| The “top-down effects” of predation and parasitism by insect enemies are important forces in regulating herbivore pest populations in agroecosystems.In traditional biological control practice,single enemy species is introduced to suppress insect pests.However,in pest management practice,the intraspecific interactions,higher-order predation or parasitism may further reduce the efficiency of the biological control systems that lack of enemy diversity.Most of pest species are often attacked by multiple natural enemy species including predators and parasitoids,and traits of predator and parasitoid are different in suppressing pests.Thus,coordinated control measures that suppress target pests using both predators and parasitoids have received great attention in biological control practice.However,most of predators not only feed on the target pest,but also engage in preying on parasitoids that develop within the hosts,which has been termed as intraguild predation(IGP).Myzus persicae(Sulzer)is considered as an important pest in agricultural systems.Harmonia axyridis(Pallas)and Aphidius gifuensis Ashmead are two common enemies of M.persicae.When combined use of these enemies in regulating M.persicae,the effect of IGP between these natural enemies may result in a negative effect on enemy dynamics,and lead to an uncertain consequence in regulating M.persicae population by these enemies(positive effects,neutral,and negative effects).This study was aimed to test the mechanism of how a series of biotic and abiotic factors could affect the behavior and predation/parasitism efficiency of these two natural enemies in IGP systems.Then,control efficiency of combined use of these two natural enemies on M.persicae was assessed.In this study,the following questions were investigated: Firstly,whether IGP would occur between H.axyridis and A.gifuensis when sharing the same resource,and whether specific biotic factors would affect the IGP intensity.Then,the mechanism of direct or indirect interactions between H.axyridis and A.gifuensis on their foraging behaviors and fecundity was further studied,these include: the mechanism of temperature and H.axyridis cues on foraging behavior and fecundity of A.gifuensis was studied in Part 2,and the mechanism of mummy consumption on the fitness and oviposition site selection of H.axyridis was investigated in Part 3.In addition,the mechanism of aphid distribution and IGP on the predatory or parasitism efficiency of H.axyridis and A.gifuensis when sharing the same prey species was studied in Part 4.Lastly,the efficiency of combined use of H.axyridis and A.gifuensis in management M.persicae was further investigated in cages under greenhouse conditions.The main results and conclusions are as follows:1.Effects of biotic factors on the IGP intensity between H.axyridis and A.gifuensisEffects of starvation period of H.axyridis,plant dimension,and extraguild prey density on IGP intensity between H.axyridis and A.gifuensis were studied.Though the no-choice test,results showed that H.axyridis could consume A.gifuensis mummies.H.axyridis starved for 24 and 48 h had a higher intensity of IGP than those that were not starved.However,the number of aphids and mummies consumed by H.axyridis did not differ between the 24 and 48 h starvation treatments.Although IGP still occurred,mummy consumption decreased with increasing aphid density and plant dimension.In addition,H.axyridis consistently preferred aphids to mummies.2.Consequence of H.axyridis cues and temperature on the searching behavior and fecundity of A.gifuensisThe frequency and duration of stinging,contacting,stationary and walking behavior of A.gifuensis did not show significant differences between aphid patches with or without H.axyridis cues under three different temperature(except for stationary and walking behavior of A.gifuensis between aphid patches with or without H.axyridis cues under 28℃).In 23℃,A.gifuensis progeny was significantly higher on aphid patches without H.axyridis cues than patches with H.axyridis cues.Duration of A.gifuensis behavior were not significantly different among three different temperature with or without H.axyridis cues.The frequency of stinging,walking and stationary behavior of A.gifuensis were increased with increasing temperature,but not for contacting behavior.Aphidius gifuensis progeny was significantly higher in 28 and 33℃ compared with 23℃ when H.axyridis cue occurred.3.Effects of mummy consumption on fitness and oviposition site selection on Harmonia axyridisCompared with H.axyridis supplied with only unparasitized aphids or a mixture of prey(unparasitized aphid: mummy = 1:1),its larval development times was prolonged,and body weight of the fourth instar larvae and newly emerged adults,and fecundity decreased when H.axyridis was reared on A.gifuensis mummies only.Harmonia axyridis laid more eggs on plants with unparasitized aphids alone than plants with mummies.In contrast,H.axyridis previously fed with A.gifuensis mummies did not show a significant different in laying eggs between plants with unparasitized aphids alone and plants with mummies.4.Effects of IGP and aphid distribution on the functional response of H.axyridis and A.gifuensisType II functional responses were observed in all experiments.Functional response curves of single H.axyridis or A.gifuensis were higher in the aggregate treatment compared with in the uniform treatment when aphid densities were between 40–180 or 70–170,respectively.When comparing between aggregate and uniform treatments with heterospecific enemy occurrence,no differences were found in the parasitism efficiency of A.gifuensis.However,H.axyridis consumed more aphids in the aggregation treatment than in the uniform treatment when aphid densities were between 50–230 in the presence of A.gifuensis.Functional response of individual H.axyridis was not affected by A.gifuensis under two aphid distributions.However,the treatment where A.gifuensis was alone had a higher parasitism rate than the treatment where A.gifuensis was sharing the experimental patches with H.axyridis in the aggregation or uniform treatment at aphid densities below 150.5.Suppression efficiency of M.persicae by H.axyridis and A.gifuensis in greenhouseAphid densities were significantly lower in treatment when A.gifuensis was paired with H.axyridis than single H.axyridis or A.gifuensis treatment in 20–45 days.In 20–60 days,H.axyridis dampened the number of A.gifuensis mummies.However,combined use of H.axyridis and A.gifuensis did not alter the ratio of A.gifuensis to M.persicae in 20–45 days.In addition,the presence of A.gifuensis did not affect the H.axyridis population.Overall,H.axyridis could consume A.gifuensis mummies.Shorter starvation period of H.axyridis,larger plant dimension,higher extraguild prey density,increasing temperature,controlling aphid distribution and density could reduce the negative effects by H.axyridis on A.gifuensis.Mummy consumption could reduce the fitness,and alter the oviposition site preference of H.axyridis,but predation rate of H.axyridis was not affected by IGP.When released H.axyridis and A.gifuensis together,although IGP were detected between H.axyridis and A.gifuensis,the control efficiency of combined use of these enemies on M.persicae were better than one of these natural enemies.In conclusion,a series of biotic and abiotic factors could be used to decrease the negative effects of IGP on H.axyridis and A.gifuensis,and IGP between these two enemies did not significantly disturb the overall control efficiency of these enemies.Therefore,H.axyridis and A.gifuensis may present the possibility that could be successfully combined to improve aphid control efficiency. |
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