Objectives were:
1) To screen potential food plants and identify those that have suitable nectar and/or pollen resources. Results showed that insects fed on Choisya ternata and Satureja hortensis survived almost 4 days; on Alyssum maritimum more than 6 days and on Fagopyrum esculentum and Nepeta cataria ~7.5 days. The species of flowers that present no interest for the parasitic wasps are Calendula officinalis and Kalanchoe blossfeldiana.
There is a current lack of studies addressing the combined effects of climate warming and food sources on herbivorous, their natural enemies and ecosystem services. For these ectotherms insects the ability to remain active under extreme climatic conditions (extremely hot summer temperatures, some greenhouse climate conditions) is a significant key to survival since extreme temperatures may constrain resource availability and influence the phenology. Thus, increasing parasitoid resistance and adaptability to current heat waves during the summer through floral nectar supply could be used to improve Conservation Biological Control.
During the growing season, extremely high temperatures and low humidity conditions, which are becoming more and more common in southern European countries over the summer time, may constrain natural enemy populations. When considering greenhouse crops this could be worse since temperatures reached inside are higher.
2) To determine aphid parasitic wasp thermal tolerance and the effect of feeding on different species of flowers on parasitoid resistance to thermal shocks. We compared parasitic wasps allowed to feed either from F. esculentum (buckwheat) nectar or from water only. Both were submitted to thermal chocks at a temperature of 36.5ºC to evaluate the effect of stressing temperature that have lethal effects (60 minutes of exposure killed 50% of the individuals) and sub-lethal effects (15 minutes of exposure). Results showed that flower fed parasitoids survived statistically longer than unfed parasitoids under all conditions demonstrating that insects fed on a good quality food can better tolerate stressing temperatures. The proportion of insects that suffered a shock (face-up insects moving only legs and antennae) after stressing temperature was also significantly lower for fed insects.
3) To evaluate the attractiveness at distance of selected flowers to parasitic wasps. We used the Y-olfactometer equipment of CEQA Institute to evaluate the attractivity at distance of buckwheat odour cues to parasitic wasps. We examined four constant temperatures 20, 25, 30 and 35°C and three type of choice experiments: Buckwheat flowsers vs Control filtered air, Flower vs Hosts (wheat plant containing aphids) and Control vs Host. The results showed that 35°C was a stressful temperature, at which most insects are not attracted anymore by the flowers and the hosts.
4) To test the selected plants in a greenhouse in order to check previous laboratory results and evaluate how they affect the parasitism rates (the death of the aphids) in real conditions. The interest of adding buckwheat banks was evaluated in a greenhouse testing different spatial distributions under optimal and hot temperatures. Since summer hot temperatures are guaranteed in Mediterranean area of Spain where they can be a serious problem for greenhouse cultivation, this part of the project was realized in Valencia (Spain) during the secondment. We placed 10 pots containing young wheat plants infested with aphids inside 2x2x2m boxes covered with insect-proof net. Each essay included either 2 or 5 pots containing buckwheats in flowering phase. We released inside each box 10 mated females of parasitic wasps. Essays were performed from February to July. Temperature and relative humidity were recorded. After 7 to 12 days, depending on temperature, we randomly took 2 pots and collected all the mummies (dead aphids, parasitized by the wasps) generated. The number of remaining alive aphids was counted. The results showed that the parasitism rates were statistically higher during the period April-May (mean 21%) compared to the periods February-March (8%) and June-July (7%) which were not significantly different. During April-May period, temperatures ranged from 13ºC to 31ºC while during February-March ranged from 8ºC to 26ºC and during June-July from 19ºC to 36ºC. Thus, parasitism rate is strongly influenced by temperature and beginning of spring can be considered optimal period. We observed that parasitism rates were always higher in boxes with flowers during the period May-June and there were statistical differences between boxes with and without flowers (mean parasitism 15% and 8% respectively) while there were no statistical differences during the remaining periods.
