فهرست مطالب علی ایرجی

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.

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.

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.

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.

Conservation of the natural enemies is an important approach to develop a biological control program. Knowledge of the side-effects of insecticides on parasitoids and using reduced-risk chemicals is a principal tool for integrating biological and chemical methods to pest management in agricultural ecosystems. Because of the important role of Trichogramma brassicae in lepidopteran pests control in agricultural ecosystems of Iran, this study was carried out to investigate lethal and sublethal effects of three concentrations of two insecticides, thiodicarb and hexaflumuron on this parasitoid under laboratory conditions.
Thiodicarb was prepared at three concentrations of 250, 500 and 1000 ppm and hexaflumuron was prepared at concentrations of 250, 500 and 750 ppm, and their effects were assessed through three exposure methods, including contaminating adult’s food (honey solution 10%) with insecticides, adult’s residual contact, and dipping parasitized (in prepupa stage of parasitoid) and unparasitized host eggs into insecticide solutions. Lethal effects on adult and immature stages of parasitoid and some sublethal effects such as male and female longevity and mean of daily oviposition were assessed for each insecticide/concentration.
Thiodicarb at all three concentrations and in both oral and contact exposure method caused 100% mortality in both male and female adults. Dipping parasitized host eggs in 1000 ppm concentration of thiodicarb caused 87.3% mortality in wasp prepupa population. In contrast, 750 ppm hexaflumuron in oral and contact exposure methods caused 30.2 and 30.54% mortality in adult female population, respectively and 60.55% mortality in wasp prepupa. Dipping unparasitized host eggs in highest concentration of thiodicarb and hexaflumuron resulted in 55.8 and 43.6% reductions in parasitism rate, respectively. Dipping unparasitized host eggs into both insecticide solutions reduced their parasitism significantly. Hexaflumuron in both oral and contact bioassay tests reduced significantly male and female longevity and mean of oviposition, but there was no significant difference among three concentrations of this insecticide.
According to the "total effect" and IOBC categories of toxicity, thiodicarb was found to be a "harmful" insecticide in both oral and contact exposure methods. Therefore, its application is not recommended in fields. Lethal effect of hexaflumuron was significantly lower than thiodicarb, categorized as a "slightly harmful" and "moderately harmful" insecticide in oral and contact exposure methods, respectively. However, because of high total effect of hexaflumuron on T. brassicae, using other less-toxic insecticides is recommended in IMP programs of lepidopteran pests, especially cotton boll worm.