Efficacy of combinations of methoprene and deltamethrin as long-term commodity protectants

Authors: Arthur, Franklin

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2019
Publication Date: 2/5/2019
Citation: Arthur, F.H. 2019. Efficacy of combinations of methoprene and deltamethrin as long-term commodity protectants. Insects. 10(2):50. https://doi.org/10.3390/insects10020050.
DOI: https://doi.org/10.3390/insects10020050

Interpretive Summary: Stored raw grains can be infested by a variety of insect pests, depending on the specific commodity, and conditions may warrant the use of a protectant insecticide applied as grains are loaded into storage bins. In the US there is a limited number of registered protectants; one product is the trade name Centynal, which was a Suspension Concentrate (SC) of the pyrethroid deltamethrin, which can be combined with an insect growth regulator, Diacon IGR. However, this combination treatment has little effectiveness against weevils. We conducted studies with a new Emulsifiable Concentrate (EC) formulation of Centynal, which was easy to mix and formulate compared to the existing product. All combinations of the both the SC and EC formulations of Centynal combined with Diacon were effective for controlling the lesser grain borer on wheat and brown rice. However, the new EC deltamethrin formulation combined with Diacon was more effective for controlling weevils on wheat, rice, and corn compared to the existing SC formulation. The new EC formulation was also better than the SC for controlling the red flour beetle. The registrant for this product, Central Life Sciences, used the data from this product to justify replacing the SC with the EC formulation, and combining it with Diacon to create a new insecticide, Diacon IGR+. Grain storage managers can use this new insecticide to help control stored product insects and reduce economic losses associated with insect infestations.

Technical Abstract: Wheat, corn, and brown rice were treated with different binary combinations of a deltamethrin suspension concentrate (SC) formulation a new emulsifiable concentrate (EC) formulation, at rates of either 0.5 or 1.0 ppm alone or in combination with 1.25 and 2.5 ppm methoprene (10 treatments in all, including an untreated control). Treated commodities were stored at ambient conditions on the floor of an empty grain bin in Manhattan, KS, USA, in 5-kg lots for individual replicates. The commodities were sampled and bioassayed every three months for fifteen months by exposing ten mixed-sex parental adults of selected adult stored product insects on 70-80 g of the commodity. For all treatments, there was no regression of declining efficacy with respect to month, therefore data were combined for analysis. On wheat and brown rice, there was no reproduction of Rhyzopertha dominica (Fauvel) in any of the treatments, and there was no weight loss in either commodity that was caused by feeding of the parental adults or developing progeny. There was reproduction of Sitophilus oryzae (L.) on wheat but for several combinations the EC formulation gave better suppression of progeny compared to the standard SC. However, on brown rice only the combination of 1.0 ppm deltamethrin EC and 2.5 ppm methoprene was different than other treatments with respect to progeny development, sample weight loss caused by feeding, and weight of the feeding damage itself. Progeny production was correlated with grain damage. No progeny of Tribolium castaneum (Herbst) developed on the treated corn, but there was some variation in insect damage, with less damage in those treatments involving the EC formulation. Progeny production of Sitophilus zeamais (Motschulsky) was lowest in the combination of 1.0 ppm deltamethrin EC and 2.5 ppm methoprene and insect damage. Results of this study were used by the registrant (Central Life Sciences) in the United States (US) to modify the commercial formulation to replace the deltamethrin SC with the EC, at label rates of either 0.5 ppm EC+ 1.25 ppm methoprene, or 1.0 ppm EC + 2.5 ppm methoprene, on wheat, corn, and rice.