Impacts of selected grain protectants against maize/rice weevil in stored field corn in Louisiana, 2022

A trial was conducted at the Macon Ridge Research Station in Winnsboro, Louisiana to evaluate the efficacy of selected stored-grain protectants against maize/rice weevil in stored field corn. Plots consisted of 30-gallon plastic drums containing triple-cleaned corn grain obtained from local suppliers. Each plot (drum) held 200 lbs of grain. Drum lids were placed onto respective drums with holes large enough to accommodate weevils and other similar-sized insects but prevented larger animals such as rodents and birds from entering. Plots (drums) were arranged in a randomized complete block design with four replications. Insecticides with labeled rates to be used as grain protectants (those applied directly to the grain) are expressed as fluid oz of product with an application volume of 5-gallon solution per 1,000 bu of grain. To provide a means of standardization, 1,000 bu of corn were assumed to have a weight of 56,000 lbs, with the exception of the Actellic treatment, which assumes 1,000 bu is equivalent to 60,000 lb. Triple-cleaned corn was purchased in Jun 2022 and allowed to sit unprotected until the trial began in Aug 2022, allowing for natural infestations to occur. Grain protectants were applied on 16 Aug 2022, using two concrete mixers large enough to hold 50 lbs of grain each. Drums of grain were stored under an open-air equipment shed for rain protection and to allow natural infestations of insect pests during the experiment. Both maize and rice weevils could have infested the grain during this experiment. Weevils were not identified to species but were identified to be either maize or rice weevils. Samples from each container were collected monthly from Sep 2022 to Aug 2023. Grain temperature and moisture content, as well as weevil density, were determined for each sample. Gain temperature was determined using a 12-inch digital thermometer inserted into the grain mass. Grain samples were collected using a standard 40-inch slotted grain probe inserted into each plot (drum) at random locations to obtain a total sample volume of 1 pint of grain. Insects were separated from grain using a 0.0833-inch mesh size sieve. Grain moisture was determined using a Dickey John GAC 2100 grain analysis computer. Data for each sample date were subjected to ANOVA and means were separated using Fisher’s protected LSD (a = 0.05).

For sample moisture contents, no differences were observed on 22 Nov 2022, 21 Mar 2023, 17 Apr 2023, 10 May 2023, 13 Jun 2023, 25 Jul 2023, or 9 Aug 2023 (Table 1). When differences were observed, Suspend + PBO and Diacon IGR tended to have a higher moisture content than the untreated check. Actellic was lower than the untreated check on 15 Sep 2022 and 12 Oct 2022 sample dates but was similar to the untreated check on all other sample dates. Diacon IGR Plus and Suspend SC moisture contents were similar to the untreated check on all sample dates. Sensat moisture content was similar to the check on all sample dates excluding 12 Oct 2022, where it was lower than the untreated check (Table 1). Significant differences were observed for sample temperature on all sample dates except for 22 Nov 2022 and 25 Jul 2023 (Table 2). Sensat and Actellic treated plots had lower temperatures than the untreated check on all sample dates from 14 Dec 2022 to 29 Aug 2023, except for 25 Jul 2023 (Table 2). There were no significant differences in weevil densities on sample dates 14 Dec 2022, 17 Jan 2023, and from 21 March 2023 through 29 Aug 2023 (Table 3). Weevil densities in all treated plots (drums) were lower than the untreated check and similar to each other, except for Diacon IGR Plus, on sample date 15 Sep 2022. On sample dates where significant differences were observed, only plots treated with Sensat, Actellic, or Suspend + PBO were consistently lower than the untreated check (Table 3).¹

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