Life-history traits of wild honey bee colonies living in forests around Ithaca, NY, USA

Original article Apidologie * The Author(s), 2017. This article is an open access publication DOI: 10.1007/s13592-017-0519-1 Life-history traits of wild honey bee colonies living in forests around Ithaca, NY, USA Thomas D. SEELEY Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA Received 7 December 2016 – Revised 27 February 2017 – Accepted 12 May 2017 Abstract – Wild honey bee colonies—both truly wild (in trees and buildings) and simulated wild (in small hives)—were studied to determine their life-history traits, to see if these traits have changed now that these colonies are infested with Varroa destructor . Most colonies (97%) survive summers, but only 23% of founder (first-year) colonies and 84% of established colonies survive winters. Established colonies have a mean lifespan of 5–6 years and most (87%) have a queen turnover (probably by swarming) each summer. A population model shows that these life-history traits produce a stable population of colonies. Remarkably, the suite of colony life-history traits found in the 2010s (with V. destructor ) matches that found in the 1970s (without V. destructor ). It seems likely that the wild colonies living near Ithaca, NY, possess defenses against V. destructor that are not costly. Apis mellifera / life-history evolution / colony survival / deformed wing virus / Varroa destructor / wild honey bees 1. INTRODUCTION Starting in 1952 in Europe and in 1987 in North America, colonies of the hive honey bee (Apis mellifera ) became infested with an ectoparasitic mite (Varroa destructor ) (reviewed by Locke 2016). Researchers found that if a colony of European-derived honey bees is not treated for V. destructor , then in a year or two, the colony’s mite population will surge, the bees’ virus titers will skyrocket, and the colony will die (Korpela et al. 1992; Fries et al. 2006). It was widely believed, therefore, that the wild colonies of European-derived honey bees living in Europe and North America had perished (Moritz et al. 2007; Potts et al. 2010). In the mid 2000s, however, reports began to surface from Europe and North America of populations of wild colonies of European-derived honey bees that are surviving despite being infested Corresponding author: T. Seeley, D. Seeley, Manuscript editor: Yves Le Conte with V. destructor : Sweden (Fries et al. 2006), France (Le Conte et al. 2007), and the USA (Seeley 2007). A balanced relationship between honey bees and V. destructor is expected wherever most colonies are living wild because in these places, there should be strong natural selection for mite-resistant bees, and maybe also for avirulent pathogens and parasites. The expectation of strong natural selection on the bees was confirmed recently by a study (Mikheyev et al. 2015) of the wild honey bees living around Ithaca, NY. It used whole-genome sequencing of bees collected from wild colonies in 1977 (before arrival of V. destructor ) and in 2011 (after arrival of V. destructor ) and found that 232 nuclear genes scattered throughout the genome underwent strong selection between the sampling dates. This study also found that these bees experienced a collapse in effective population size between 1977 and 2011, probably the result of massive colony mortality following the arrival of V. destructor in the mid 1990s. The census population size has, however, recovered (Seeley 2007). This paper shifts from the genetics to the ecology of the wild colonies living around Ithaca, NY. S. D. Thomas It reports a 7-year study (2010–2016) of how this population is surviving even though all colonies are infested with V. destructor . A study conducted 40 years ago (1974–1977) found that wild colonies living around Ithaca possessed a certain suite of life-history traits: slow development, long lifespan, and few but costly offspring (Seeley 1978). The present study sees if the life-history traits of these colonies have changed since the 1970s because now they must invest in immunological, behavioral, and other defenses against V. destructor and the viruses that this mite vectors. Presumably, the costs of these additional defenses are traded off against other fitness components—reviewed by Minchella (1985), Sheldon and Verhulst (1996), and SchmidHempel (2003)—so it may be that wild colonies now have slower development, shorter lifespan, or fewer (or less costly) offspring. Another possibility, though, is that wild colonies have low-cost defenses against the mites and viruses. If so, then their life-history parameters are expected to be unchanged. To find out which scenario pertains to the wild colonies living around Ithaca, NY, I studied 33 nest sites (in trees and buildings) occupied by wild colonies and 22 hives occupied by simulated wild colonies. 2. MATERIALS AND METHODS 2.1. Study area and bees’ ancestry This study is based on colonies living in the heavily forested countryside south of Ithaca, NY, USA (42° 26′ 36″ N 76° 30′ 0″ W) . This region was settled in the early 1800s and until the late 1800s the region was devoted primarily to agriculture. It is no longer farmed, and large acreages that were once cultivated have grown into forests ranging from old-field successional forests to mature forests. It has a moderate continental climate: summers are short and warm (temperatures rarely exceed 32° C); winters are long and cold (temps often reach −18°C). The ancestry of the wild colonies has been analyzed (Mikheyev et al. 2015). Both old (1977) and modern (2011) populations are primarily descendants of Central European races (Apis mellifera carnica and A . m . ligustica ), but also have a sizable ancestry from Western European (A . m . mellifera ) and Central Asian (A . m . caucasica ) races. 2.2. Finding wild colonies Thirty-three nest sites of wild colonies were followed; 14 were found by bee hunting (Seeley 2016) and 19 were found by chance. As before (Seeley 1978), I distinguished two categories of colonies: founder and established. Founder colonies are ones that have not yet survived a first winter; established colonies are those that have. I categorized a colony as founder or established only if the owner of the tree or building housing the colony reported that it had moved into the site that summer (hence a founder colony), or had lived there the previous summer and had survived the previous winter (hence an established colony). All other colonies were categorized as Bambiguous^. A founder or ambiguous colony that survived winter was reclassified as an established colony the following spring. 2.3. Inspecting nest sites and statistical analyses I inspected each nest site three times a year: spring, summer, and fall. Spring inspections were made before May 10, thus before the main swarming season. Summer inspections were made after July 20, thus after the main swarming season. Fall inspections were made after September 20, thus after the secondary swarming season. In the Ithaca area, 80% of swarms (...truncated)