Reproductive capacity of the grey pine aphid and allocation response of Scots pine seedlings across temperature gradients: a test of hypotheses predicting outcomes of global warming Jarmo K. Holopainen and Pirjo Kainulainen Abstract: The effects of the expected increase in growing season temperature on the performance of the aphid Schizolachnus pineti (Fabricius) (Homoptera: Lachnidae) and on the nutritional quality of its host plant (Pinus sylves- tris L.) were studied under a daytime temperature range of 20–28 °C, with nighttime temperature that was either fixed at 12 °C or 8 °C below the daytime temperature. Fecundity had a curvilinear response, with an optimum at 24 or 26 °C, which is 4 to 6 °C above the local mean daytime temperatures. Longevity of nymphal stage was negatively and linearly correlated (r 2 = 0.967) with daytime temperature. Intrinsic rate of population increase (r m ) and relative growth rate were significantly higher at 26 °C than at 20 °C. Fecundity and r m were negatively correlated with total phenolic concentration in needles. Temperature affected concentrations of some individual resin acids in needles and stems, while concentrations of monoterpenes, total phenolics, starch, and total nitrogen in needles were not affected by tem- perature. Seedlings grown at 24 °C achieved the greatest biomass. Results support the protein competition hypothesis, which predicts no changes in the concentration of plant phenolics with small temperature increases. However, at 26 °C the low starch/nitrogen ratio and low total phenolic concentration may partly explain increased fecundity of aphids. Résumé : Les effets de l’augmentation anticipée de la température pendant la saison de croissance sur la performance du puceron Schizolachnus pineti (Fabricius) (Homoptères : Lachinidés) et sur la qualité nutritive de son hôte (Pinus sylvestris L.) ont été étudiés à des températures diurnes variant entre 20 et 28 °C. La température nocturne était soit fixe à 12 °C, soit 8 °C plus basse que la température diurne. La fécondité réagit de façon curviligne avec un optimum à 24 ou 26 °C, soit 4 à 6 °C au-dessus de la température diurne locale moyenne. La longévité du stade nymphal est négativement corrélée (r 2 = 0,967) à la température diurne. Le taux intrinsèque d’augmentation de la population (r m ) et le taux de croissance relative sont significativement plus élevés à 26 °C qu’à 20 °C. La fécondité et r m sont corrélés négativement avec la concentration en composés phénoliques totaux dans les aiguilles. La température affecte la con- centration de certains acides résiniques en particulier dans les aiguilles et la tige, alors que la concentration en mono- terpènes, en composés phénoliques totaux, en amidon et en azote total dans les aiguilles n’est pas modifiée par la température. Les semis cultivés à 24 °C produisent le plus de biomasse. Ces résultats soutiennent l’hypothèse de la compétition protéinique, selon laquelle de faibles augmentations de la température ne produiraient aucun changement dans la concentration des composés phénoliques dans la plante. Cependant, à 26 °C, le faible rapport amidon : azote et la faible concentration en composés phénoliques totaux pourraient en partie expliquer la fécondité accrue des pucerons. [Traduit par la Rédaction] Holopainen and Kainulainen 102 Introduction Temperature is one of the most important physical factors of the environment affecting the physiological and behav- ioural interactions of insects and plants. Climate-change sce- narios for Europe forecast an increase in mean annual temperatures (relative to 1958) of 3.5–5 °C by ca. 2050 (Carter et al. 1991). For northern latitudes, it has been pre- dicted that the mean annual temperature increase between 1990 and 2100 due to anthropogenic activity could reach 6 °C (Carter et al. 1995). Expected changes in temperature will affect plant susceptibility to herbivores directly (Har- rington et al. 2001; Bale et al. 2002) and indirectly by changing host-plant quality (Veteli et al. 2002; Sallas et al. 2003). However, surprisingly little is known about the ef- fects of temperature changes on plant allocation patterns or resistance to herbivory (Ayres 1993; Veteli et al. 2002). Currently two partly competing hypotheses have been pre- sented to predict plant secondary metabolite concentrations under increasing temperature. The protein competition model (PCM) (Jones and Hartley 1998) predicts no effects in plant phenolic concentrations with small temperature in- creases. According to this model, decreases in leaf carbon due to increased growth and respiration are compensated by increased leaf carbon from increased photosynthesis across this small temperature increase. Ayres (1993) presented a generalized model of plant carbon balance response (CBR) Can. J. For. Res. 34: 94–102 (2004) doi: 10.1139/X03-203 © 2004 NRC Canada 94 Received 12 December 2002. Accepted 22 August 2003. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 15 January 2004. J.K. Holopainen 1 and P. Kainulainen. Department of Ecology and Environmental Science, University of Kuopio, P.O. Box 1627, FIN-70211 Kuopio, Finland. 1 Corresponding author (e-mail: jarmo.holopainen@uku.fi).