The pecan weevil, Curculio caryae (Horn), is a major pest of pecans (Harris 1985, Pp. 51–58. In Pecan Weevil: Research Perspective, W. W. Neel (ed.), Quail Ridge Press, Brandon, MS). These insects have a 2- or 3-yr life cycle with most adult weevils emerging from soil beneath trees from late July through September to feed on and oviposit in the developing fruit (Harris 1985). Fourth instars drop to the ground and burrow to a depth of 8–25 cm, form a soil cell, and overwinter. During the following autumn, approximately 90% of larvae pupate and spend the next 9 mo in the soil as adults (Harris 1985). The remaining population (about 10%) spends approximately 2 yr in the soil as larvae and emerges as adults in the third year.Currently, control recommendations for C. caryae primarily consist of canopy applications of chemical insecticides (e.g., carbaryl and certain pyrethroids) to suppress adults (Acebes et al. 2021, Pg. 5, In Wells (ed.), Commercial Pecan Spray Guide. Univ. Georgia-Extension Bull. 841). Although these chemical insecticide applications are effective in controlling C. caryae in conventionally managed orchards, there is a lack of knowledge regarding C. caryae management in organic pecan systems. Additionally, due to problems associated with aphid and mite resurgence that often result from chemical insecticide applications that target C. caryae (Dutcher and Payne 1985, Pp. 39–50 In Pecan Weevil: Research Perspective, W. W. Neel (ed.), Quail Ridge Press, Brandon, MS), as well as other environmental and regulatory issues, research on developing alternative control strategies in both organic and conventional systems is necessary.In prior research, we discovered that that the microbial insecticide Grandevo® (based on Chromobacterium subtsugae Martin, Gundersen-Rindal, Blackburn & Buyer), applied in pecan orchards at 3.36 kg per ha, can control C. caryae at similar levels compared with recommended chemical insecticides (Shapiro-Ilan et al. 2017, J. Econ. Entomol. 106: 257–266). Moreover, Grandevo applications conserved natural enemies in the orchard and reduced populations of black pecan aphid, Melanocallis caryaefoliae (Davis) (Hemiptera: Aphidae) (Oliveira-Hofman et al. 2021, Biol. Contr. 161, 104709). However, the cost of 3.36 kg per ha Grandevo is prohibitive to most growers. Therefore, the objective of our study was to determine the ability of Grandevo to control C. caryae under field conditions when applied at a lower rate.The experiments were conducted over two consecutive years (2019 and 2020) in pecan orchards at the USDA-ARS Southeastern Fruit and Tree Nut Research Station (Byron, GA, USA). The experiments were organized in randomized complete block designs and methodology was based on Shapiro-Ilan et al. (2017) and Oliveira-Hofman et al. (2021, Biol. Contr. 161: 104709). There were four blocks (1 ha each) separated by at least 100 m each. Blocks 1 and 2 consisted of trees approx. 70 yr old with a cultivar mix of Stuart and Schley, Block 3 contained 32-yr-old trees with mixed cultivars of Cheyenne, Sumner, and Farley, and Block 4 contained 80-yr-old trees with a single cultivar (Moneymaker). All blocks had 18 × 18-m spacing. No insecticide sprays were applied in the blocks other than those described below.The plots were split in half such that one half received the treatment (Grandevo) and the other was the control (no treatment). Grandevo was obtained from Marrone Bio Innovations (Davis, CA). In 2019, a granular formulation of Grandevo was applied at 2.24 kg per ha and in 2020 a liquid formulation was applied at 1.9 L per ha; the manufacturer indicated that the two rates (2.24 kg and 1.9 L) were deemed to be equivalent. Four applications were made in each year (between 16 August and 4 October 2019 and between 9 August and 20 September 2020). Applications were made with approximately the same interval and were timed based on emergence captures of adult weevils in Circle traps (Shapiro-Ilan et al. 2017).Treatment effects were evaluated by determining the percentage of nuts infested with pecan weevil after harvest. Nuts were harvested between November and January each year. In 2019, at least 95 nuts were randomly sampled per treatment (range of 23 to 100 per plot) and in 2020, 400 nuts were randomly sampled per treatment (100 per plot). The difference in number of nuts sampled in the 2 years was due to a much lower crop load in 2019. Only sample trees buffered by at least one other tree on each side within each plot were used (i.e., trees on the edge of the plot were not sampled). Nuts with obvious C. caryae exit holes were counted as infested. The remaining nuts were cracked and checked for the presence of C. caryae larvae.Treatment effects were analyzed using two-way analysis of variance (ANOVA) based on percentage C. caryae infested nuts (α = 0.05) (2016, SAS Institute Inc., Cary, NC). Percentage infested nuts were arcsine transformed (arcsine of square root) before analysis (Southwood 1978, Ecological Methods __ Chapman and Hall, London, U.K.); residuals of the transformed means were plotted to indicate that equality of variance and the normality assumptions were met. Nontransformed means are presented in Fig. 1 and 2.In both 2019 and 2020, the Grandevo treatment significantly reduced the percentage of infested nuts (F= 8.73 df = 1,27, P= 0.0064 for 2019 and F= 56.56, df = 1, 27, P < 0.0001 for 2020) (Figs. 1 and 2). Additionally, Abbott's formula applied to the data (Abbott 1925, J. Econ. Entomol. 18: 265–267) indicated 93% control in each year.The reduced rate of Grandevo (2.24 kg and 1.9 L) was found to be highly effective in reducing C. caryae damage in both years that field trials were conducted. Hence, relative to prior research indicating efficacy with 3.36 kg per ha, the lower rates of Grandevo enhance economic feasibility of the approach. Based on this current research and the recent findings that Grandevo also contributes to M. caryaefoliae reduction (as opposed to flaring aphids as chemical treatments for C. caryae) makes Grandevo an attractive option for C. caryae control.We thank Stacy Byrd and Danny Howard for technical assistance. Mention of a proprietary product name does not imply USDA's approval of the product to the exclusion of others that may be suitable.