Establishing Economic Threshold and Epidemiology for Nosema Ceranae, A Relatively New Species of Microsporidian Parasite in the Honey Bee for PNW

Honey bee colonies in the U.S have been recently reported to be infested with Nosema ceranae, a gut parasite which is prevalent in the Asian honey bee. Apparently this new species has jumped hosts and is now dominant in honey bees in the U.S and Europe. Nosema ceranae results in increased winter mortality. Some studies have implicated Nosema ceranae for the colony collapse disorder. Currently not much is known about the biology and epidemiology of this new Nosema species. Nosema ceranae has threatened the U.S beekeeping industry for the past two years. Beekeepers have been forced to treat their colonies prophylactically with the antibiotic Fumagillin to protect their colonies from N. ceranae without information on the presence or absence of Nosema or its threshold. This prophylactic treatment has significantly increased hive management costs for beekeepers, as fumagillin is expensive. Prophylactic use of fumagillin is not only a financial burden for beekeepers, but its excessive use may lead to the development of resistance.

In this study we focus on undestanding the epidemiology and phenology of Nosema ceranae in the Pacific Northwest (PNW) region, evaluate efficacy of Fumagillin (antibiotic) delivery methods and develop a reliable sampling protocol to detect nosema infections. Our results suggest that Nosema ceranae is prevalent in honey bee colonies in the PNW region throughout the year, with a peak infection during May. Among the three Fumagillin delivery methods, spraying method (spraying bees with Fumagillin mixed in sugar syrup) was the most effective. A sample of 100 bees obtained from the brood area for counting spores was the most reliable method for detecting Nosema infection when compared to a sample size of 25 and 50 bees.

Introduction

Pollination requirement is huge for commercial agriculture in the PNW. A strong and healthy beekeeping industry is of immense value to the PNW agricultural economy that is vast and diverse. Recent honey bee colony losses attributed to colony collapse disorder and a steady decline of colonies for past two decades have caused serious concern and alarm. According to recent PNW honey bee colony winter mortality survey, the colony loss was 30%, which is alarming and unsustainable (Burgett 2009). Currently not much is known about the health of honey bee colonies in the PNW. Hence, for sustainable pollination and apiculture there is an urgent need to understand current honey bee health problems and develop appropriate management tools to overcome these problems. Honey bee colonies are constantly under threat from existing pests and disease complex and also from other new species invasions. Recently honey bee colonies in the U.S have been reported to be infested with Nosema ceranae, a gut parasite which is prevalent in the Asian honey bee. Apparently this new species has jumped hosts and is now dominant in honey bees in the U.S and Europe. Nosema ceranae results in increased winter mortality (Higes et. al 2006). Some studies have implicated Nosema ceranae for the colony collapse disorder. Currently not much is known about the biology and epidemiology of this new Nosema species. Nosema ceranae has threatened the U.S beekeeping industry for the past two years. It appears to be a silent killer without much overt symptoms compared to Nosema apis, which was more common in the U.S. Studies have confirmed that N. apis has been displaced by N. ceranae in the past couple of years. Beekeepers have been forced to treat their colonies prophylactically with the antibiotic Fumagillin to protect their colonies from N. ceranae without information on the presence or absence of Nosema or its threshold. This prophylactic treatment has significantly increased hive management costs for beekeepers, as fumagillin is expensive. Prophylactic use of fumagillin is not only a financial burden for beekeepers, but its excessive use may lead to the development of resistance and the beekeepers might loose this Nosema control tool from their limited arsenal. Also, excessive and unwarranted use of fumagillin can contaminate honey that is consumed by humans. Economic thresholds for Nosema ceranae are not established yet for PNW nor for the U.S. An economic threshold allows the beekeeper or producer to take appropriate and timely control measures and also allows them to pursue non-chemical control tactics to the extent possible. Development of a threshold will primarily help the beekeepers to assess Nosema levels and intervene with chemical treatment accordingly. Thus, it will not only significantly reduce the operating costs but will also aid in judicious use of antibiotic Fumagillin that will help delay the development of Fumagillin resistant strains of Nosema. One of the research and education proposals (SW07-055) funded by Western SARE reports that N. ceranae has almost replaced N. apis within PNW honey bee populations, and colony health ramifications of this novel pathogen are currently unknown. That study was the first one to establish the fact that Nosema ceranae is wide spread in the PNW, but that study has not focused on establishing economic threshold levels for Nosema, its epidemiology and efficacy of existing treatments. Our study focused on these exclusive objectives. Here we proposed to develop an economic threshold for N. ceranae for the PNW and examine epidemiology of Nosema ceranea. We also proposed to develop a reliable sampling protocol to estimate Nosema spore counts from infested bee colonies. The long term goals are to reduce beekeepers' excessive reliance on chemical treatments, decrease the probability of resistance development for the existing chemical treatments and promote cleaner hive products.