Impacts of three insect growth regulators and the particle barrier film, Kaolin, on alfalfa weevil, Hypera postica (Gyllenhal), secondary pest, pea aphid, Acyrthosiphon pisum (Harris) & natural enemy complex
Tharp, Cecil Irwin
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Studies were conducted in Montana to evaluate the impacts of the insect growth regulators novaluron, diflubenzuron, azadirachtin and the particle barrier film, kaolin, on the primary pest, alfalfa weevil (AW, Hypera Postica [Gyllenhal)], natural enemies of alfalfa weevil and the secondary pest, pea aphid, Acyrthosiphon pisum (Harris). Kaolin, diflubenzuron and azadirachtin treatments caused low (<53%) AW mortality and did not prevent AW feeding damage across 5 field sites. Novaluron caused the highest mortality (74 ± 3% at one field site) while significantly reducing feeding damage across two of five field sites (P < 0.05) and two greenhouse trials. Plants treated with novaluron weighed significantly more than untreated plants at harvest in either greenhouse study with a final harvest weight of 2.7 ± 0.2 and 3.4 ± 0.3g / pot in the novaluron treated pots compared to 2.2 ± 0.1 and 2.4 ± 0.3 g / pot in the untreated; however harvest yields were not increased in field trials. All experimental treatments provided some pre-harvest benefits to the predator-alfalfa weevil and predator-pea aphid complex at various field sites; however novaluron treatments provided significantly higher predator-alfalfa weevil ratios consistently across four of five field sites when compared to the synthetic pyrethroid, lambda cyhalothrin (P < 0.05). At these four field sites, novaluron treated plots harbored an average predator-alfalfa weevil ratio of 0.15 ± 0.07 compared to 0.02 ± 0.02 in lambda cyahlothrin treated plots in the first harvest cycle. Parasitism rates were significantly higher when experimental treatments were used compared to the lambda cyhalothrin treated plots across five field sites (P < 0.05). The added benefit of conserving predators and parasitoids in combination with direct pesticide efficacy never reduced densities of AW or pea aphid to that of the synthetic pyrethroid treatment in the first or second harvest cycle. While novaluron had little benfit on reducing AW or pea aphid poulations to that of the synthetic pyrethroid treatment, it offers the best potential for developing a soft-chemical/biological system for protecting alfalfa from this key arthropod pest. Future studies taking advantage of novalurons mode of action as a feeding deterrent should be explored.