Douglas-fir beetle lipid levels in relation to tree physical characteristics

Journal of the Entomological Society of British Columbia, Dec 2004 by Dodds, Kevin J, Ross, Darrell W

ABSTRACT

The relationship of Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins, brood adult lipid levels and position of development along infested tree boles was investigated. In addition, the effects of phloem and bark thickness on brood adult lipid levels were also tested. There were no significant differences (P > 0.05) in brood adult lipid levels in relation to bole position, phloem thickness, or bark thickness found in this study. Numbers of attacks, larval mines, brood adults, and parasitoid cocoons did not differ significantly by tree bole position. Results from this study suggest Douglas-fir beetle does not benefit, in the form of increased lipid levels, from oviposition at different bole positions.

Key Words: Dendroctonus pseudotsugae, lipids, phloem thickness, optimal habitat

INTRODUCTION

Bark beetles are economically important insects and knowledge of factors that affect their flight and dispersal behavior could be useful for improving existing management techniques or developing new ones. Within a population beetles display varying degrees of flight capabilities from extended flight periods to those incapable of flight (Atkins 1966; Jactel 1993). This variation in flight capability can be related to a beetle's physiological state.

Lipids are a source of energy for insect flight (Canavosa et al. 2001) and have been correlated with flight capabilities in bark beetles (Atkins 1966; Slansky and Haack 1986; Jactel 1993). Atkins (1966) found that Douglas-fir beetle (DFB), Dendroctonus pseudotsngae Hopkins, with high lipid levels were least likely to respond to pheromones and hence disperse, while beetles with low lipid levels responded immediately to pheromones. Bennett and Borden (1971) found that a 90 minute flight was required before DFB responded to pheromones, suggesting the need to metabolize lipids before pheromone arrestment occurred (Atkins 1969). Relationships between lipid levels and responsiveness to host chemicals, pheromone arrestment, or dispersal behavior have been found in other bark beetle species as well (Hagen and Atkins 1975; Hedden and Billings 1977; Wallin and Raffa 2000). Because of their association with bark beetle dispersal potential, a better understanding of factors that influence lipid levels is important for understanding population movements.

Bark beetle lipid levels are influenced by temperature (Atkins 1967), attack density (Atkins 1975; Botterweg 1983; Anderbrandt et al. 1985), mycangial fungi (Coppedge et al. 1995) and phloem thickness (Slansky and Haack 1986). However, with the exception of Ips calligraphus (Germar) (Slansky and Haack 1986), it is unknown whether host tree characteristics affect lipid levels in bark beetle brood adults. Nutrient levels (N, P, Mg, Fe, Zn) vary by bole height on Douglas-fir, Pseudotsnga menzesii (Mirb.) Franco, tree boles and could influence patterns of insect colonization (Schowalter and Morrell 2002). In several Dendroctomts species, initial attacks occur at or near mid-bole (Miller and Keen 1960; Fargo et al. 1978; Safranyik et al. 1992) possibly due to phloem nutrients at these heights. Consequently, brood developing at mid-bole could have higher lipid levels than brood developing elsewhere along the tree bole.

Other factors, such as parasitism or predation rates, could influence colonization behavior. Studies investigating the relationship between parasitoid density and tree height have produced mixed results. Several studies found relationships between parasitoid density and height on tree boles (Ryan and Rudinsky 1962; Mills 1986; Wermelinger 2002), while others have not (Gargiullo and Berisford 1981). Our objectives were to determine if DFB brood development position along the length of tree boles and bark and phloem thickness affected lipid levels in brood adults and to determine the influence of bole position on attack density, larval mines, brood adults, and parasitoids.

MATERIALS AND METHODS

Tree Sampling. On 27 to 29 April, 2002, prior to the DFB flight period, nine Douglas-fir trees infested the previous year were felled and sampled from a small stand (

The portion of each tree bole infested by DFB was distinguished by the presence of successful egg galleries and brood adults. Total length of infested tree boles and dbh were recorded. Infested tree bole lengths ranged from 6.7 to 14.6 m (x = 9.7 m, SE ± 0.8). Bark samples were collected at three positions along the infested tree bole: 2 m up from the bottom of the infestation, the mid-point of the infested tree bole, and 2 m down from the top of the infestation.

Four bark samples were collected from each bole position. Bark samples were taken randomly around the circumference of each tree. A 100-cm^sup 2^ hole saw attached to a power drill was used to remove bark samples from the infested tree. Bark samples were removed and placed individually in labeled plastic bags. Samples were transported to the laboratory on ice and stored in a freezer at -10 °C until processed. Phloem thickness was measured on a subsample of bark samples before and after freezing. No differences in the average phloem thicknesses before and after freezing were found (KJD, unpublished data).

Bark Analysis. Brood adults were removed from bark samples and placed individually in numbered 7 ml glass vials with caps attached. Numbers of DFB entrance holes, larval mines, parasitoid cocoons, and bark and phloem thickness were recorded for each bark sample. Bark thickness was measured on four locations around each bark sample, while phloem thickness was measured in two locations. To account for bark thickness variability, the minimum and maximum thicknesses on each sample were recorded along with two random measurements.