Methodology for assessing progeny production and grain damage on commodities treated with insecticides

Authors: Arthur, Franklin; Morrison, William - Rob

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/29/2020
Publication Date: 6/5/2020
Citation: Arthur, F.H., Morrison III, W.R. 2020. Methodology for assessing progeny production and grain damage on commodities treated with insecticides. Agronomy. 10:804. https://doi.org/10.3390/agronomy10060804.
DOI: https://doi.org/10.3390/agronomy10060804

Interpretive Summary: Evaluations of grain protectants, insecticides applied directly to the grain to reduce the ability of insects to feed and develop on the grain during storage, can be done by exposing parental adults for a given time period, removing them from the commodity, and assessing progeny production, or continuously exposing parental adults and then counting progeny after 6-8 weeks. However, there are no studies comparing these two methods. In a long term study (1 year) that evaluated residual activity of two rates of a methoprene + deltamethrin insecticide and one rate of a new methoprene + deltamethrin + piperonyl butoxide synergist insecticide on wheat, corn, and brown rice it was demonstrated that lesser grain borers and red flour beetles were susceptible to all insecticides with no progeny production demonstrated using either experimental method. Rice weevils on wheat and maize weevils on corn were initially controlled by the new insecticide, but after 3 months of corn storage when adults were added to the corn there was extensive initial adult survival and progeny production. Brown rice was very susceptible to the rice weevil and none of the insecticides were effective. Parental mortality was correlated with progeny production, and progeny production and grain damage were correlated with the two exposure methods. Results also indicate that brown rice is extremely susceptible to the rice weevil, managers may need additional insecticidal inputs to control this species on brown rice and to a lesser extent on wheat. The two exposure regimes used for evaluating grain protectants did not appreciably alter the ability to determine the relative differences among treatments. Progeny production, sample weight loss, and insect feeding damage were all correlated with the two exposure regimes for all target species on all three commodities. Removing the adults tended to have lower progeny production, suggesting this method might be preferred in situations where over production of progeny results in contamination issues. These results can be used by grain managers in determining which products might provide the best efficacy.

Technical Abstract: In evaluating insecticides, progeny production on grain commodities can be evaluated by either exposing adults on a commodity for a period, then assessing mortality and progeny production, or by leaving the adults on the commodity continuously, and then assessing progeny production. Little research directly compares these methodologies. Thus, our aims were to determine: 1) residual efficacy of Diacon IGR+ (methoprene+deltamethrin) and Gravista (methoprene+deltamethrin+piperonyl butoxide) on wheat, corn, and brown rice, using select stored product insects over a year, and 2) directly compared the two different methods of parental adult exposure on progeny production. Adults were either exposed for 7 d, then removed and assessed for survival, and the commodities held for 6–7 weeks, or continually exposed on the commodities for 6–7 weeks. Commodities were aged and sampled every 3 months for 12 months. Afterwards, samples were examined for progeny, sample weight loss, and insect feeding damage. Each insecticide killed exposed adults and prevented progeny of Rhyzopertha dominica on wheat and brown rice, and Tribolium castaneum on corn, but there was extensive survival of Sitophilus spp on some commodities. Gravista initially increased suppression of S. oryzae on wheat and S. zeamais on corn compared to Diacon IGR+. Progeny, weight loss, and insect feeding damage was positively correlated in the 7-d compared with continual parental exposures. Both insecticides will control R. dominica and externally-feeding insects, but may exhibit reduced effectiveness for Sitophilus spp., especially S. oryzae. Food managers can utilize this data to more effectively plan management programs.