# Evaluating the Long-Term Efficacy of Four Active Ingredients Against Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) and Sitophilus oryzae (L.) (Coleoptera: Curculionidae) on Stored Sorghum in the United States

## TL;DR

This study tested four insecticides on sorghum to see how well they protect against two common pests over 28 weeks.

## Contribution

The study provides new insights into the long-term efficacy of grain protectants against Rhyzopertha dominica and Sitophilus oryzae under controlled storage conditions.

## Key findings

- Spinosad was most effective, causing 100% mortality in Rhyzopertha dominica and 30–39% in Sitophilus oryzae.
- Rhyzopertha dominica was more susceptible to all insecticides compared to Sitophilus oryzae.
- Kernel moisture content declined over time, reducing progeny in both pest species.

## Abstract

Stored product pests such as the lesser grain borer and the rice weevil cause significant damage to grain products during post-harvest storage. In the United States, grain protectants are commonly applied to grain prior to storage to prevent infestation, but their long-term performance depends on environmental conditions inside the storage structure and the target pest. In this study, we evaluated the residual effectiveness of four commercially available grain protectants applied to sorghum stored for 28 weeks in a grain bin. We evaluated adult mortality, progeny production, frass weight, and the percentage of insect-damaged kernels every four weeks. The lesser grain borer adults were more susceptible to each treatment compared to rice weevils, and the insecticide that contained the active ingredient spinosad was the most effective against both species. In addition, the overall moisture content of the sorghum declined over the course of experiment, resulting in the reduction in progeny of both species in both the control and insecticide-treated sorghum. Overall, these findings highlight the importance of kernel moisture content and the varying efficacy of grain protectants in reducing pest damage from different species of stored product pests.

Grain protectants are insecticide formulations applied directly to raw grain to prevent and control insect infestations in bulk storage; however, their efficacy depends on insect species, commodity, and grain quality. The objective of this study was to determine the residual efficacy of four commercially available grain protectants, Gravista® (deltamethrin + methoprene + PBO), Diacon® IGR (methoprene), SensatTM (spinosad), and EverGreen® (pyrethrin), applied to sorghum and held for 28 weeks, against Rhyzopertha dominica (F.) and Sitophilus oryzae (L.). Subsamples were collected every four weeks and infested with 10 adults of either species. Adult mortality was assessed after seven days and progeny, frass, and insect-damaged kernels were evaluated after eight weeks. Rhyzopertha dominica was more susceptible to all insecticides compared to S. oryzae, and had fewer progeny compared to control sorghum. Spinosad-treated sorghum resulted in 100% adult mortality in R. dominica compared to 30–39% in S. oryzae. There was an overall decline in kernel moisture content, which may have impacted progeny of both species in the control and insecticide-treated sorghum. These findings highlight the influence of insecticide formulation, insect species, and grain moisture content on the long-term efficacy of grain protectants applied to sorghum.