| Lepidopteran larvae perform a rich variety of behaviors to defend against attack by parasitoids,yet bear the cost of compromising their fitness.The fitness cost constitutes part of the effect on the host of attacking parasitoids,the so called‘non-consumptive effects'(risk effects,sublethal effects).Non-consumtive effects(NCEs)and consumptive effects(lethal effects)are united to influence the host population.Up to now,most of studies of host defensive behaviors against parasitoids have been descriptive rather than quantitative,and far less is known about the effcicacy of these behaviors in defense against parasitoids and fitness cost the host bears.This thesis study firstly examined defensive behaviors of Mythimna separata(Walker)larvae in response to two species of braconid parasitoids,Meteorus pulchricornis(Wesmael)and Microplitis mediator(Halidag),and measured their efficacy by resolving direct and indirect factors.It then investigated non-consumptive effects of the two parasitoids on the host on different food plants.This study increases ours understanding of interactions between caterpillars and their parasitoids,and provides data for evaluation of overall effects the parasiotoids have on this herbivorous pest.1.Defense behaviors and their efficacy of M.separata larvae in response to two parasitoidsThis experiment was to examine defensive behaviors and the efficacy M.separata larvae performed in response to two spcies of parasitoids that have different oviposition behaviors.We firstly measured the effect of M.separata larval body size and two different parasitoids on the probability of larval dropping behavior,the probability of larval escaping behavior,and the rate of larval thrashing behavior.Then we estimated larval defense efficacy through the sting probability and host-handling time,as affected by larval body size,larval thrashing frequency,and parasitoid species.? M.separata larvae,whether small or large in body size,exhibited escaping and dropping behaviors in the same probability to defend against both parasitoid species,but performed thrashing behavior more frequently against M.mediator than against M.pulchricornis.Thrashing frequency was nearly 10 times higher in response to M.mediator than in response to M.pulchricornis.? The efficacy of defensive behaviors varied depending on host body size and parasitoid species.The likelihood of stinging decreased with thrashing frequency of both small and large hosts for M.pulchricornis,while this was true only for large hosts for M mediator.While host-handling time increased with host thrashing frequency at the same rate for both parasitoid species,it was overall shorter for M.mediator than for M.pulchricornis.? In addition,host-handling time decreased with host body size for M.pulchricornis but did not change with it for M.mediator.Our study results suggest that M.separata larval defense efficacy are depended on host body size and parasitoid species.2.Direct and indirect factors for the efficacy of defensive behaviorsThis experiment examined direct and indirect effects of host defensive behaviors on parasitism success of two parasitoids in relation to host and parasitoid density.Structural equation model(SEM)was applied to determine the path-diagram and indirect effects of host defensive behaviors on parastism Yule'Q(a transformed variable ranged from-1 to 1,measuring success of defensive behaviors and parasitism)of M.pulchricornis and M.mediator.? Yule's Q of hiding,dropping or escaping to ground,or alerting behaviors increased with the density of M.mediator females,whereas Yule's Q of resting or feeding behaviors decreased with the density.Yule's Q of thrashing,alerting,and dropping or escaping behavior to the ground increased with host density,yet parasitism Yule's Q decreased with Yule's Q of hiding behavior or host density.?Two indices,defensive behavior syndrome(PA1)and elusive behavior syndrome(PA2),were obtained to comprise six host defensive behaviors in response to M.pulchricornis.PA1 was positively correlated with Yule's Q of dropping and thrashing behaviors,but negatively with resting Yule's Q.PA2 was positively correlated with Yule's Q of hiding or resting behavior.Both PA1 and PA2 were negatively correlated with feeding Yule's Q.PA1 increased with either host or parasitoid density.PA2 increased with parasitoid density but decreased with host density.Parasitism Yule's Q decreased with host density but increased with parasitoid density;both indices had only direct effects.The results suggest that M separata larval hiding,feeding activity-inhibiting,and resting behaviors can decrease the risk of parasitism,and the caterpillar may perform collective defense behavior.3.Non-consumptive effects of two parasitoids on corn and wheat plantsTwo plants(Zea mays and Triticum aestivum)and three parasitization risks(M.mediator,M.pulchricornis,and CK)were fully crossed to obtain six combined treatments to observe host larval survival and duration,adult body weight and longevity.? Host larval survival in the presence of M.pulchricornis(76.45%)was significant lower than the control(91.92%),but not different from that in the presence of M.mediator(84.42%),which did not differ from the control.Plants did not have an effect on host survival.? Mortality of host larvae from both consumptive and non-consumptive effects varied with parasitoids.For M.pulchricornis,non-consumptive effects contributed heavily to host larval mortality,which did not differ between corn(39.22%)and wheat(50.80%).For M.mediator,however,consumptive effects played a major role for host larval mortality,which did not differ between corn(20.32%)and wheat(18.94%).?Host larvae at the risk of either M.pulchricornis(31.48 mg)or M.mediator(32.03 mg)were lighter than that in the control(35.80 mg).Host pupae were influenced by the interaction between plant species and risk treatments.On the corn,the pupa at the risk of M.mediator(382.63 mg)was smaller in body weight than in the control(406.67 mg),yet was similar at the risk of M.pulchricornis(395.07 mg)to that in the control.On the wheat,host pupae were similar between risk treatments.? Host larval development time and pupal duration were influenced by the interaction between plant species and risk treatments.When being on the corn,larval and pupal times were not influenced by risk treatments.When being on the wheat,larval time at the risk of M.pulchricornis(21.31 d)and M.mediator(21.38 d)was longer than that in the control(18.33 d);pupal time at the risk of M.pulchricornis(13.50 d)was significant longer than that in the control(15.23 d),but the time at the risk of M.mediator(14.23 d)did not differ from that in the control.Host adult longevity was significant influenced by the risk treatment but not by plant species.The longevity at the risk of M.pulchricornis(7.31 d)was shorter than that at the risk of M.mediator(8.49 d)and the control(9.89 d).Host adult longevity was not influenced by plant species.The results suggest that non-consumptive effects take a major role for host mortality than consumptive ones do for M.pulchricornis,and stronger than that fo M.mediator;plant species have consequences on non-consumptive effects.4.Mechanisms for non-consumptive effects of parasitoidsThe risk of M.separata was manipulated as behavioral by using male wasps or physiological by using Co60 sterolized(80 Gy Co60 radiation dose at 2 Gy/min)females,with the control without wasps.? Host adult emergence rate was lower under the physiological risk(61.11%)than in the control(89.66%),and not different between the behavioral risk(83.33%)and the control.? M separata under the physiological risk performed longer host larval duration(28.14 d vs 20.73 d)or pupal time(17.05 d vs 12.88 d),and larger pupal weight(0.3047 g vs 0.4055 g)than in the controls,yet was similar under the behavioral risk to that in the control.Host adult longevity were not affected by the risk treatment.The results suggest that M.separata ovipositor stinging may play a major role in causing non-consumptive effects on M.separata larvae.5.Non-consumptive effects of parasitoids in the fieldA field plot was laid out to comprise 64 clusters of 3 plants of corn seedlings,arragned in a square with 0.7 m apart between clusters and rows.Each plant cluster was allocated 10 M.separata larvae,and then M.pulchricornis and M.mediator each 25 were released in the plot.On following successive 5 days,all plant clusters were examined for the presence of insects and some samples of host larvae were brought to laboratory to observe their developmental performances.? A percentage of 22.09%host larvae(n=86)was parasitized by M.pulchricornis(3 hosts),M.mediator(5),and Tachinid flies(11).Using partial Mental test for spatial auto-correlation of parasitoids in the plot showed a significant effect at 0.7 m apart.?The analysis with mixed effects model showed that spatial distribution of parasitoids did not influence M separata larval survival,host larval body weight,and the ratio of numbers of hosts on central to side leaves,but influenced host development time,extending for two days.The results suggest that the activity of parasitoids may be spatially correlated at local areas and parasitoids have non-consumptive effects on M separate.6.SummaryThis thesis study indicates that M.mediator larvae can adjust their defensive behaviors in response to attack by M.pulchricornis and M.mediator.The efficacy of these defensive behaviors in reducing the risk of parasitization was evaluated using path analysis.Non-consumptive effects of the two parasitoids differed in importance relative to consumptive effects,manifested by higher mortality,smaller host size,extending larval development time,and decreased adult longevity.In addition,these effects may vary with parasitoid and plant species.This study deepens our understanding of interactions between caterpillars and their parasitoids and provides data for a full evaluation of effects of parasitoids on the herbivorous pest M.separate. |
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