Karen M. Clancy

The role of sugars in western spruce budworm nutritional ecology

Abstract. 1 The western spruce budworm, Choristoneura occidentalis Freeman, and Douglas‐fir, Pseudotsuga menziesii (Mirb.) Franco, have been used to test the hypothesis that variation in levels of foliar sugars form part of the basis for plant resistance to herbivore attack. 2 Budworm population growth was evaluated on artificial diets with 2–45% sucrose using a three generation bioassay. Diets with 1.2% and 3.9% N were tested to determine if responses to sugar were dependent on levels of protein. The 3.9% N diets were supplemented with a mineral salt mixture, so they had high levels of N and minerals. 3 The response of budworm population growth to sucrose concentrations ≤20% was convex at 1.2% N and flat for 3.9% N. Population growth on the 1.2% N diet, which had levels of N and minerals similar to host foliage, was good with only 2% sugar, but optimal at the 6% sucrose level; the number of F1, F2 and F3 larvae produced declined substantially when sugar was increased to 11% and 20%. On the 3.9% N diets, population growth was equivalent for diets with 2% and 11% sucrose. Sucrose concentrations ≥29% were detrimental to the budworm at both N levels. 4 Sugar concentrations in Douglas‐fir foliage varied between 5.7% and 18.4%. Thus, results from the 1.2% N experiment indicated that budworm performance was best on diets with sugar concentrations near the lower limit observed for host foliage. This implies that plants with higher foliar sugar may be inferior hosts for the budworm. Field observations supported this conclusion, as putatively resistant Douglas‐fir trees had significantly higher levels of sugars in their foliage than nearby paired susceptible trees. Variation in foliar sugars among individual trees may be part of the mechanism in Douglas‐fir resistance to C.occidentalis damage.

Purification and characterization of the western spruce budworm larval midgut proteinases and comparison of gut activities of laboratory-reared and field-collected insects

Three proteolytic enzymes, trypsin, chymotrypsin, and aminopeptidase-N (APN), were purified from laboratory-reared western spruce budworm, Choristoneura occidentalis [Freeman], larvae. Budworm trypsin exhibited a high degree of substrate specificity, was inactivated by DFP and TLCK, and was inhibited by trypsin inhibitors. The western spruce budworm chymotrypsin hydrolyzed SAAPFpNA and SAAPLpNA, but not SFpNA, SGGFpNA, SGGLpNA or BTpNA. The chymotrypsin was inactivated by DFP, and was inhibited by chymostatin and the chymotrypsin inhibitor, POT-1. Purified budworm chymotrypsin exhibited little BTEE esterolytic activity and was insensitive to inhibition with TPCK. The N-terminal sequence of budworm trypsin, chymotrypsin, and APN were obtained. Similar levels of trypsin and APN gut activities were found in laboratory-reared and field-collected larvae. However, in comparison to laboratory-reared insects, considerably less chymotrypsin activity, and a much higher level of gut carboxypeptidase activity were found in field-collected western spruce budworm larvae.

Influence of Elevation on Bark Beetle (Coleoptera: Curculionidae, Scolytinae) Community Structure and Flight Periodicity in Ponderosa Pine Forests of Arizona

Abstract We examined abundance and flight periodicity of five Ips and six Dendroctonus species (Coleoptera: Curculionidae, Scolytinae) among three different elevation bands in ponderosa pine (Pinus ponderosa Douglas ex. Lawson) forests of northcentral Arizona. Bark beetle populations were monitored at 10 sites in each of three elevation bands (low: 1,600–1,736 m; middle: 2,058–2,230 m; high: 2,505–2,651 m) for 3 yr (2004–2006) using pheromone-baited Lindgren funnel traps. Trap contents were collected weekly from March to December. We also studied temperature differences among the elevation bands and what role this may play in beetle flight behavior. Bark beetles, regardless of species, showed no consistent elevational trend in abundance among the three bands. The higher abundances of Ips lecontei Swaine, I. calligraphus ponderosae Swaine, Dendroctonus frontalis Zimmerman, and D. brevicomis LeConte at low and middle elevations offset the greater abundance of I. knausi Swaine, D. adjunctus Blandford, D. approximatus Dietz, and D. valens LeConte at high elevations. I. pini (Say) and I. latidens LeConte were found in similar numbers across the three bands. Flight periodicity of several species varied among elevation bands. In general, the flight period shortened as elevation increased; flight initiated later and terminated earlier in the year. The timing, number, and magnitude of peaks in flight activity also varied among the elevation bands. These results suggest that abundance and flight seasonality of several bark beetles are related to elevation and the associated temperature differences. The implications of these results are discussed in relation to bark beetle management and population dynamics.

Oviposition preference and larval performance of the western spruce budworm (Lepidoptera: Tortricidae)

Abstract Many preference and performance studies have been done on latent species of insect herbivores, but few studies have examined whether eruptive species exhibit preferences for oviposition sites that affect larval survival. We used choice experiments to test whether female moths of an eruptive species (the western spruce budworm, Choristoneura occidentalis Freeman) exhibited oviposition preferences for needle age class, for age of host trees, and for host vigor among individuals of one species, Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco. We also evaluated oviposition preferences among three common host species (Douglas-fir, white fir, Abies concolor Lindl., and Engelmann spruce, Picea engelmannii Parry). Females laid more egg masses on older branches (45–60 yr old) than on younger branches (10–15 yr old) of Douglas-fir trees. Also, proportionately more eggs were laid on branches from white fir than on branches from Douglas-fir or Engelmann spruce. There were no differences in the distribution of egg masses when females were offered a choice between 1- and 4-yr-old Douglas-fir needles or between long and short shoots of Douglas-fir. These general patterns were obtained across two generations. However, there were no differences in viability of the F1 or F2 egg masses or weights of F1 female pupae (i.e., fecundity) among treatments for any experiment. Consequently, although female moths of this eruptive species appeared to use foliar cues to determine oviposition sites, we could not link these preferences to any effects on offspring performance because performance did not vary. This contrasts with the tight linkages between preference and performance documented for many latent species.

Mechanisms of Resistance in Trees to Defoliators

At least 10 mechanisms are known to be important in resistance of trees to insect defoliators: 1) Phonological asynchrony between host trees and insect herbivores; 2) Host tree tolerance of defoliation, which is linked to host tree vigor; 3) Host tree compensatory photosynthesis and growth in response to defoliation; 4) Toughness of leaves and needles; 5) Low nutritive quality of foliage; 6) Defensive compounds (or allelochemicals) in foliage; 7) Induced defenses in host trees; 8) Induced susceptibility in host trees; 9) Three-trophic-level interactions among the trees, their insect herbivores, and natural enemies of the herbivores (i.e., predators, parasites, pathogens); and 10) Host tree microbial mutualists such as mycorrhizae and fungal endophytes.

Western spruce budworm response to different moisture levels in artificial diets

Western spruce budworm (Choristoneura occidentalis Freeman) performance (survival, larval development time, and pupal biomass) was evaluated for three consecutive generations on artificial diets with four moisture levels: 72%, 76%, 84%, and 88% moisture, for three levels of nitrogen: 1.2%, 1.9%, and 3.9% N dry biomass. Budworm performance was generally negatively associated with food moisture content, rather than positively associated as reported by others. Furthermore, increased moisture in the artificial diets did not improve the efficiency with which the budworm could utilize the nitrogen in the diets. Budworm survival, pupal biomass, and development time were best on diets with low N and low water or high N and low water. Water appeared simply to be acting as a diluent in these experiments. For the western spruce budworm the importance of plant water and N as key nutrients for predicting herbivore performance may not be direct cause-and-effect but, rather, to be related to other factors that are linked to these characteristics in plant tissues. Water is not likely to be a critically limiting nutrient when budworm larvae feed on the new, current-year needles of their conifer hosts. The higher moisture levels tested, 84% and 88%, did not improve larval performance; thus, the budworms enhanced growth on early-season, new foliage may not be directly associated with the higher moisture content of new, expanding needles, but is probably related to other factors linked to plant water content.

Evaluation of Funnel Traps for Characterizing the Bark Beetle (Coleoptera: Scolytidae) Communities in Ponderosa Pine Forests of North-Central Arizona

Abstract Lindgren funnel traps baited with aggregation pheromones are widely used to monitor and manage populations of economically important bark beetles (Coleoptera: Scolytidae). This study was designed to advance our understanding of how funnel trap catches assess bark beetle communities and relative abundance of individual species. In the second year (2005) of a 3-yr study of the bark beetle community structure in north-central Arizona pine (Pinus spp.) forests, we collected data on stand structure, site conditions, and local bark beetle-induced tree mortality at each trap site. We also collected samples of bark from infested (brood) trees near trap sites to identify and determine the population density of bark beetles that were attacking ponderosa pine, Pinus ponderosa Douglas ex Lawson, in the area surrounding the traps. Multiple regression models indicated that the number of Dendroctonus and Ips beetles captured in 2005 was inversely related to elevation of the trap site, and positively associated with the amount of ponderosa pine in the stand surrounding the site. Traps located closer to brood trees also captured more beetles. The relationship between trap catches and host tree mortality was weak and inconsistent in forest stands surrounding the funnel traps, suggesting that trap catches do not provide a good estimate of local beetle-induced tree mortality. However, pheromone-baited funnel trap data and data from gallery identification in bark samples produced statistically similar relative abundance profiles for the five species of bark beetles that we examined, indicating that funnel trap data provided a good assessment of species presence and relative abundance.

Lack of genetic differentiation in aggressive and secondary bark beetles (Coleoptera: Curculionidae, Scolytinae) from Arizona

Abstract Bark beetles (Coleoptera: Curculionidae, Scolytinae) play an important role as disturbance agents in ponderosa pine (Pinus ponderosa Douglas ex Lawson) forests of Arizona. However, from 2001 to 2003, elevated bark beetle activity caused unprecedented levels of ponderosa pine mortality. A better understanding of the population structure of these species will facilitate analysis of their dispersal patterns and improve management strategies. Here, we use fluorescently labeled amplified fragment length polymorphism (fAFLP) analysis to resolve genetic variation among and within sampling locations in northcentral Arizona of Ips pini (Say), Dendroctonus brevicomis LeConte, and D. frontalis Zimmermann. We generated genetic fingerprints for >500 beetle specimens and analyzed genetic diversity. For all species, gene flow estimates among sampling locations were high, and significant population subdivision was not discernible across a large portion of ponderosa pine forests in Arizona. However, a weak relationship was detected with I. pini population structure and elevation. Because of the lack of genetic differentiation detected throughout the large study area, our findings suggest these insects are capable of long distance dispersal and exhibit a high degree of gene flow across a broad region. We conclude that our results are consistent with strong dispersal patterns and large population sizes of all three species.