نوع مقاله : مقالات پژوهشی
1 دانشگاه علوم کشاورزی و منابع طبیعی گرگان
2 علوم کشاورزی و منابع طبیعی گرگان
عنوان مقاله [English]
Introduction: Bracon hebetor Say is a well-known cosmopolitan ectoparasitoid that attacks larvae of the various lepidopteran pests, in both grain storage and field conditions. This parasitoid mass-reared on Anagasta kuehniella (Zeller) larvae in many insectaries of Iran and released annually into cotton, soybean, tomato and maize fields especially to control Helicoverpa armigera (Hübner) in augmentative biological control programs. Cold storage has been considered as an important part in augmentative biological control programs. This technique enables insectaries to store a sufficient number of biocontrol agents for a prolonged period, thus allowing the release of natural enemies concurrent with the critical stages of the pest and minimizing the cost of insect colony maintenance when they are not in demand. In an effort to improve the techniques used in the mass rearing and release of this parasitoid we assessed the storage feasibility of pupa and adult stages of this wasp at a low temperature and the effects of cold storage on its biological and reproductive parameters.
Materials and Methods: B. hebetor adults were initially obtained from a commercial insectarium in Gorgan, northern Iran and reared on Anagasta kuehniella (Zeller) larvae as factitious host for about five generations. Cold storage experiments were carried out on pupal and adult stages of parasitoid separately. Five hundred 5-days old pupae and five hundred couples of both sexes (female and male) of one-day-old adult parasitoids were collected from the colony and divided into ten groups of 50 parasitoids. Each group (considered as a replication) was released into plastic vials separately, fed adults for 24 hours with a 30% honey solution and then stored in a refrigerator (5±1˚C, and full darkness) for 7, 14, 21, 30, 45, and 60 days. Vials of non-refrigerated control groups were kept at 26±1˚C, 60±5% RH, and a photoperiod of 16L: 8D h. After each storage period, the pupa eclosion percent and adult mortality percent were subsequently assessed. To evaluate the effect of cold storage on longevity and fecundity of parasitoid, 30 female-male pairs were chosen randomly from the emerging adults (in pupae cold storage experiment) or surviving adults (in adults cold storage experiment), and each pair was placed in a plastic vial separately. Daily, the paired wasps in each container were provided 10 last instars A.kuehniella larvae on a piece of paper, as well as several drops of honey solution. The number of eggs laid was recorded on a daily basis until the female parasitoids died. To evaluate the effect of cold storage on parasitoid performance in the next (F1) generation, 50 newly laid eggs of parasitoid were removed from parasitized host larvae, placed in 10 cm diameter petri dishes individually (one egg/dish) and allowed to develop to adult stage. Developmental time (the period from egg to adult emergence) was recorded for male and female progeny separately and sex ratio (female percentage) of emerging adults was then determined. All experiments were carried out using a completely randomized design (CRD) and data were analyzed using one-way ANOVA and LSD test.
Results and Discussion: Eclosion rate of cold-exposed pupae decreased significantly with increasing cold storage duration. After 7 days of cold storage, about 93 percent of pupae were emerged, whereas all pupae died and no adult wasp was emerged, after 30 days of cold storage. However, cold storage of pupae even for a short duration (e.g. 7 days), had a significant adverse effects on longevity, fecundity and sex ratio of emerged adult parasitoids and means of these parameters were reduced by 55.8, 53.4 and 20%, respectively, compared to unstored control pupae. In adult cold storage experiment, survival rate of both male and female parasitoids was reduced significantly, according to cold duration. Adult mortality percent increased with storage duration and reached 100 and 97.14 % in 60 days storage of male and female parasitoids, respectively. One week storage at 5˚C resulted in 4.42 percent mortality in females which was not significantly different from that of the control. Cold storage duration had no adverse effect on the longevity of both male and female parasitoids. Reproductive parameters including mean of daily oviposition (eggs/day/female) and clutch size (eggs/larva) were also not significantly affected by cold storage durations. Low temperature storage of parental parasitoids had no significant effect on developmental time (the period from egg to adult emergence) and sex ratio in the F1 generation. Developmental time of progeny had a little variation among the treatments and changed from 11.9 to12.77 days in male and 12.07 to 12.8 days in female progeny.
Conclusion: Cold storage of pupae even for a short period, reduced significantly their eclosion rate and longevity and fecundity of adults emerged from these cold-exposed pupae. Cold storage of adult B. hebetor, affected negatively their survival, whereas other fitness traits such as longevity, fecundity, developmental time and sex ratio were not significantly reduced after cold storage. In conclusion, B. hebetor pupae is not recommended to be stored at low temperature, even for a short period but adult parasitoids can be cold stored for up to 7 days, with negligible mortality (4.41 %). The results of this study can be used in parasitoid mass rearing and cold storing in insectaries.
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