Effects of Combining Methoprene and Aeration Against Stored Grain Insects

1. Rhyzopertha dominica Lesser grain borer, LGB Tribolium castaneum Red flour beetle, RFB Plodia interpunctella Indianmeal moth, IMM Effects of Combining Methoprene and Aeration Against Stored Grain Insects Siwei Liu1, Thomas W. Phillips1, Franklin H. Arthur2 and Doug Vangundy3 (1)Oklahoma State University, Stillwater, OK (2) USDA ARS, Manhattan, KS (3)Wellmark International 12200 Oenton Dr. Dallas, TX Introduction Over the past decade IPM has been implemented for stored grain. And the uses of reduced-risk insecticides and non-chemical controls have increased. Insect growth regulators such as JH analogues have very low toxicity for humans, but affect or disrupt the normal growth and development of insects by interfering with development. The first objective of our study was to compare and contrast methoprene, Diacon II, used as a top- dressing alone, as a complete grain mass treatment alone, or complete treatment combined with grain aeration to prevent insect infestation of wheat stored in field. The second objective was to determine the stability of Diacon IIresidues on stored wheat and to verify its activity against insects by laboratory bioassays. Results and Conclusions Tab. 3. P. interpunctella emerge to normal adults at different storage times Tab. 1. Mean adults (SE) captured during 7 days in probe trap in wheat bins treated with methoprene and aeration at six times during storage (N=4 ) Treatments. A: aeration, T: to-dress methoprene. M: methoprene total, MA: methoprene total + aeration. Tab. 4. The mean (SE) F1 adults per 50 parent adults produced in bioassasy of grain removed from experimental bins at different storage times (N = 4) Materials and Methods Insects: Cryptolestes ferrugineus Rusty grain beetle, RGB Sitophilus oryzae Rice weevil, RW Treatments. A: aeration, T: to-dress methoprene. M: methoprene total, MA: methoprene total + aeration. Bioassays of grain samples taken soon after the grain was treated, and at 5 and 10 months of storage, showed that any grain treated with methoprene was very active against species such as IMM (Tab. 3) and RFB (Tab. 4), for which all larval stages are externally feeding. Methoprene was also active against LGB, normally considered an internal feeder, but whose first instar larvae feed freely in the grain before going inside kernels (Tab. 4). However, RW, whose larvae are entirely internal feeders, generally escaped effects of methoprene. Chemical residue analysis showed good coverage in our treated bins and very little degradation over 10 months of storage. Diacon II is relatively expensive compared to other insecticides, so a low dose is necessary. Top dressing is a good method to apply. Diacon II does not control all pests. Therefore, it applied in combination with aeration is a good idea, which can enhance population suppression of internal feeders in stored grain. Means in the same column with the same capital letters are not significantly different, and mean in the same row with the same small letters are not significantly different. LSD procedure at P<0.05. Field study:Sixteen 500-bu bins (Fig. 1) filled with new hard red winter wheat were used at the Oklahoma State University Stored Product Research and Education Facility. The treatments were aeration only as a control, treatment of the top 50 cm methoprene (top-dressing), complete grain mass treatment of methoprene and a combination of aeration and methoprene. Methoprene was applied at the label rate of 1 ppm in all cases. After all bins were treated, 100 adults each of RFB, LGB, and RGB were added weekly into each bin for four weeks. The OPI system for temperature monitoring and aeration-control was used to turn the fans on or off to cool grain in designated bins using ambient cool air (night time). Fans were programmed to turn on when air temperature dropped below grain temperature.  Probe trap (Fig. 2) and Grain trier (Fig. 3) samples were taken monthly. Tab. 2. Mean (SE) adult density ( per Kg) from grain trier samples taken in wheat bins treated with methoprene and aeration at six times during storage (N=4) Fig. 4. Mean temperature °C each week from aeration and non- aeration bins Fig. 1. 500-bu bins Fig. 2. Probe trap Fig. 3. Grain trier Summer aeration was able to cool grain in this study 10-15 °C below that of non-aerated grain (Fig. 4). A cost-benefit analysis of the use of aeration fans in this study showed our fans ran for an average 893.8 hours in 10 months. It cost $ 0.0026 per kg grain, or about $0.07 per bu. Aeration is expensive and it is very difficult to get the grain below 20 °C during summer in Oklahoma. Aeration may be more cost-effective during the fall and winter time. Most insect pest species are very sensitive to temperature below 20 °C and the moisture contents below 12-13 %. In Oklahoma it is possible and worthwhile to store grain at temperatures close to 5 °C in the winter season following fall aeration. Laboratory bioassay: Bioassay samples of grain from experimental bins were taken immediately after treatment and 5 and 10 months following treatment. Twenty-five 0-1 day old eggs of IMM were placed in 120 ml jars with 40 g wheat (of which 20 g were crushed) and held a in growth chamber. Fifty RFB, LGB, or RW adults were placed in 240 ml jars by species with 100 g wheat (5 g crushed for RFB), and placed in a growth chamber (Fig. 4.). IMM egg-hatch was checked after 1 week and count normal adult development was made after 6 – 8 weeks. Parent beetles were removed after 1 week and F1 progeny were counted after 6 weeks. Grading and methoprene residue analysis were carried out at both the beginning and the end of this study. Means in the same column with the same capital letters are not significantly different, and mean in the same row with the same small letters are not significantly different. LSD procedure at P<0.05. Insect numbers in grain bins varied significantly by treatment and over time as indicated by probe trap and grain trier samples (Tab. 1 and 2). Diacon II at 1 ppm resulted in reduced numbers of insects of all three species studied, and the methoprene treatment to the total grain mass seemed most effective, whether with or without aeration (Tab. 1 and 2). Analysis of grain damage, as measured by the mean number of insect-damaged kernels per 100 g, at the end of the experiment was 9.0, 4.3, 3.3 and 2.0 for bins with aeration control, top-dressing, total methoprene, and methoprene combined with aeration, respectively. Acknowledgments USDA-CSREES PMAP and RAMP programs for partially funding this work. Mr. Edmond Bonjour provided support.