ASSESSING MANAGEMENT STRATEGIES TO CONTROL MAJOR INSECT PESTS OF STORED SORGHUM GRAIN

Occurrence and diversity of insect pests of stored sorghum, Sorghum bicolor (L.) Moench, were monitored in Niger. Efficacy of botanicals was evaluated to control red flour beetle, Tribolium castaneum Herbst, in Niger and maize weevil, Sitophilus zeamais Motschulsky, in Texas. Hermetic storage methods to control insect pests were assessed in Niger. Cost-benefit analysis determined measures of project worth, Net Present Value and Benefit-Cost Ratio, for benefit from investing in hermetic storage methods.
Monitoring by sieving 200 g of infested grain found means of 15 to 17 red flour beetles in three improved sorghum varieties. Three traps detected as many as 10 insect pests in storage facilities in Niger. A water bottle trap caught 60% of the storage species trapped by a ‘Dome’ Pheromone trap and 85% of the species in the sticky glue trap. Four botanicals were effective in controlling red flour beetles in sorghum grain in Niger. Means of 1.5-2.0 adults died with 0.0125 and 0.025 g of kernels of neem, Azadirachta indica A. Juss. African locust beans, Parkia biglobosa Jacq., hibiscus, Hibiscus sabdariffa L., and baobab pulp, Adansonia digitata L. were less effective after 7 days. The cumulative number of beetles dead was 3.2 at 35 days after infestation with 0.05 g of neem. Damage scores were low (1.3-2.6 on a 1-5 scale) for sorghum treated with ≥0.1 g of botanical powder compared to a 3.9 score for the check (not treated). With three botanicals assessed, less than 0.1 g of milkweed, Asclepias speciosa J. Torrey, killed 1.3-1.8 maize weevils by Day 2, while mesquite, Prosopis glandulosa J. Torrey, killed 1.5 adult by Day 4. Few adults (≤0.8) were killed by neem bark. Two to 2.3 weevils were killed by 0.1 and 0.2 g of neem and 2.0 were killed by 0.2 g of milkweed by Day 6. Neem bark acted slowly for 14 days and killed 2.0 maize weevil adults when grain was treated with 0.05 g of powder.
The cumulative percentage (94 and 91) of dead red flour beetles was greatest with 0.2 g of milkweed or mesquite. With 0.1 g, mesquite killed 87.5% compared to milkweed (78%) and or bark (65.6%).
Percent mortality of beetles (78%) was greatest on sorghum grain treated with 0.2 g of neem compared to 0.05 and 0.1 g that killed 62.5 and 66%. Percent corrected mortality was 75-93% when maize weevils were fed sorghum treated with milkweed or mesquite at >0.1 g. Percent corrected mortality was 86% with 0.1 g of mesquite. Neem at 0.2 g lessened the grain damage score by weevils to 25.5%. Any botanical at any dose reduced damage to less than the mean of 3.5 for the check.
Correlation coefficients showed that an increase in percentage of adult mortality was followed by an increase in percent corrected mortality. When percent mortality or corrected mortality increased, percentages of grain damage and weight loss and the damage score decreased. Percentage of grain damage was very correlated with weight loss and damage score. Regression (Probit) analysis of the cumulative mean number of dead maize weevils was used to calculate LD50 and LD90 lethal doses of 0.02 and 0.2 g of mesquite or milkweed were required to kill 50 and 90% of maize weevils. More neem was required for lethal doses of 0.26 and 1.27 g to kill 90 and 99% of maize weevils.
Hermetic storage methods assessed to control insect pests of sorghum grain showed that triple and double bagging killed 7.6 and 8.0 red flour beetles by the 7th day. Means were greater than the 1.2 red flour beetle killed when a polypropylene bag was used as a check and greater than the overall mean of 5.2. Means of 10.6, 9.0, and 8.2 storage insects were killed when a double bag, triple bag, and clear plastic bucket, respectively, were used for 7 days. Means were greater than overall means of 6.8 (Day 7) and 8.1 (Day 14) rice moths killed. At 28 and 35 days, few (2.2 and 0.4) moths were dead in the polypropylene check, but 2.5 moths were dead in the double bag at 35 days. Fewer red flour beetles than rice moths survived in hermetic storage. With polypropylene and polyethylene bags, 8.0 and 25.2 red flour beetles and 21.6 and 33.2 rice moths were alive in sorghum stored 35 days. Overall, double bagging lessened survival of both storage insect species, with 3.4 red flour beetles and 9.6 rice moths alive. Mean weight losses were 8.5 and 3.2% when polypropylene and polyethylene bags were used, compared with 1.4% with other storage methods.
Cost-benefit analysis assessing project worth by Net Present Value and Benefit-Cost Ratio showed that for any hermetic storage method, percentage weight loss increased during time, especially after 6 months. Simple linear regression analysis showed that for all alternative hermetic storage methods, coefficients were less than for the polypropylene bag (check). Storage costs per ton were $15.00 for a polyethylene bag, $20.00 for a double bag, and $34.60 for a triple bag for 1 year. Benefit gained from any hermetic storage method increased linearly during time. At a discount rate of 4.5% and 10-year investment period, Net Present Value was positive (Year 1-10) for all hermetic storage methods. Overall, greater Net Present Value was calculated for triple bagging ($4,344.81), followed by double bagging ($4,343.22). The Net Present Value for a polyethylene bag ($4,186.55) was $158.26 less than for a triple bag. Benefit-Cost Ratio for the three hermetic storage methods differed slightly, with 1.19 for polyethylene and 1.20 for double and triple bagging of 1 ton of sorghum stored 1 year. Sensitivity analysis using discount rates of 4.5, 6.5, or 8.5% with varying 2-year increment period during 10 years of investment found hermetic storage methods had Benefit-Cost Ratio >1, indicating the alternatives were good investments. For double and triple bagging, Benefit-Cost Ratio ≥1.20 was calculated with all discounting rates at Year 8-10, but less with the polyethylene bag. Hermetic storage methods (triple and double bags) can be used as alternatives to traditional polypropylene bags to preserve more sorghum grain of greater quality. Monitoring storage insect pests was effective and key to deciding when to apply integrated pest management strategies. Botanicals and hermetic storage methods are alternatives to insecticides and polypropylene bags, and can be used to safeguard large quantities of sorghum to retain grain quality and generate more revenue.