries was measured. viously (Hay 1956, Wood 1963), and Wood Data were subjected to analysis of variance suggested that the difference is at least partly en- (ANOVA) to determine if brood characteristics dif- vironmental rather than genetic. The results of this fered significantly among hosts. The Hartley mul- study support that statement. Whether there is any tiple range test was used to determine significant dif- advantage conveyed to a specific beetle size has not ferences among means (Snedecor 1956). been demonstrated. However, large Douglas-fir beetles, D. pseudotsugae Hopkins, contained a Results and Discussion greater quantity of lipids than small beetles (Atkins Brood Production 1967). Because of the greater lipid content, such Brood production differed significantly (0.025 > beetles may be able to disperse farther and, hence, P > 0.010) among hosts, with the greatest produc- be better able to find suitable host trees. Also, large tion occurring in the five-needle pines, western mountain pine beetles oviposit more eggs than small white followed by whitebark, and least in the three- ones (Amman 1972b, McGhehey 1971, Reid 1962). and two-needle pines, ponderosa followed by lodgepole, However, only the means of lodgepole Sex ratio and western white differed significantly (Table 1). No difference in the proportion of females in Phloem thickness was significantly different (0.05 > emerging brood adults was observed. However, P> 0.025) among hosts and shown to be strongly re- brood adults from lodgepole departed greatest from lated to brood production in lodgepole pine a 1:1 sex ratio, with 66.5% female. Broods from the (Amman 1972a, Amman and Pace 1976). This, how- other three hosts ranged between 57.4% female in ever, did not account for differences in brood pro- whitebark to 62.3% in western white. The sex ratio duction, since phloem thickness of lodgepole and for field populations of mountain pine beetles western white