Insecticide Efficacy Against Alfalfa Weevil, 2018 And 2019

The objective of the study was to evaluate insecticide efficacy for the management of alfalfa weevil (AW) larvae under Intermountain West growing conditions, specifically Utah. Field trials were conducted in on-farm commercial alfalfa hay fields in Monroe and Price, Utah during 2018 and 2019, respectively. An RCB design was used consisting of four replicates per treatment with an untreated control plot and seven insecticide treatments during 2018, and five insecticide treatments during 2019 (insecticide treatments and rates detailed in Tables 1 and 2). Plots measured 10 feet by 30 feet. Treatments were applied 22 May 2018 and 31 May 2019 using a Bell Spray Inc. JR-201S-JR handheld CO₂ boom sprayer fitted with six TJ-8002VS nozzles, spaced 16-inches apart, operating at a target 20 PSI and 17 gallons per acre spray volume. AW populations were assessed by pooling five 180° sweeps using a standard 15-inch sweep net down the center of each plot. During 2018, samples were collected on 29 May (7 DAT), 5 Jun (14 DAT), and 12 Jun (21 DAT). During 2019, samples were collected on 3 Jun (3 DAT), 14 Jun (14 DAT), 21 Jun (21 DAT), and 28 Jun (28 DAT). Samples were placed in labeled bags, stored in a cooler for transport, and then frozen until processed in the laboratory. Data were analyzed using a two-way ANOVA using the PROC GLM procedure (SAS Studio) followed by Tukey’s HSD posthoc means comparisons using transformed data where appropriate.

In 2018, AW populations were well below the economic threshold (20 larvae per sweep) in the untreated control plots throughout the trial. Specifically, AW at the start of the trial was 5.7 larvae per sweep. There were significant differences in AW suppression across all treatments and sample periods (Table 1). Treatments producing the highest levels of control 7 DAT were Steward (10 oz.) and Cobalt (24 oz.). Whirlwind (24 oz.) provided moderate control while dimethoate, Steward (5 oz.), Mustang Maxx (4 oz.), and malathion (5 oz.) did not provide significant control (Table 1, see 7 DAT column). Compared to the untreated control, this pattern remained until 21 DAT when statistical differences were no longer detectable. Overall, Cobalt plots were similar to the high rate of Steward (Table 1 see PTA column). The high rate of Steward (10 oz.) significantly reduced AW compared to the untreated control overall (Table 1). Whirlwind provided moderate but significant control. Several products including Mustang Maxx and Malathion provided no noticeable suppression in comparison to controls. In addition, plots treated with Steward at a reduced rate (5 oz.) had significantly more larvae compared to the high rate (see Table 1, PTA column) and was not significantly different than the untreated control. It is important to note, however, that AW populations were low and variable throughout the trial and although post-hoc testing of % control data did not exhibit statistically significant groupings (see Table 1, PTA % control column), our results may not reflect the efficacy targeting higher AW densities.

In 2019, AW populations in untreated controls were much higher than in 2018 and were well above the economic threshold. Specifically, AW had a recorded 77.72 larvae per sweep at the start of the trial in untreated control plots. All treatments suppressed AW populations compared to the untreated control plots across all sample dates (Table 2, see PTA column). At 3 DAT, all treatments were providing statistically significant levels of control compared to the untreated control. By 7 DAT continuing to 28 DAT, Cobalt (24 oz.) and Mustang Maxx (4 oz.) provided the highest level of control (Table 2, see individual columns of respective times, PTA column, and PTA % control column). Whirlwind (24 oz.), Steward (11.3 oz.), and Steward (6.7 oz.) performed similarly beginning 7 DAT and provided similar efficacy until 28 DAT. Cobalt, Mustang Maxx, and Whirlwind provided high levels of AW control across all sample dates compared to untreated controls (Table 2, see PTA column). Moreover, the high (11.3 oz.) and low (6.7 oz.) rates of Steward significantly reduced AW compared to the control. There was no significant difference in larval densities between the rates of Steward.¹

Footnotes