Residual efficacy of a deltamethrin emulsifiable concentrate formulation against Rhyzopertha dominica (F.) and Sitotroga cerealella (Olivier) after partial treatment of brown rice

Authors: Arthur, Franklin

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2019
Publication Date: 4/1/2019
Citation: Arthur, F.H. 2019. Residual efficacy of a deltamethrin emulsifiable concentrate formulation against Rhyzopertha dominica (F.) and Sitotroga cerealella (Olivier) after partial treatment of brown rice. Insects. 10(4). https://doi.org/10.3390/insects10040095.
DOI: https://doi.org/10.3390/insects10040095

Interpretive Summary: Treating grain with insecticides prior to storage is a widely used management tactic, but how variation in coverage of the individual kernels or mixing of treated and untreated grain impacts efficacy in not well understood. Here we evaluated how different ratios of treated and untreated grain impacted two important internal feeding pest species; Rhyzopertha dominica (Fab.), the lesser grain borer, and Sitotroga cerealella (Olivier), the Angoumois grain moth. Females of both species lay eggs on the exterior of grain kernels, and the neonate larvae bore into a kernel to complete development to the adult stage. In this test, brown rice was treated with 0.5 and 1.0 ppm of a new emulsifiable concentrate (EC) formulation of the pyrethroid deltamethrin and stored for 12 months at ambient conditions and evaluated 1 day after treatment, and every 3 months for 12 months. At the percentage ratios of treated to untreated brown rice of 0:100, 10:90, 25:75, 50:50, 75:25, and 100:0 (untreated controls), there was no upward trend in progeny production with the time after treatment or with decreasing percentage of treated kernels for either species. Impact of treatment on progeny production of S. cerealella was limited and ranged from 105.6 on untreated brown rice to 69.4 on 100% treated rice, but there was little feeding damage or weight loss. In contrast, progeny production of R. dominica declined from 177.4 on untreated rice to 9.8 on 100% treated rice, and weight loss and feeding damage was correlated with progeny production. Results show that the new deltamethrin formulation could be used for protection of brown rice, however; S. cerealella may be less susceptible to deltamethrin compared to R. dominica. In addition, complete treatment of brown rice will be more effective than partial treatment, so efforts should be made to ensure that proper application procedures are followed when brown rice is treated with insecticides.

Technical Abstract: When the rough rice hull is removed during the milling process to produce brown rice, the loss of the protective husk can facilitate infestations by insects, particularly internal feeders such as Rhyzopertha dominica (Fab.), the lesser grain borer, and Sitotroga cerealella (Olivier), the Angoumois grain moth. Females of both species lay eggs on the exterior of grain kernels, and the neonate larvae bore into a kernel to complete development to the adult stage. In this test, brown rice was treated with 0.5 and 1.0 ppm of a new emulsifiable concentrate (EC) formulation of the pyrethroid deltamethrin and stored for 12 months at ambient conditions on the floor of an empty grain bin. Untreated brown rice was held in the same manner. One day after treatment, and every 3 months for 12 months, the treated rice was mixed with untreated brown rice in the follow percentage ratios: 0:100, 10:90, 25:75, 50:50, 75:25, and 100:0 (untreated controls). The weight of individual samples was about 80g. Bioassays were conducted by exposing 10 parental adults of each species on the various mixtures and assessing progeny production, feeding damage, and weight loss. For both species, there was no regression trends with sample times or differences between insecticide rates, and data were combined for analysis. Progeny production of S. cerealella ranged from 105.6 on untreated brown rice to 69.4 on 100% treated rice, but there was little feeding damage or weight loss. In contrast, progeny production of R. dominica declined from 177.4 on untreated rice to 9.8 on 100% treated rice, and weight loss and feeding damage was correlated with progeny production. Results show that the new deltamethrin formulation could be used for protection of brown rice, however; S. cerealella may be less susceptible to deltamethrin compared to R. dominica.