| Oviposition strategy of parasitoids is an important domain of behavioral ecology,and reflects the adaptability of parasitoids to abiotic and biotic factors.Numerous studies have been done about the effects of host size and competition among conspecific parasitoids on oviposition strategy of parasitoids,but most of the studies are focused on solitary parasitoids.Superparasitism behaviour and its effect on fitness gain of parasitoids have got many researchers' attention.Gregarious parasitoids are ideal materials to study superparasitism,because more than one offspring of gregarious parasitoids can complete development on a host.While,for it is difficult to distinguish offspring of the two parasitizing parasitioids in a superparasitized host,there is little research on superparasitism of gregarious parasitoids.Oomyzus sokolowskii(Kurdjumov)is a gregarious larva-pupal endoparasitoid of the diamondback moth Plutella xylostella(L.),which is a severe pest causing tremendous economic losses to brassica crop production.As one of the effective biological control agent of P.xylostella,O.sokolowskii has great potential in biological control of P.xylostella.In this study,we first determined a time dependent standard for a successful parasitization of O.sokolowskii,and then examined the effects of host size,female wasp body size,and oviposition sequence on the oviposition tactics of O.sokolowskii,and investigated the oviposition strategy of O.sokolowskii when parasitized on a parasitized host,and finally evaluated the effect of superparasitism on the fitness of O.sokolowskii.Major results and conclusions obtained from this study are summarized below.1.Parasitizing behaviors of O.sokolowskii.To obtain a time dependent standard for a successful parasitization of O.sokolowskii,one ovipositor stinging of O.sokolowskii was artificially interrupted at intervals of 30 seconds apart in time,obtaining different treatment levels in oviposition duration:30,60,90,120 seconds,and uninterrupted one(>120 s).Successful parasitized host number,brood size,and sex ratio in each treatment were recorded.The results showed that>120 s treatment yielded 97.5%successful parasitization,and 30 and 60 s treatments got less than 50%parasitization success,and in 90 and 120 s treatments the parasitization success ranged from 50%to 60%.So,an uninterrupted oviposition bout with the ovipositor stinging for>120 s was employed as a standard for successful parasitization.Brood size and sex ratio of O.sokolowskii also had some difference among treatments.The brood size in 30 s treatment was 5.7 in average and was significantly smaller than that in 120 and>120 s treatments.The brood size among 60,90,120 and>120 s treatments had no significant difference.All female broods,of which size is less than 5,were observed in 30,60,90,and 120 s treatments.Based on the results we hypothesize that O.sokolowskii may lay the egg cluth in a short time;the sex allocation sequence of O.sokolowskii in a bout of oviposition is that O.sokolowskii lays some fertilized eggs(female)firstly,and lays 1-2 unfertilized eggs(male)after that,and lays some fertilized eggs in the end.2.Effects of host size,female wasp size,and oviposition sequence on oviposition strategy of O.sokolowskii in single parasitism.To investigate whether O.sokolowskii could adjust oviposition strategy based on different host size and wasp body size or not,we used O.sokolowskii,of which the hind tibia length ranged from 0.32 mm?0.64 mm,sequentially parasitized on three host larvae,of which the weight ranged from 0.21 mg?6.91 mg,in the same stadium.Survival of parasitized hosts,brood size,sex ratio,and the development time of O.sokolowskii were recorded.The results showed that the survival of parasitized hosts did not vary with host size,wasp body size,and oviposition sequence.The survival of parasitized hosts ranged from 70.9%to 72.2%and 67.1%to 85.1%with different host size and wasp body size,respectively.The average brood size was 9.1 and was not influenced by host size and wasp body size.Oviposition sequence had a marginally significant effect on the brood size,which linearly decreased by 11%with an increase of oviposition bout.The sex ratio of O.sokolowskii was stable and did not vary with host size,wasp body size,and oviposition sequence.The sex ratio of O.sokolowskii was 13.1%,12.7%,and 15.5%in three oviposition bouts,respectively.The number of males per brood changed as a step function of brood size,which split at the threshold of 12 with a significant difference between the two split groups,maintaining 1 below the threshold but increasing exponentially when brood size ranged from 13 to 21.The development time of O.sokolowskii was negatively correlated with host size and brood size,and was not influenced by wasp body size and oviposition sequence.The development time of O.sokolowskii was 15.8 d,15.9 d,and 15.9 d in three oviposition bouts,respectively.Our results suggest that "host-size quality" hypothesis is not suitable to suggest the oviposition strategy of O.sokolowskii,and our results support Waage's hypothesis for koinobiont parasitoids which proposed that host size at oviposition time was not a good predictor of the amount of resources available to support the offspring development.3.Discrimination of O.sokolowskii between healthy and parasitized hosts.A series of dual-choice experiments were performed to examine preference of O.sokolowskii between healthy and parasitized host larvae,which were attacked previously 0,24,or 48 hours before.The results showed that O.sokolowskii preferred healthy hosts by 68.8%over ones parasitized 0 h before.O.sokolowskii showed no preference between healthy and parasitized hosts,which were parasitized 24 h or 48 h before.The brood size and sex ratio were 13.4 and 22.7%respectively,from superparasitized hosts that were previously parasitized 0 hour before,higher than those parasitized 48 hour before.The results suggest that O.sokolowskii may have the ability to discriminate between healthy and parasitized hosts,and the ability is dependent on the time that the host has previously been parasitized;time interval between two parasitizations in superparasitism may affect the survival of the offspring in the superparasitized host and sequentially affect sex ratio.4.Oomyzus sokolowskii superparasitization strategy.To investigate oviposition strategy of O.sokolowskii in response to a parasitized host,two methods were used to obtain parasitized hosts,which did not produce viable offspring.We either used irradiated wasps,with 60Co by 40 Gy at the dose rate of 2 Gy/min to sterilize them(offspring couldn't survive),or constrained wasps to sting hosts for less than 30 s(without laying eggs),to obtain parasitized hosts,which were then subjected to be superparasitized by a healthy wasp.Offspring emerged from the superparasitized host was recorded.Single parasitism was used as the control.The results from the experiment with the irradiation treatment showed that(1)the average brood size was 7.1 significantly lower than that in the control(9.6)(single-parasitized hosts);(2)both sex ratio(48.5%)and development time(16.3)significantly differed from the control(12.3%and14.1 d);(3)both female and male offspring size from superparasitized hosts were significantly smaller than those from the control.The results from the stinging-time limitation trial showed that the brood size(9.5)did not significantly differ from that in the control,and so as to sex ratio(18.3%)which although was larger than that in the control.The development time was significantly prolonged compared to the control,while offspring adult size was similar with that in the control.The results suggest that O.sokolowskii decreases brood size but increases sex ratio at superparasitism,lending support to the "local mate competition" hypothesis,and suggest that female O.sokolowskii may have the ability to discriminate between hosts that have been successfully parasitized or not.5.Comparison in developmental parameters between different types of superparasitism.To examine offspring developmental performances from different types of superparasitism,we compared self-superparasitism with conspecific superparasitism,which was sub-divided into that by either naive or experienced wasps.The results showed that the brood size in self-superparasitism was 13.4,same as that in conspecific superparasitism by naive wasps(14.8).Sex ratio in self-superparasitism(17.6%)was significantly smaller than that in conspecific superparasitism by naive wasps(23.3%).Both the brood size(12.2)and sex ratio(14.6%)from conspecific superparasitism by experienced wasps were significantly different from those by naive wasps.The development time for self-,and conspecific-superparasitism by naive and experienced wasps was 14.8 d,14.5 d,and 14.6 d,respectively.The results suggest that O.sokolowskii may decrease sex ratio in self-superparasitism compared to conspecific superparasitism,and female wasp with the experience of parasitism may lay fewer eggs compared to those without the experience when parasitized on a parasitized host.6.Body size and fecundity of O.sokolowskii offspring from superparasitized hosts.To investigate the effect of superparasitism on the fitness of O.sokolowskii,we observed male and female size(hind tibia length)and fecundity of O.sokolowskii emerged from superparasitized hosts.The results showed that both female and male body sizes emerged from superparasitized hosts were significantly smaller than those from single parasitized hosts,with female and male size decreasing in body size by 0.1 mm and 0.05 mm,respectively.The fecundity of O.sokolowskii from superparasitized hosts was 51,significantly less than that from single parasitized hosts(93.2).The offspring sex ratio and development time of O.sokolowskii did not differ between superparasitized and single parasitized hosts.Our results suggest that superparasitism may decrease the fitness of O.sokolowskii. |
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