| While parasitoids of phytophagous insect pests have received considerable attention,those of predatory insects have attracted scant attention in their biology,behavior,and ecology.In this thesis study,a field investigation was firstly carried out to uncover parasitoid natural enemies attacking two dominant ladybirds Coccinella septempunctata and Harmonia axyridis.Among the parasitoids attacking the seven-spotted ladybird(C.septempunctata,C7)larvae,were collected two gregarious parasitoids(Hymenoptera:Eulophidae),Oomyzus scaposus and its closely related conspecific,not yet named scientifically and temporarily named as O.flavitibialis.These two parasitoids were compared in morphology,barcode of COI,and parasitism performances;hybridization trials between were made.Following studies focused on progeny allocation of these two parasitoids in response to the host already parasitized by each other,with host and wasp body sizes as covariates.The final study examined the performance of parasitized C7 larvae in cannibalism and intra-guild predation by H.axyridis.This thesis study provides new data for a comprehensive understanding of behavioral ecology of gregarious parasitioids attacking predatory ladybirds.What follows are main results and conclusions from this study.1.Survey of parasitoid natural enemies of predatory ladybirds and comparison of two sibling eulophid parasitoidsThe survey in suburbs of Nanjing uncovered 8 parasitoid species from rearing C.septempunctata and 2 from H.axyridis.The most numerous species and highest parasitism rate were found in the habitat with least disturbance among three types of habitat varying at the level of anthropogenic disturbance.Pupae of H.axyridis were parasitized at the highest rate(5.31%)only by the Diperan phorid,Phalacrotophora sp.,while C.septempunctata larvae were attacked by 26.1%and its pupae by 36.8%,to which Oomyzus parasitoids(>15%)contributed most.Two species of Oomyzus parasitoids were found together attacking C7 larvae,O.scaposus and another one named temporarily as O.flavitibialis for the yellow hind tibia,which was differentiated from the former with black hind tibia.The differentiation between these two Oomyzus parasitoids was further supported by the results from hybridization trial,sequencing mitochondrial COI gene,and observation of parasitism performances.The scaposus had a longer egg-to-adult time(average 18.48 d)than theflavitibialis(16.46 d),a smaller brood(8.9)than the flavitibialis(11.62),and smaller offspring adults(female HTL:0.47 mm,male HTL:0.40 mm)than the flavitibialis(female HTL:0.44 mm,male HTL:0.36 mm).The two species were not different in offspring sex ratio(proportion males)(the scaposus,0.18;the flavitibialis,0.15).Host instar choice trials showed that O.scaposus did not prefer older hosts while O.flavitibialis preferred 3rd(90%)to 1st(10%)instar larvae.The residence time on host body of the 1st to 3rd instars was significantly longer for O.flavitibialis than for O.scaposus.The results suggest that O.scaposus and O.flavitibialis were the most important parasitic wasps of C7 larvae,and they were sister species.2.Host discrimination and progeny allocation in multiparasitismThe dual-choice trials showed that both naive and experienced O.scaposus females had no preference between healthy and heterospecifically parasitized C7 larvae.While naive O.flavitibialis females did not show the prefence the experienced preferred healthy to parasitized hosts.By the residence time on host body,0.scaposus,either naive or experienced,did not show any preference between the two hosts,but experienced O.flavitibialis stayed longer on healthy than parasitized host.A comparison between a single bout of parasitization and of stinging(the way of attack)showed different progeny allocation performances between the two parasitoids.For O.scaposus,brood size and sex ratio(proportion males)did not chage with the way of attack;for O.flavitibialis,brood size of a bout of parasitization was 1.75 times as large as a bout of stinging,though sex ratio same.In response to a parasitized host,25%(n=24)of O.flavitibialis females made the progeny allocationon the host attacked by a bout of parasitization and 71.4%(n=24)on that by a bout of stinging,while 80.8%(n=26)and 91.3%(n=23)O.scaposus females made the allocation on the respective hosts.Females of these two parasitoid species did not change their progeny allocation in brood size and sex ratio in response to the host previously parasitized by each other.The results suggest that while O.scaposus may not be able to discriminate between healthy and heterospecifically parasitzed C7 larval hosts,O.flavitibialis may have such ability;these two parasitoids make progeny allocation irresponsive to the host previously parasitized by each other.3.Body size dependant progeny allocation of O.scaposusWe investigated the combined effect of host size across successive host larval stages and maternal wasp size on fitness-correlated performances of offspring parasitoids,using the larval-pupal gregarious parasitoid Oomyzus scaposus attacking C7.The results showed that the probability of parasitism success decreased with ladybird body weight at the time of parasitism when small sized wasps made the attack,but maintained a high level across the full range of host weights when large sized wasps made it.Offspring development time to adulthood decreased,while brood size increased,asymptotically with host body weight,but they did not vary with maternal wasp size.Sex ratio was female-biased and did not vary with ladybird body weight or maternal wasp size.Body size of F1 adult parasitoids did not vary with ladybird body weight or maternal wasp size.By extending the contact time between the host and the wasp,we can get samples with larger variation in brood size,which is much more close to the parasitic situation in the field,and evaluate the affects from another feature.The results showed that proportion males and egg-to-adult time were significantly affected by brood size.As brood size increased,the proportion males increased and the egg-to-adult time was shortened,while offspring size and emergence holes were not affected by brood size.Our findings suggest that small C7 ladybird larvae are preferred hosts for small O.scaposus females,while large sized ladybird larvae appeared to have little or no effect on parasitism success of large female parasitoids;proportion males increased with brood size.4.Performances of parasitized C7 larvae in cannibalism and intra-guild predationThis study aimed at examining the effect of parastized C7 larvae on intra-(cannibalism)and inter-specific(intraguild predation)competition.The results showed that the odds of winning of parasitized and healthy larvae did not differ under all treatments imposed.Cox proportional hazard model showed the predation risk of parasitized 4th instar larvae did not differ between being attacked at the 1st and 3rd instars.The median death time of them was 22 h and 22.5 h.Besides,the predation risk of 4th instar larval hosts separately parasitized at 1st between 12 h starvation and 24 h starvation was not significant as well.However,the larval median death time with 24 h starvation treatment(4 h)was shorter than that with 12 h starvation treatment(20 h).Then we recorded and examined the predation risk and behaviors of C7 larvae with different treatment in response to H.axyridis.The results showed that the probability of C7larvae being preyed was significantly influenced by the treatments.The probability of C7 larvae being preyed at the 3rd instar was highest(21%),but it was 9%at the 1st or 10%in healthy larvae.The dropping probability was 50%for the 1st instar,43%for the 3rd and 47%for healthy ones.The results from the present study suggest that parasitized C7 larvae were not at disadvantage over competitors in cannibalism,yet have a higher risk of predation by H.axyridis in the guild. |
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