Comparing distribution of willow psyllids (Cacopsylla spp.) on Salix lapponum in 2018 with 1995 along an alpine altitudinal gradient

The psyllid (Cacopsylla spp.) population on Salix lapponum in the alpine areas of southern Norway, in an altitudinal gradient between Haugastøl and Finse, were thoroughly studied in 1995. The results were presented and published in Hill et. al. 1998. With rise in global air temperature and observed changes in insect communities over the last years, a prediction could be made that there have been changes in the psyllid population in this area from 1995 to 2018. Current findings were compared with the ones from 1995. I predicted changes to have happened in egg-laying location on the Salix shrubs, with a higher proportion of eggs laid on leaves rather than catkins, and changes in psyllid species composition. I resampled the same sites along the gradient, which is located between 998 and 1222 m a.s.l. The methodology described in Hill et. al. 1998 was followed closely. Measurements were done of selected Salix shrubs while taking samples of leaves and catkins in early June. Enclosures were attached on leaf and catkin branches, trapping the developing psyllids inside. Further enclosures were added on leaf branches in July. All enclosures were collected in August, after any entrapped specimens would have developed into adults. The collected samples were measured and counted in the lab, followed by statistical analyses looking for trends and comparing the data with the 1995 findings. Some differences were found in the egg placement on the shrubs, but the findings were not significant. The number of eggs on catkins had a significant relationship with catkin developmental stage and observed catkin density. Only one species of the adult psyllids were found in the resampling, and their mean abundance was significantly lower for most sites in 2018. I interpret the difference in the results as an effect of phenology having shifted from 1995 or the methods being slightly different. The temperature increase will likely also be a part of the explanation, with the 2018 summer months being abnormally warm and sunny