Climate driven shifts in aphid phenology does not affect predator appearance and chemical defense of a grass-endophyte association

Background/Question/Methods

Due to global warming, climate has changed within the last 50 years with a temperature increase of about 0.13°C per decade. Climate-driven shifts in plant and insect phenology can disrupt plant-microorganism and plant-herbivore interactions. Species with a short reproductive cycle might adapt faster to climate change than species with a longer reproductive cycle. In a common garden experiment we examined the effects of a 2 week advanced occurrence of aphids in spring on (i) the physiological response of an endophyte-infected and uninfected host grass and on (ii) aphid predator occurrence. As study system we used the grass Lolium perenne, either infected with the wild type endophyte Neotyphodium lolii or uninfected. We measured the physiological response by recording growth of the plant (biomass), growth of the fungus using a quantitative PCR on DNA basis, and alkaloid concentrations using a UPLC-MS. Aphid densities and predator occurrence and densities were recorded throughout the season.

Results/Conclusions

Aphid densities on grass pots with advanced aphid occurrence had overall three times higher densities than pots with regular occurrence. Predators appeared independently of aphid density at the same time on all pots. Within one week after predator occurrence the aphid densities of all pots were levelled. Growth of the endophytic fungus and alkaloid concentrations were correlated throughout the season, but neither the growth of the fungus nor the alkaloid concentrations were affected by aphid density. The fungus and alkaloids increased until mid-summer and decreased afterwards. We conclude that neither predator occurrence nor endophyte growth adapted to higher aphid densities earlier in the year. However at the end of the season (mid-summer), when aphid densities were significantly decreased by predators on all grass pots, endophyte growth and alkaloid concentrations decreased significantly.