The impact of Varroa destructor Anderson & Trueman (Mesostigmata: Varroidae) on colonies of Apis mellifera L. (Hymenoptera: Apidae) in southern Louisiana was evaluated by analyzing changes in swarming and longevity of colonies for 17 yr. Swarming rates were calculated from yearly captures of swarms in bait hives placed in five areas of Louisiana from 1991 to 2006. Colony longevity was monitored in 104 swarms established from 1990 to 2000 and followed until 2004. In the first years, before V. destructor, average swarm capture rates ranged from 0.85 to 0.95 swarms per bait hive-year, and survival of colonies established from swarms averaged 14 mo. In years immediately after the arrival of V. destructor (1993–1996), swarming rates and colony longevity decreased to 0.36–0.60 swarms per bait hive-year and 10 mo, respectively. After ≈5 yr in the presence of V. destructor, both rates recovered to levels at least as high as those seen before varroa arrived; swarm capture rates were 0.75–1.04 swarms per bait hive-year and average longevity was 26 mo. Analysis of varroa infestations in three colonies established from swarms in 1997 showed the presence of varroa at oscillating densities for 5 to 8 yr. Possible causes for this apparent recovery are natural selection for resistance in honey bees, introgression of selected resistant genetic material or reduced virulence of the mites.
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Vol. 101 • No. 5