The nitrogen content of plant tissue is low relative to that of herbivores; as a consequence, dietary N can limit the growth and reproduction of herbivores and select for attributes that increase N acquisition. Bark beetles face a particularly severe challenge because the phloem that they consume is very low in nitrogen and phosphorus relative to their requirements. We quantified variation in the phloem concentrations of N and P in the host tree, Pinus taeda, and evaluated the following hypotheses regarding the role of symbiotic fungi in nutrient budgets of the herbivore Dendroctonus frontalis: D. frontalis experience variation in phloem nutrient concentrations across several spatial scales (H1); mycangial fungi enhance the diet of D. frontalis larvae by contributing to the acquisition of N and P (H2); Ophiostoma minus, an apparently antagonistic fungal symbiont, hinders D. frontalis larvae because it does not enhance nutrient concentrations of the phloem as much as mycangial fungi do (H3); and larvae of bark beetle species that lack mycangial fungi must consume more phloem to accomplish the same growth as larvae of D. frontalis (H4). In addition, we developed a general model for the N budgets of herbivorous insects that identifies the possible combinations of dietary and physiological parameters that can allow developmental success on low‐nutrient diets. Spatial variation in phloem N was mostly at the level of trees within sites (a scale of meters) while P mostly varied among sites (a scale of kilometers). Trees with higher N content produced larger D. frontalis adults. Prior to infestation by beetles, phloem nutrient concentrations were very uniform within trees and very low relative to that of the bark beetles (N and P concentrations of D. frontalis adults were 28 and 8 times greater, respectively). During infestation, phloem nutrient concentrations increased overall and became highly variable within trees. Nitrogen concentrations increased from 0.40 ± 0.01% (mean ± 1 se) in uninfested phloem to 0.86 ± 0.03% in the phloem surrounding successfully developing D. frontalis larvae, which are typically associated with one or two species of mutualistic mycangial fungi. Nitrogen concentrations were intermediate in other microhabitats within infested trees, including regions with no adult colonization, with failed larval development, or colonized by the antagonistic bluestain fungus O. minus. We parameterized a general nutrient‐budget model for D. frontalis and a sympatric non‐mycangial bark beetle, Ips grandicollis, which indicated that (1) mycangial fungi provide their benefits by concentrating dietary N for larvae; (2) O. minus may exert its antagonistic effects on D. frontalis larvae by failing to concentrate dietary N as much as mycangial fungi do; (3) non‐mycangial bark beetles meet their N budgets through high consumption of unaltered, low‐N phloem; and (4) larvae should easily meet their P requirements with any combination of consumption rate and development time that allows them to meet their N requirements. A...
1 Bursaphelenchus xylophilus (Steiner & Buhrer) (Nematoda: Aphelenchoididae), the pinewood nematode and the causal agent of the pine wilt disease, is a globally important invasive pathogen of pine forests. It is phoretic in woodborer beetles of the genus Monochamus (Megerle) (Coleoptera, Cerambycidae) and has been able to exploit novel indigenous species of Monochamus (but only Monochamus) in newly-invaded areas. North America (NA) is the continent of origin for the B. xylophilus/Monochamus spp. phoretic system. NA also contains the largest number of Monochamus species known to act as vectors for B. xylophilus. Understanding this phoretic system in its native geographical area helps to explain the evolutionary ecology of pine wilt disease. 2 In the present study, we measured the flight phenology, size, sex ratios and species identity of Monochamus species in five geographically distant forests in NA. We also measured phoresy by B. xylophilus. 3 We found the nematode to be abundant across eastern NA but rare or absent in western NA. In eastern forests, nematode phoresy was highest on the Monochamus species that flew earliest in the year. However, in the southeast, where Monochamus is most likely multivoltine with a long flight season, we found vectors with high nematode loads throughout the season, indicating that B. xylophilus can be transmitted to new hosts during most part of the year. The frequency distribution of nematode dauers on Monochamus was highly aggregated. Bursaphelenchus xylophilus in NA appears to be able to use all available Monochamus species as vectors. 4 In native NA pine forests, the pinewood nematode appears to have an ecology that is sufficiently flexible to exploit different species (and both genders) of Monochamus, and disperse at different times of the year. This flexibility may contribute to its recent success in invading Eurasian pine forests.
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