Search

The jumping plant-lice fauna of seven different willow species growing adjacent to the Rhine River and nearby was investigated in a two-year study. In four of five distinct sites all the seven plant-lice species were found, which feed on willows in central Europe. Number and density of species per host varied tremendously depending on the year, host plant species and site. There was no plant-lice species which characterised a distinct host or a distinct site. Cacopsylla pulchra (Zetterstedt 1838) was the most common psyllid species: it dominated at four of the five sites studied, and preferred either Salix viminalis, S purpurea or S alba at each site, even when the other host plants were also present. The canonical correspondence analysis (CCA), which takes the year of study, the site and the host plant species including secondary plant chemistry into account, resulted in 45% of the variance in psyllid diversity, leaving up to 55% to unexplained variance which could potentially be accounted for by a stochastic dispersion pattern and the low ability to find host plants. The number of species and individuals of the Salix-feeding psyllids usually surpassed that one of the tourist species. However, several examples of this study exhibited higher tourist guild inputs than of psyllid species associated with their hosts. This was found not only for single host trees but also for total groups of single willow species growing at a site. The study clearly demonstrates that plant-lice assemblages and herbivory are dynamic processes that reflect influences of variable environmental conditions and erratic colonization.

Choleva agilis Illiger 1789 is a cold season species which reproduces and develops in winter months and obligatorily aestivates in the early adult stage from IV until IX. Aestivation delays, and tends to synchronize, the developmental cycle with the season rather than avoids deteriorating climatic conditions. Females [FF] oviposit for about 150 days. However, at the end of pre-imaginal development in the spring the adults emerge within a relatively narrow time interval of about 35 days. Synchronisation of individual development within the population is caused by the differences in thermal thresholds in subsequent pre-imaginal stages, and by the changing temperatures during winter months as well. The narrow time interval of adult emergence is not maintained until the age of first reproduction. By contrast, diapause intensity varies and gives the capability to spread the onset of reproduction over a time interval of more than 200 days, even between full sibs. Spreading the age at first reproduction even within families, and the relatively low heritabi-lity in the duration of diapause (h 2 = 0.12 + 0.018), are interpreted as bet-hedging mechanisms to cope with the vagaries of weather conditions in an unpredictable environment. Temporal spreading of reproduction is so pervasive that it provides the opportunity for at least some daughters of each single dam to match the correct date of oviposition, independently of birth dates and weather conditions which could be expected in the area of study. There is insufficient synchronisation of the developmental cycle with the season by diapause intensity when (a) oviposition is very early in the season, and (b) the entire pre-imaginal development occurs at extremely high temperatures. Under these circumstances an ecological tuning by photoperiod and temperature is neccessary.