Efficacy of Selected Bio- and Reduced-Risk Insecticides on Mint Root Borer, 2014

Mint root borer (MRB) is an economically important pest of peppermint in California. The trial was conducted at the University of California-Intermountain Research and Extension Center, Tulelake, California on peppermint of variety ‘Black Mitcham’ using RCB design with 10 treatments and four replications. The experimental plot size was 20 × 20 ft. The overall growth of the mint was good with plant height ~20–25 inches when treatments were applied. Treatments were applied twice (3rd wk of Jul and 1st wk of Aug) except Coragen treatment, which was applied only at the first application. CO₂ pressurized backpack sprayer with nozzle TJet 8002 VS was used. The spray boom size was 10 ft, and spray was conducted at 30 psi. Fields were irrigated night before treatment applications. Post-harvest rhizome samples were taken during Oct to assess the damage by MRB larvae. Rhizomes were collected from ~1 sq. ft area of the ground using a small garden tool (trowel). Two samples were taken diagonally from each plot. Each rhizome sample was placed inside a zip-locked plastic bag, and the bag was placed inside a cooler with ice packs. Samples were transported to Davis lab and stored at 50°C until processing. Five rhizomes (~15–20 cm long consisting of five internodes) were randomly selected from each of the two samples taken from the individual plots (i.e., 10 rhizomes per plot) and examined for rhizome damage by MRB using a dissecting microscope. The percentage internode damage and the number of damage holes in rhizomes were calculated and subjected to ANOVA with LSD mean separation (P < 0.05) procedure. Statistical analysis was conducted in JMP Pro. 10.

Bt galleriae, Met 52, EPN Sc, EPN Hb, and EPNs (Hb + Sc) significantly reduced internode damage by MRB compared with an untreated check (Table 1). However, these treatments were not statistically different from CoStar, BotaniGard, and PFR-97, which in turn were not different from untreated check. Coragen provided significantly more reductions in internode damage than all other treatments (Table 1). Rhizome infested with MRB larvae contained entering and exiting holes. All treatments resulted in significantly fewer holes in rhizomes compared with untreated check except BotaniGard (Table 2). The mean number of holes per rhizome varied from 0.35 in Coragen treatment to 1.55 in untreated check. The numbers of damage holes per rhizome in the remaining treatments were between these two extremes (Table 2).

Disclosure of Potential Conflicts of Interest: This research was funded by California-Specialty Crop Block Grant Program. Some insecticides used in this study were made available by industries as gifts.