ONG Su Ping,袁瑞玲,陈鹏

(1.云南省林业和草原科学院,云南 昆明 650201;2.马来西亚林业研究所, 吉隆坡 雪兰莪州，52109)

The Oriental fruit flyBactroceradorsalis(Hendel)(Diptera:Tephritidae)is a pest of quarantine concern that has established and become widespread in many tropical and subtropical countries in the Asian and African continents including Hawaii and some Pacific Islands[1].B.dorsalisis known to infest over 300 species of fruits and vegetables whereby the adult females puncture the fruit skin to insert their eggs inside the ripe fruits.The hatched larvae then feed on the fruit pulp causing the fruit to rot and damage[2].In India,B.dorsaliscaused considerable yield losses up to 80% in mango and guava orchards[3]while in China,damage rate on fruits such as guava can reach 100% during their peak breeding season[4].Since the 1980s,B.dorsalishas colonized most of the southern provinces in mainland China and gradually expanding its range northwards into the cooler provinces due to increase trade and global warming[2,5-6].Its widespread dispersal is also aided by its ability to fly long distances[7-8],which is associated with its strong flight muscles[9].

In China and elsewhere,management strategies rely heavily on chemical control besides other prevention and control measures which include fruit bagging,quarantine for exported fresh agricultural commodities,methyl eugenol-baited traps and sterile insect technique.SinceB.dorsalisdeveloped insecticide resistance[10],exploration of biological control agents becomes important as the alternative,long-term solution to theB.dorsalisproblem.For example,the introduction and establishment of the egg-pupal parasitoid,Fopiusarisanusto controlB.dorsalisin Hawaii was a remarkable success,which serves as a biocontrol model to other countries[11].In China,techniques to improve mass rearing and parasitism efficiency of parasitoids such asF.arisanusandDiachasmimorphalongicaudataonB.dorsalisegg,larva and pupa are continuously developed[12-15],so as to increase the quality and performance of the parasitoids in the laboratory and field.

While these species of braconid wasps are well-known endoparasitoids ofB.dorsalisin China,the lesser-known parasitoid e.g.Spalangiaendius,which is mainly used as a biological control agent of the house flyMuscadomestica[16],has also been studied for its suitability to controlB.dorsalisin Thailand[17]and China[18].Natural enemies’ diversity can strengthen pest control when different species of natural enemies complement each other by occurring at different niches and times[19].Therefore,the knowledge on other potential parasitoids ofB.dorsaliscan potentially help increase the efficiency of its biological control agents.

In this experiment,we tested the potential ofSclerodermusharmandi,an ectoparasitoid of the cerambycid larvae,to parasitize and useB.dorsalisas a host.AlthoughSclerodermusparasitoid is not known to attack dipterans,it has been reared on non-wood boring coleopteran,lepidopteran and hymenopteran larvae,pupae and adults[12,20-21].

1 Materials and methods

1.1 Testing Insects

Sclerodermusharmandiwasps were obtained from the rearing facility at the Forest Protection Laboratory in Yunnan Academy of Forestry and Grassland.The wasps were divided into two groups:one group without feeding,other group was fed with 30% honey-soaked cotton wool placed at the top of the glass vials prior to the experiment.

TheBactroceradorsalistest insects were obtained from laboratory-reared colonies at the Forest Protection Laboratory in Yunnan Academy of Forestry and Grassland.The colonies have been maintained on an artificial diet[9](see Chen et al.2015)for at least 20 generations.The larvae ofB.dorsaliswere reared in rectangular plastic containers(17 cm length×12 cm width×7 cm height)and kept in a controlled environment at 28-30℃,40% relative humidity and a photoperiod of 16︰8(L︰D)h.In consideration of the sufficient nutrients needed by parasitic wasps for reproduction,we selected the 3rd instar larvae and 2-day-old pupae as host materials for this experiment.

1.2 Suitability of B.dorsalis as a host of S.harmandi wasp

In this no-choice test,three female wasps from two groups(1 week old and over 6-month-old),maintained on without honey solution and honey solution for 1-,3-,5-,7-,9- and 11 days respectively,were introduced into each glass vial(4.8 cm height×0.9 cm width,flat bottom)with five 3rd instarB.dorsalislarvae.The adult wasps were removed from the vials once oviposition occurred to reserve the host resources for the hatched wasps’ larvae.Ten replicates were set up and each replicate was paired with a negative control,which contained onlyB.dorsalislarvae.We also observed the general behaviour of the wasps as they were introduced into the test arenas for about 10 minutes.

2 Results and Analyses

The experiment showed that only wasps fed with 30% diluted honey was observed to lay eggs on the larva ofB.dorsalis(Table1).Feeding wasps with diluted honey for several days prior to experiment may help to increase fitness and egg production.Our observations showed that the wasps did not attack the pupa ofB.dorsaliswhile attack and oviposition were observed on 10-12 days old larvae.Young larvae such as 1stinstar and 2rd instar larvea,could possibly provide lower nutrients and resources to the wasps.

Tab.1 Oviposition by S.harmandi wasps on B.dorsalis larvae/pupa in the no-choice tests

We observed that the wasps started attackingB.dorsalislarvae upon introduction into the test arenas by jabbing the larvae repetitively with their ovipositor for about 2-3 minutes until the larvae were paralyzed.The wasp begun to examine and probe the paralysed host with its antennae and occasionally resting over the host’s body.Host feeding by the wasps occurred for about 5 minutes by biting and lapping on the body tissue of the larva.This feeding activity continued for 3-4 days.Oviposition occurred on day 4-9 after introduction of the wasps into the test arenas(Tab.1,Fig.1).The eggs hatched 2-7 days later,however the wasps’ larvae were short-lived and only survived between 2-8 days.In the negative controls,emergence ofB.dorsalisadults was 70-100%(Tab.1).

The experiment indicated that the pupae ofB.dorsaliswere not attacked by the young(1week)and aged wasps(over 6 months),suggesting wasps were not interested inB.dorsalispupae as the host(Tab.1).

Fig.1 Testing of Bactrocera dorsalis as a potential host for Sclerodermus harmandi

3 Discussion and Conclusion

In this study,we found that honey water satiation of wasp is important forS.harmandito parasitize.Both number of eggs laid onB.dorsalislarvae and number of eggs hatched increased with the increased in the number of days adult wasps maintained on honey solution.Feeding wasps with diluted honey for several days prior to experiment may help to increase fitness and egg production.The female ofS.harmandiparalyses its host by stinging and injecting venom through its ovipositor and acquires the host’s nutrient for egg production[21,22].Despite being able to oviposit on its new host,the eggs and larvae ofS.harmandicould not complete development onB.dorsalislarvae.The small-sizedB.dorsalislarvae could have provided lower nutrients and resources to the wasps as the wasps are mainly reared on Coleopteran larval hosts[21-22]with large body sizes.Reducing the number of wasps in the test arenas could possibly increase host survival by limiting the frequency of venom injection and host feeding.

In parasitoid rearing experiments,parasitoids such asHabrobraconhebetor[23-24]andSclerodermuspupariae[25]that were supplemented with diluted honey were found to have longer lifespan and higher fecundity.This could possibly explain the observations in our experiment in which wasps maintained for 11 days on 30% diluted honey laid the highest number of eggs.

In this study,we also found that mortalities increased with the increased in the number ofB.dorsalislarvae and parasitoids.This could be due to repeated venom injection and host feeding by multiple wasps.In the testing,the survival rates for the host and wasp were less than 10% and 20%,respectively.Most of the larvae in negative control replicates had successfully pupated and emerged as adults under the same rearing conditions.Nevertheless,mortality ofB.dorsalislarvae or pupation are likely to occur when they are kept in the experimental vials without any food provided.

AlthoughS.harmandiwasps were able to parasitize and lay eggs on the larvae ofB.dorsalis,the wasps’ larvae that hatched from the eggs could not develop successfully.Therefore,B.dorsalisis not a suitable host for the development of the betylid wasp.