| Plant volatiles are important olfactory cues affecting the host discrimination and host selection of herbivorous insects.The exploration of the effect of natural plant background odour on the host selection of herbivorous insects could provide a scientific accordance for controlling insect pests by the alteration of agroecosystem biodiversity such as the scientific arrangement of intercrops.In this dissertation,a specialist Helicoverpa assulta(Guenée)was used as the subject,two suitable host plant species(Nicotiana tabacum and Capsicum annuum),two unsuitable host plant species(Lycopersicon esculentum and Gossyoium hirsutum),and two non-host plant species(Arachis hypogaea and Zea mays)were selected as the test plants.To avoid confusion,the plants will be referred to by their common names hereinafter.The author studied the effects of different plant combinations on the olfactory approaching response and ovipositional choice response of the adult females in a specially designed flowing-air bioassay setup,as well as that on the host selection of neotates,respectively.The main results are summarized as follows:(1)I tested the approaching choice responses of the female moths and the neonates to different plant odours under the dual-choice condition of six plants paired with clean air,respectively.The results show that the female moths exhibited significant ovipositional preference to all the plants except for tomato without the disturbance of other odours.Among them,peanut and corn received the most deposited eggs(223.50 and 176.55/night,respectively),followed by hot pepper received the mean eggs of 172.90/night,suggesting that the females could perceive the presence of these plants during egg-laying but could not judge their suitability.The only exception is tomato,whose volatiles showed extremely significant ovipositional repellence to the females analyzed by paired t test(t = 8.04,P < 0.0001),suggesting that tomato is not a suitable host species for H.assulta.The choice frequency percentages of neonates to the two suitable host plants,hot pepper and tobacco,were the highest(72.5% and 67.5%,respectively)among the test plants,and reached to the levels of extremely significance and significance when compared with their corresponding controls(?2hot pepper = 7.23,Phot pepper = 0.0072;?2tobacco = 4.23,Ptobacco = 0.0397).There was no significant difference between the choice frequencies of the other plants comparing with their corresponding controls.Therefore,the neonates could precisely judge plant suitability via olfactory cues.(2)I tested the oviposition choice response and approaching response of the females and neonates to different plant odours under the dual-choice conditions of tobacco paired with the other five plants.The results show that the females exhibited strong ovipositional preference to tobacco volatiles in all the five paired groups,among them the most discriminative group was cotton versus tobacco,with 328.83 eggs received by tobacco.The statistics analyses show that the mean eggs deposited on tobacco and peanut were not significant(t = 2.04,P = 0.0639),significantly differed between those of on tobacco and hot pepper(t = 3.09,P = 0.0115),and extremely significant differed between those of on the remained pairs(t tobacco vs.corn = 4.00,P tobacco vs.corn = 0.0018;t tobacco vs.cotton = 5.56,P tobacco vs.cotton = 0.0002;t tobacco vs.tomato = 6.39,P tobacco vs.tomato = 0.0002),suggesting that the females could effectively discriminate the difference of volatiles of tobacco and those of all the other plants but peanut,and this might be due to the presence of attractive compound(s)in peanut volatiles.The choice frequencies of neonates between tobacco and tomato reached to significant level(29 and 12,respectively,?2 = 6.24,P = 0.0125),suggesting that the neonates could effectively discriminate the difference between tobacco and tomato leaves and tomato might be a non-host of H.assulta.No significant difference were found between tobacco and the other four plants,suggesting that the attractiveness of these plants were comparable to tobacco.A comprehensive comparison of the choice responses between the two stages showed that the overall discriminative ability to suitable host plants of the females were stronger than that of the neonates.(3)I tested the masking effect of the odours emitted from the other five plants on tobacco selection of the females and the neonates under the dual-choice conditions of tobacco versus tobacco plus each of the other plants.The results show that the presence of tomato and corn adjacent to tobacco could significantly inhibit egg-laying on the latter(43.56 and 93.30 eggs on the treatment substrates,respectively,t corn = 2.59,P = 0.0294;t tomato = 2.74,P = 0.0253),suggesting that the odours of tomato or corn added to that of the tobacco could inhibit egg-laying.While the presence of peanut adjacent to tobacco could significantly stimulate egg-laying(171.60 eggs on the treatment substrates,t = 2.35,P = 0.0435),and the presence of hot pepper and cotton adjacent to tobacco had not significant effect on egg-laying on tobacco.The choice frequencies of the neonates had not significant difference between the pairs in all the groups,suggesting that tobacco exhibited strong attractiveness to the neonates and which not affected by the presence of other plant odours.(4)I tested the preference of females and neonates under the presence of all the six plants.The results show that the tobacco received the most eggs(44.83),followed by hot pepper and peanut(37.00 and 30.16,respectively),but ANOVA showed no significant difference among all the odour sources(F5,30 = 1.15,P = 0.3573),suggesting that the host selection discrimination of the females decreased dramatically under over-complex odour environment.However,the choice frequencies of the neonates among the leaf discs prepared from different plants presented extremely significant difference(?2 = 23.22,P = 0.0003),among them the choice frequency percentage of suitable host tobacco was highest(34%),while those of non-host corn and peanut were least(both only 11%),suggesting that the neonates could precisely judge the most suitable host plant under the presence of all the six plants.(5)I observed daily eggs produced by females after emergence under the odour environments of the two suitable host plants(tobacco and hot pepper),and compared with the control.The result shows that the ovipositional duration under tobacco odour environment was longest(25 d),while those under hot pepper and control were only ranged from 14 d to 16 d.The mean cumulative eggs deposited per female under tobacco and hot pepper odour environments were 352.32 and 238.13,respectively,while that under control was only 184.48,suggesting that the odours from suitable host plants could stimulate egg-laying,especially,tobacco odour played a dual role of elongation of ovipositional duration and oviposition stimulation.(6)I tested the effect of egg-hatching environment on the selection of different parts from different hosts of subsequent hatched neonates.The result shows that egg-hatching experience of tobacco floral parts or hot pepper leaf could dramatically increase the choice frequencies of the subsequent hatched neonates to hot pepper fruits(51 and 53,respectively),being about three-fold of those of the other three materials and differed extremely significant with the control group(?2 = 14.59,P = 0.0022;?2 = 16.22,P = 0.0010).While egg experiences of tobacco leaf or hot pepper fruits had not significant effect on food selection of the larvae,and the two experienced group did not differ from the control group(?2 = 3.79,P = 0.2855;?2 = 7.61,P = 0.0547),suggesting that hot pepper fruit elicited the strongest olfactory preference of the neonates regardless of their egg-hatching experiences.I also tested feeding experience of 3rd instar larvae on tobacco or hot pepper leaf on their subsequent approaching response and feeding behaviour.The result showed that choice frequency of tobacco leaf were always more than that of hot pepper leaf,even no significant difference between them(?2 = 0.00;P = 1.0000),suggesting that feeding experience did not affect the approaching response of 3rd instar larvae.However,the feeding consumption of tobacco leaf was extremely significant more than that of hot pepper leaf(t tobacco experience group = 4.20,P = 0.0005;t hot pepper experience group = 5.36,P < 0.0001),suggesting that tobacco leaf is the most preferred food regardless of previous feeding experience. |
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