Impacts Of Long-term Water-deficit Stress On Tritrophic Interactions In The Wheat-sitobion Avenae-aphidius Gifuensis System

Increasing temperature and CO₂ concentrations can alter tritrophic interactions in ecosystems,but the impact of increasingly severe drought on such interactions is not well understood.We examined the response of a wheat-aphid-parasitoid system to variation in water-deficit stress levels.Our results showed that arid area clones of the aphid,Sitobion avenae?Fabricius?,tended to have longer developmental times compared to semiarid and moist area clones,and the development of S.avenae clones tended to be slower with increasing levels of water-deficit.Body sizes of S.avenae clones from all areas decreased with increasing water-deficit levels,indicating their declining adaptation potential under drought.Compared to arid area clones,moist area clones of S.avenae had a higher frequency of backing under severe water stress only,but a higher frequency of kicking under well-watered conditions only,suggesting a water-deficit level dependent pattern of resistance against the parasitoid,Aphidius gifuensis?Ashmead?.The number of S.avenae individuals attacked by the parasitoid in 10 min showed a tendency to decrease with increasing water-deficit levels.Clones of S.avenae tended to have lower parasitism rates under treatments with higher water-deficit levels.The development of the parasitoid tended to be slower under higher levels of water-deficit stress.For long-term water-deficit stress,our results demonstrated that after 10consecutive generations of water-deficit stress,S.avenae tended to have prolonged DT5?i.e.,total developmental time of nymphs?with increasing levels of water-deficit in source areas and in different water-stress treatments.Clones of S.avenae from all areas tended to have declining body sizes with increasing water-deficit levels in water treatments.Arid area clones of S.avenae showed lower parasitism rates by A.gifuensis than moist area clones.The parasitism rates for any developmental stage of S.avenae clones from different areas tended to decrease with increasing water-deficit extents in water treatments.The developmental time of A.gifuensis on all aphid stages showed a tendency to increase with increasing levels of water-deficit in water treatments,suggesting that parasitoids could become increasingly vulnerable with lowered fitness under drought.For behavioral responses,our results showed higher frequency of A.gifuensis plant contacts of moist area clones than arid clones under the well-watered treatment and moist area clones had a lower frequency of A.gifuensis plant contacts under the severely stressed treatment than under the well-watered treatment.Frequency of A.gifuensis attacks in 30 min on moist area clones of S.avenae was significantly higher than that on arid and semiarid area clones.Numbers of S.avenae individuals attacked by A.gifuensis in 10 min declined with increasing water-deficit level.Arid area clones of S.avenae showed higher frequency of backing under severe water stress compared to semiarid area clones,and higher frequency of dropping under well-watered conditions only.Such results exhibit clear water-deficit level dependent pattern of resistance against Aphidius gifuensis?Ashmead?.Comparisons between 1^st generation and 10^th generation of this aphid showed that after continuous exposure of water-deficit for 10 generations,the body size of S.avenae clones for arid and moist area clones declined significantly under the well-watered and intermediately water-stressed treatment,respectively.The developmental time of S.avenae clones increased significantly after long-term water-deficit exposure.The developmental time of A.gifuensis also increased in aphid individuals under long-term water-deficit.Furthermore,number of attacks on single aphids had a tendency to decline compared to 1^st generation,showing poor suitability for A.gifuensis.This resulted in a decrease in parasitism rates.Thus,the bottom-up effects of water-deficit stressed plants were negative on S.avenae.However,the top-down effects via parasitoids were compromised by water-deficit,which could favor the growth of aphid populations.Overall,the first trophic level under water-deficit stress was shown to have an indirect and negative impact on the third trophic level parasitoid,suggesting that parasitoids could be increasingly vulnerable in future warming scenarios.