Even though predation is a contributing factor in the suppression of alfalfa weevil densities, arthropod predators are currently ignored in alfalfa weevil management recommendations. These programs have been developed to suppress alfalfa weevil with insecticides or harvesting without considering the post-treatment effects on predators. Additionally, the effect of management practices on other plant feeding insects, such as pea aphids, which serve as prey for the same predator insects is ignored.
Studies were conducted in Iowa during 1994-1995 to examine the effects of full and reduced rates of Pounce 3.2EC and Lorsban 4E, and strip harvesting. Previous research documented that reduced rates of insecticides and strip-harvesting of alfalfa resulted in alfalfa weevil control without disrupting the activity of the insect-attacking fungus, Zoophthora phytonomi, or parasitism by Bathyplectes spp. wasps. In 1994 predators (lady beetles, damsel bugs, predatory stink bugs, and spiders) were reduced in strips treated with full or half rates of Pounce 3.2EC, and harvested strips (swathed and crimped). In 1995 no significant differences in predator densities were observed in any treatments when rainfall occurred on the day of the insecticide applications, and harvested alfalfa was not crimped. In both years, Lorsban 4E applied at reduced or full rates did not result in consistent, significant reductions in predator abundance. Reduction in prey (alfalfa weevil and pea aphids) densities after treatments may have contributed to lower abundance of predators in treated alfalfa strips.
Rates of Lorsban 4E that reduce alfalfa weevil larval densities do not consistently reduce arthropod predator abundance when applied in strips. These results indicate that strip applications for alfalfa weevil may be the basis for an integrated control program.
Giles, K. & J. Obrycki. 1997. Reduced insecticide rates and strip-harvesting: Effect on arthropod predator abundance in first-growth alfalfa. J. Kansas Entomol. Soc. 70: 160-168.
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