RESUMEN
A high diversity of plant defenses may be a response to herbivore diversity or may be collectively more toxic than single compounds, either of which may be important for understanding insect-plant associations. Monoterpenes in conifers are particularly diverse. We tested the fumigant toxicity of four monoterpenes, alone and in combination, to mountain pine beetles, Dendroctonus ponderosae, in the context of the beetles' individual body traits. Chemical structures of tested monoterpene hydrocarbons had modest effects on beetle survival, mass loss, water content and fat content, with (R)-(+)-limonene tending to be more toxic than (-)-α-pinene, (-)-ß-pinene, and (+)-3-carene. Monoterpene diversity (all qualitative combinations of one to four monoterpenes) did not affect toxicity. Concentration (0 to 1200 ppm) of individual monoterpenes was a strong determinant of toxicity. Beetle body size and body condition index strongly and positively affected survival during monoterpene treatments. Larger beetles in better condition lost proportionally less mass during exposure, where proportion mass loss negatively affected survivorship. Toxicity was much more associated with water loss than with fat loss, suggesting that a main cost of detoxification is excretion, a process that has received little attention. These results provide insight into the determinants of beetle success in historic and novel hosts that differ in monoterpene composition and concentration. We also suggest that water availability will affect beetle success directly through their ability to tolerate detoxification as well as indirectly through host responses to drought.
Asunto(s)
Escarabajos/fisiología , Monoterpenos/química , Monoterpenos/toxicidad , Pinus/química , Animales , Monoterpenos Bicíclicos , Tamaño Corporal , Compuestos Bicíclicos con Puentes/química , Compuestos Bicíclicos con Puentes/toxicidad , Ciclohexenos/química , Ciclohexenos/toxicidad , Limoneno , Terpenos/química , Terpenos/toxicidadRESUMEN
1. Given sexual size dimorphism, differential mortality owing to body size can lead to sex-biased mortality, proximately biasing sex ratios. This mechanism may apply to mountain pine beetles, Dendroctonus ponderosae Hopkins, which typically have female-biased adult populations (2 : 1) with females larger than males. Smaller males could be more susceptible to stresses than larger females as developing beetles overwinter and populations experience high mortality. 2. Survival of naturally-established mountain pine beetles during the juvenile stage and the resulting adult sex ratios and body sizes (volume) were studied. Three treatments were applied to vary survival in logs cut from trees containing broods of mountain pine beetles. Logs were removed from the forest either in early winter, or in spring after overwintering below snow or after overwintering above snow. Upon removal, logs were placed at room temperature to allow beetles to complete development under similar conditions. 3. Compared with beetles from logs removed in early winter, mortality was higher and the sex ratio was more female-biased in overwintering logs. The bias increased with overwinter mortality. However, sex ratios were female-biased even in early winter, so additional mechanisms, other than overwintering mortality, contributed to the sex-ratio bias. Body volume varied little relative to sex-biased mortality, suggesting other size-independent causes of male-biased mortality. 4. Overwintering mortality is considered a major determinant of mountain pine beetle population dynamics. The disproportionate survival of females, who initiate colonisation of live pine trees, may affect population dynamics in ways that have not been previously considered.
RESUMEN
1. In group-living organisms, individuals that initiate aggregations, termed pioneers, may suffer higher mortality costs than individuals that join established aggregations. Here we examine the hypothesis that aggregation initiators achieve higher reproductive success in the early phases of colonization, potentially through lower competition and increased access to the resource (finder's advantage), and that this benefit is sufficient to outweigh the costs of pioneering. 2. We also examine the role of arrival date (irrespective of order within the aggregation) on reproductive success, because individuals in seasonal environments may gain an advantage by arriving early. We test these hypotheses using mountain pine beetles, Dendroctonus ponderosae (Coleoptera: Curculionidae, Scolytinae), an obligately aggregating insect wherein pioneers suffer high mortality due to tree defences. We measured reproductive success at the start of winter when most components of final offspring number were likely to be determined. 3. Surviving pioneers that successfully recruited conspecifics had smaller broods than individuals that joined aggregations, refuting our hypothesis. The later that a beetle settled within an aggregation, the higher its reproductive success. However, beetles that settled early in the season produced more offspring than those that settled later in the season, and this effect was generally stronger than settlement order within an aggregation. Our study highlights the importance of examining the effects of both settlement order and arrival date on the costs and benefits of pioneering aggregations.