Mukti N. Ghimire and Thomas W. Phillips

1. Mass Rearing and Augmentative Releases of Bracon hebetor to Suppress Indianmeal Moth, Plodia interpunctella Populations in Stored Wheat Mukti N. Ghimire and Thomas W. Phillips Fig. 1 Department of Entomology and Plant Pathology, Oklahoma State University, Stillwater, OK 74078 USA Introduction Bracon hebetor (Say) (Hymenoptera: Braconidae) is a synovegenic, gregarious, ecto-parasitoid that attacks larvae of several species of Lepidoptera, mainly Pyralid moths infesting stored products including the Indianmeal moth, Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae) (Fig. 1), a destructive pest of stored commodities. This parasitoid is considered as one of the potential biological control agents of stored product moths (Brower et al. 1996). Bracon hebetor females first paralyze their host larvae by stinging (Fig. 2) and then laying variable numbers of eggs singly on or near the surface of paralyzed hosts (Fig. 3) (Antolin et al. 1995). The paralyzed host larvae are then used as food sources for developing wasp (Fig. 4, 5) and also for the adult females. In this study, we explore the potential of this parasitoid for the management of Indianmeal moth in a series of laboratory and field experiments. In a laboratory study, we investigated whether the parasitoid and host density affect the sex ratio and adult progeny production of B. hebetor. In a field study, we evaluated whether the B. hebetor releases affect the suppression of Indianmeal moth populations in stored wheat. Results In the B. hebetor density experiment, the total number of parasitoid progeny produced from 50 IMM larvae differed significantly in response to B. hebetor release density (df = 3, 39, F = 7.83, P = 0.0003). The highest number B. hebetor adults was produced from a density of eight pairs of B. hebetor followed by and four, two and, one pairs (Fig. 11). The B. hebetor progeny produced in this experiment were slightly female biased and this proportions did not differ significantly with the parasitoid release density (df = 3, 39, F = 2.02, P = 0.127) (Fig. 12). In the IMM host density experiment, the total number of B. hebetor adults produced in response to different host densities was found significant (df = 4, 42, F = 28.15, P = 0.0001), whereas sex ratio of the parasitoid did not differ with the host density (df = 4, 42, F = 0.41, P = 0.797) (Fig. 13 and 14). In the augmentative release experiment, the numbers of Indianmeal moth adults and larvae did not differ significantly (df = 3, 8, F = 1.41, P = 0.309, and df = 3, 8, F = 0.62, P = 0.623 for IMM adults and larvae population, respectively) among the treatments (Table 1). However, the number of B. hebetor adults on sticky traps did differ significantly among the treatments (df = 3, 8, F = 99.8, P = 0.0001). Bracon hebetor numbers were highest in the early released bins (Table 1). Fig. 8 Material and Methods Parasitoid Origin and Rearing The parsitoid, B. hebetor studied in this experiment was collected from grain bins at the Stored Product Research Laboratory and Education Center (SPREC), in Stillwater, Oklahoma in November 2003 and was associated with Indianmeal moth (IMM). The parasitoid was cultured and mass reared on last-instar larvae of IMM in the laboratory at 29  1C, 65  5% RH, and a photo-period of 14:10 h (L:D). Last-instar larvae of P. intepunctella were obtained from a laboratory culture that was reared on a standardize diet of corn meal, chick laying mash, chick starter mash, and glycerol at a volumetric ratio of 4:2:2:1, respectively at a temperature of 28  1C, 65  5% RH, and a photo-period of 16:8 h (L:D). Parasitoid and host density experiment Plastic yogurt cups (Fig. 6) were used as experimental arena and were fitted into glass jars (Fig. 7) for easy handling and better aeration through the metal screen. In both experiments, B. hebetor within 48 hr of emergence were released into each experimental arena and allowed to sting and oviposit for five days. In the parasitoid density experiment, 50 last-instar IMM larvae were placed in an arena and four different densities (one, two, four and eight pairs) of B. hebetor were released. In the host density experiment, two pairs B. hebetor were introduced in to five different densities of host (10, 20, 30, 40, and 50 larvae per cup). Arenas were kept in a growth chamber and sex ratio and number of adult parasitoid progeny were recorded. Both experiments were conducted at the same conditions used for rearing. Augmentative Release Experiment Twelve 500-bu bins (Fig. 8) were used at the SPREC facility in Stillwater, Oklahoma and filled with  125 bu hard red winter wheat in May 2005. On June 14th,  1000 P. interpunctella 1st instar larvae along with 250 g of infested wheat media were introduced over the surface of wheat in each bin and a subsequent introduction was made after three weeks. 100 pairs of B. hebetor within 72 hr of emergence were released into the bins at three different time intervals, 22nd June, 20th July, and 17th August, as early, mid, and late season releases, respectively. A subsequent release of B. hebetor was made 15 days after the each initial release. Two weeks after the 1st introduction of the parasitoid, host and parasitoid populations were monitored at 15-day intervals using un-baited sticky traps (Fig. 9) and cardboard rolls (Fig. 10). The parasitoid-free bins served as control and the experiment was replicated three times. Table 1.Total number (mean ± SE) adults and larvae of Indianmeal moth (IMM) and B. hebetor adults in B. hebetor released and control bins Discussion and Conclusions Although observations in the augmentative release experiment are still being recorded, the results as of now show that B. hebetor females were unable to suppress the IMM population in the early release bins. There is a possible explanation behind this. First, the IMM population did not establish well enough, even in control bins, therefore we made the 2nd subsequent release of IMM larvae three weeks after the 1st introduction of IMM. So we suspect B. hebetor females probably were deprived of hosts in these early release bins. Second, although we placed screens on fans and openings of bin lids, parasitoids may have escaped with air flow while running the aeration fan in order to decrease the grain temperatures during summer time. Third, we observed high density of beetle populations; these may have hindered the population of IMM and subsequently affected the B. hebetor activities. In the B. hebetor density experiment, a density of eight pairs produced a higher number of progeny (188 adults) than did density of one and two females. Sex ratio was female biased and density with eight, one, and two pairs of B. hebetor produced significantly higher proportion of females than four pairs. In the host density experiment 50 IMM hosts produced a significantly higher number of parasitoid progeny (160 adults) among the tested host densities. The host density with 30 and 40 IMM larvae produced almost equal numbers of parasitoid progeny. Similarly, the number of parasitoid progeny did not differ significantly between host density of 10 and 20 larvae. Fig. 2 Fig. 3 Fig. 4 Fig.5 Literature cited Brower, J. H, L. Smith, P. V. Vail, and P.W. Flinn. 1996. Biological control, pp. 223-286. In B. Subramanyam and D.W. Hagstrum (eds.), Integrated Management of Insects in Stored Products. Dekker, New York, USA. Antolin, M. F., P. J. Ode, and M. R. Strand. 1995. Variable sex ratio and ovicide in an outbreeding parasitic wasp. Anim. Behav., 17: 1-7. Fig. 7 Fig. 6 Fig. 9 Fig. 10