Evaluation of tank mix partners on the efficacy of the novel biopesticide Calantha (ledprona) against first-generation Colorado potato beetle, 2023

Abstract

This trial was conducted at the University of Wisconsin’s Hancock Agricultural Research Station on field E-28 at coordinates 44.11356, −89.53641 on a Plainfield sand soil (PfA) in 2023 to determine if certain fungicide or fertilizer products reduce the efficacy of the novel dsRNA biopesticide Calantha (ledprona) as an insecticide when combined in a tank mix and applied at a low spray volume. Potato, Solanum tuberosum cv. ‘Snowden’ B-size tubers were machine planted on 27 April in 2-row strips separated by 1 unplanted row with an approximate seed spacing of 12 inches and a row spacing of 3 ft. Fertilizer granules containing no insecticide were introduced at the time of planting per standard commercial practices (see field management record for details). Two-row plots, 20 ft in length, were later established by tilling 8 ft alleys across solid-planted rows. From these plots, 4 replicates of 17 treatment plots were arranged in an RCB design. Plots were maintained according to standard commercial practices including fertilizer and herbicide applications but did not receive additional fungicide applications except those specified in the treatment program until the conclusion of the experiment. Except for Trt 1, the untreated check, all treatments included an insecticide (either Calantha, Trt 2-14 or Delegate, Trt 15-17) plus 1 or more fungicides or fertilizer products to evaluate the possible detrimental effect of the low-volume tank mix on the insecticidal activity of the treatment. See Table 1 for complete treatment details. Spray mixes were prepared at the field site immediately prior to application in a 2-L mix size. All treatments were initiated at approximately 50% egg hatch and applied 3 times on a 7-day interval using a CO2-pressurized backpack sprayer equipped with a 6-ft boom, operating at 30 psi and delivering approximately 6.8 gpa through 4 flat-fan nozzles (Tee Jet 8001VS) fitted with 100-m mesh screens spaced 18 inches apart while traveling at 4 mph. A low spray volume was chosen intentionally to replicate conditions in aerial spray rigs. On 20 June (Appl. B), pH paper was used to measure the pH of each treatment mix prior to application (Table 2). Some fungicide products led to more basic conditions in the mix (e.g., Badge, pH 8.0), while some of the fertilizer products created more acidic conditions in the mix (e.g., NUE Manganese and Premium 21, pH 6.0). After the conclusion of insect counts (5 July), Minecto Pro was applied at 10 fl oz/acre to the entire field to kill any remaining beetles, prevent spillover into adjacent trials, and allow remaining potato foliage to continue to be rated for signs of early blight (ratings performed 30 June, 13 July, and 25 July). aAll treatments applied as foliar sprays on 3 dates: 12 June (50% egg hatch), 20 June, and 27 June. bAn “X” below a treatment number indicates the inclusion of the product in that treatment mix. apH measured using paper test strips. Colorado potato beetle (CPB), Leptinotarsa decemlineata, populations were assessed on 10 randomly selected plants in the center of each plot for the following life stages: adults, egg masses, small larvae (first and second instars), and large larvae (third and fourth instars). CPB counts and whole-plot defoliation estimates were performed on 16, 19, and 26 June and 5 July (2, 5, 12, and 21 days after the first foliar application). In addition, early blight (Alternaria solani) ratings were performed on 30 June, 13 July, and 25 July where each plot was given a disease severity rating based on the 0–11 Horsfall–Barratt rating scale to evaluate differences in fungicide efficacy due to tank mix interactions. Evaluation data were analyzed in R version 4.3 (R-Core Team, 2023). Insect counts were log(x + 1) transformed, and defoliation percentages were arcsine square root transformed to satisfy assumptions of normality. Treatment main effects were determined using ANOVA with treatment and block (replicate) as explanatory factors. Means separation letter codes were generated using Tukey’s HSD procedure (α = 0.05, R package “agricolae”). Some difficulty was experienced with mixing and application of the treatments. We found that the dry ingredients (Messenger Gold, Delegate) would not dissolve well after other products were added to the mix water, so dry ingredients were added first and given sufficient time to dissolve. Treatments that included Premium 21 would quickly clog because the product contained grit and precipitate that would clog the spray filters. We also noted after the trial that the Premium 21 label specifically suggested a minimum dilution of 1 part in 20, while the protocol that we followed called for 3 parts in 5. The Premium 21 label also warns against including it in tank mixes containing insecticides as it can reduce their efficacy, though we observed no apparent influence on Calantha’s efficacy when combined with Premium 21. Overall CPB pressure was very high for this trial (see counts by life stage, Table 3). Final defoliation ratings were performed on 5 July, prior to the emergence of second-generation adults. At this time, the untreated plots had reached nearly complete defoliation. Calantha (5.8% mean defoliation on 5 July) appeared to lose considerable efficacy when combined with the fungicides Bravo (11.5% defoliation) or Badge (13% defoliation), but not Luna Tranquility (5.8% defoliation). The 6-12-2 potato foliar mix also caused a drop in Calantha’s efficacy (14.5% defoliation). Delegate performed very well (2.3% defoliation) and lost no efficacy when combined with Bravo and Luna Tranquility (3% defoliation), but when included in the 8-member tank mix (Trt 17), Delegate lost substantial efficacy (15.5% defoliation). No significant differences in early blight ratings were observed on any of the 3 rating dates (30 June, 13 July, and 25 July), although the average rating increased each week (Table 4). In conclusion, Calantha can be included in tank mixes with some other products but certain partners (e.g., Bravo, Badge, and the 6-12-2 potato foliar fertilizer mix) substantially reduced CPB management performance.1 aMeans generated from per-plot cumulative counts across 4 evaluation dates (16 June, 29 June, 26 June, and 5 July). bTreatment main effect P-values determined by ANOVA. Means followed by same letter code(s) are not significantly different (Tukey’s HSD, α = 0.05). aEarly blight severity was rated between 0 and 11 based on the Horsfall–Barratt rating scale. bTreatment main effect P-values determined by ANOVA. Means followed by same letter code(s) are not significantly different (Tukey’s HSD, α = 0.05).

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