عنوان مقاله [English]
Introduction: Parasitoid wasp, Trichogramma brassicae (Bezdenko) (Hym.: Trichogrammatidae) has excellent position in mass production technology, scope of application, wide geographical distribution and unmatched talent to adapt to different climatic conditions that can be easily reared on Mediterranean flour moth (MFM), Ephestia kuehniella Zeller (Lep.: Pyralidae). The parasitoid use is in order to control Chilo suppressalis (Walker) (Lep.: Pyralidae), Helicoverpa armigera (Lep.: Noctuidae), Ostrinia nubilalis (Hübner) (Lep.: Pyralidae), Ectomyelois ceratoniae (Zeller) (Lep.: Phycitidae) and Cydia pomonella (Lep.: Tortricidae) (38). The studies have been shown that the quantity and quality of the host diet affect growth period, size of adults, adult longevity, fecundity and sex ratio of the parasitoid. The objective of current research was to evaluate the effect of four different MFM diets on the fitness of second and forth generations of the parasitoid wasp, T. brassicae.
Materials and Methods: The four diets ((I) wheat flour, wheat bran, corn flour, bread yeast and glycerin; (II) wheat flour, wheat bran, barley flour, bread yeast and glycerin; (III) barley flour, bread yeast and glycerin and (IV) wheat flour, barley flour and corn flour) were sterilized at 51 °C for 24 hours. After cooling at ambient temperature, the diets were contaminated with the eggs of MFM and then were maintained at 25±1° C, 60±5 % RH and a photoperiod of 14 L: 10 D. The MFM adults emerged after 35 to 40 days and their eggs were collected daily to use for investigation of the parasitoid biology. For rearing the parasitoid wasps on the MFM eggs obtained from each diet, some cards containing the parasitoid pre-pupae were put inside cages (25 × 25 × 25 cm) and reared for one generation. The adults were fed honey (20%) for one day and then were provided with one-day-old sterile MFM eggs. To prevent egg hatching and sterile them, they were kept at 15°C for four h. The tests were conducted with 10 replicates for each treatment at 25±1°C, 60±5 % RH and a photoperiod of 14 L: 10 D. Each glass tube (8cm diameter, 11 cm height) as an experimental unit was consisted of 40 and 20 wasps for the second and fourth generation of the parasitoid and was provided with the card carried 200 and 100 eggs of MFM, respectively. The number of emerged parasitoid wasps along with the number of parasitized eggs on each card w recorded daily till the death of the adult wasp. Also, the number of emerged wasps and females were counted.
Results and Discussion: The results showed that the lowest parasitism rate was occurred in the treatment I (9.8±0.011 %) for the second generation of the parasitoid. But, in the fourth generation, the treatment II and III presented the highest (11.05±1.23 %) and lowest parasitism rate (5.58±0.70%), respectively. In the previous report, the parasitism rate of T. brassicae fed on eggs of Sitotroga cerealella was assessed higher than that in the present study, which occurs probably because of the differences in insect host.
The results obtained from the second generation showed that the variation in the diets had significant effect on the percentage of parasitoid emergence and the highest percentage of the parasitoid emergence was observed in treatment I (64.5±4.78 %) but there was no significant difference between the treatment I and III. The percentage of adult emergence of Trichogramma pretiosum Riley (28) and Trichogramma maidis Pint, et Voeg. (13) reared on the MFM eggs were higher than the present research. The difference may be related to the parasitoid density and the wasp species. However, the variation in diets had no significant effect on the percentage of parasitoid emergence in fourth generation and sex ratio in both generations. In second generation, there were no significant differences among the treatments in daily oviposition of the female parasitoid but in fourth generation, various diets had significant effect. The highest and lowest daily oviposition were found for treatment II (0.77±0.07) and IV (0.42±0.06), respectively. The treatment III and II meaningfully caused the longest parasitoid lifetime in the second (12.31 ± 0.09 days) and forth generation (12.36±0.03 days), respectively. Nevertheless, no significant variation in lifetime of the parasitoid between the other treatments was found in the second and forth generations, separately. Several biotic and abiotic factors can influence T. brassicae lifetime such as temperature, adult feeding, humidity, the quality of the host egg and the size of parasitoids. Also, the proportion of wasps surviving at time x (lx) were almost similar among all treatments in the second and forth generations, separately. The survival rate fluctuations of T. brassicae reared on eggs of Ch. suppressalis, were similar on different rice varieties and after eight days all adults were dead which was similar to our results.
Conclusion: By referring to the results, treatment II, included wheat flour, wheat bran, barley flour, bread yeast and glycerin (ratios: 41.5: 2: 43.5: 3: 10%, respectively) is proposed to optimize mass rearing of the parasitoid wasp, T. brassicae as an important biological control agent. The treatment II caused to increase the life span and parasitism rate of T. brassicae and is suitable for mass production of the parasitoid to promote the quality of biological control programs.