Niche Divergence Of Two Sibling Species Of Oomyzus Parasitoids Attacking Predaceous Ladybird Beetles

To date,studies of parasitoid natural enemies have been focused on the taxa parasitizing phytophagous insects,while much less attention has been paid to the taxa attacking predaceous insects.This bias in favor of certain taxa impedes a comprehensive understanding of food web ecology and an objective evaluation of predaceous natural enemies at play in suppressing herbivorous pests in biological control programs.Ladybird beetles are important predaceous natural enemies of insect pests of a minute size such as aphids,whiteflies and mealybugs,but they also have parasitic natural enemies of their own.Little is known about the biology,behavior and ecology of parasitic natural enemies of predaceous ladybugs.The cryptic species Oomyzus scaposus and O.spiraculus(Hymenoptera: Eulophidae)both attack the 7-spot lady beetle Coccinella septempunctata(Coleoptera: Coccinellidae),but it is not clear how they avoid the overlap in the trophic niche.The aim of this study is to address questions:how high temperature and host density affect the interactions between the two parasitoids.What follows is a summary of main results and conclusions.1.Effects of constant high temperature stress on life history of two Oomyzus waspsTemperature treatment was applied at three levels(26 ℃,30 ℃ and 33 ℃)to O.scaposus and O.spiraculus each attacking 7-spot lady beetle larvae alone and offspring developmental performances were observed and compared between the two parasitoid species.The analysis of developmental performance of the parasitized host showed that body weight of host pupa decreased by 8.8% and pre-pupation time shortened by 14.9% under 26 ℃ compared to those under 30 ℃.The analysis of offspring parasitoids showed that offspring failed to emerge from the host under 33 ℃.Developmental period of offspring brood was shortened by 8.1%,longevity of adults by 16.1%,and the number of offspring parasitoids decreased by 13.7%,at 30 ℃,compared to those at 26 ℃.Parasitism rate and brood size were 1.6 and 1.2 times higher for O.scaposus than for O.spiraculus at26 ℃ and 30 ℃,respectively.The results suggest that O.scaposus develops better at a higher temperature than O.spiraculus does.2.Effects of heat wave on life history of two Oomyzus waspsHeat wave(33 ℃)treatment in time for 0(the control),1 and 3 hours was applied to two Oomyzus species each attacking 7-spot lady beetle host alone.The results showed that the heat wave treatment did not have a significant effect on body weight of host pupa,pre-pupal time of the host,parasitism rate and brood size of both parasitoid species.O.scaposus increased by 35.1% offspring developmental duration compared to O.spiraculus under the control.Sex ratio and longevity of offspring female adults did not change linearly as a function of heat wave duration.O.scaposus increased by 14.4% parasitism rate as compared to O.spiraculus.The results suggest that heat wave stress may have a smaller negative impact on O.scaposus than on O.spiraculus.3.Effects of high temperature and oviposition sequence on intrinsic competition between two Oomyzus waspsTwo constant high temperatures(26 ℃ as control and 30 ℃)and two oviposition sequences(first and last)were factorially combined to apply to parasitism simultaneously by two Oomyzus species and then offspring developmental performances were observed.Parasitoid species affected the parasitism rate.O.scaposus effected 1.4 times higher parasitism rate than O.spiraculus did.Offspring brood size was affected by the interaction between parasitoid species and oviposition sequence.O.scaposus produced brood that was1.3 and 3.0 times larger than did O.spiraculus when ovipositing both first or both last,respectively.O.spiraculus produced brood smaller by 57.1% with first oviposition than the last,but O.scaposus produced brood was no significant difference between first oviposition than the last.Sex ratio was affected by the interaction between parasitoid species and oviposition sequence.Sex ratio was no influence between O.scaposus and O.spiraculus when ovipositing first,but was 2.1 times larger for O.scaposus than that for O.spiraculus when ovipositing last.Sex ratio of O.spiraculus ovipositing last was 2.7 times larger than that ovipositing first.,but sex ratio of O.scaposus was not affected between ovipositing first and last.Longevity of offspring adults was affected by temperature and was shorter at30 ℃ than at 26 ℃.The results suggest that O.scaposus is more competitive than O.spiraculus and the later has the capability of host discrimination.4.Effects of host density on intrinsic competition between two Oomyzus waspsTwo types of competition were set up by having the two wasps either alone(intra-specific)or together(inter-specific)in making parasitism of variable host densities 4,8,12,and 16 lady beetle larvae within one petri-dish.Offspring performances were observed.Under intra-specific competition,the parasitism rate was affected by the interaction between parasitoid species and host density.The parasitism rate of O.spiraculus was decreased by 19.3% at the12-hosts density compared to that at the 8-hosts density,but was no significant difference among other host densities.There was no significant difference in parasitism rate of O.scaposus under different host densities.The parasitism rate of O.scaposus was 22.5% higher than O.spiraculus at the 12-hosts density,but there was no significant difference between O.scaposus and O.spiraculus at other host densities.Offspring brood size was affected by the interaction between parasitoid species and host density.O.scaposus produced a smaller brood than O.spiraculus did at 4-hosts density and8-hosts density,but the pattern reversed at 12-hosts density and 16-hosts density.Sex ratio was affected by parasitoid species and host density.Sex ratio of was affected larger for O.scaposus than for O.spiraculus.There was significant difference in sex ratio among different densities.The highest sex ratio was found at 4-hosts density,and the specific sex ratio was 5.4% higher than that at 12-hosts density.Under inter-specific competition,the parasitism rate was affected by parasitoid species and host density.Parasitism rate by O.scaposus was 11% higher than that by O.spiraculus.The parasitism rate was 9.0% and9.9% higher at 8-hosts density and 16-hosts density that at 4-hosts density respectively,while there was no significant difference among other densities.Offspring brood size was affected by the interaction between parasitoid species and host density.At different densities,brood size of O.scaposus was 1.6(4-hosts density),2.3(8-hosts density),5.9(12-hosts density)and 1.3(16-hosts density)times higher than that of O.spiraculus,respectively.Proportion of offspring was affected for O.spiraculus by host density,but not for O.scaposus.Proportion of offspring of O.spiraculus increased by 1.5 times at the16-hosts density compared to that at the 8-hosts density.Sex ratio was affected by host density,and was affected larger for O.scaposus than for O.spiraculus.The results suggest that O.scaposus is relatively stronger in competition with O.spiraculus.