Relative impacts of Varroa destructor (Mesostigmata:Varroidae) infestation and pesticide exposure on honey bee colony health and survival in a high-intensity corn and soybean producing region in northern Iowa.

The negative effects of Varroa and pesticides on colony health and survival are among the most important concerns to beekeepers. To compare the relative contribution of Varroa, pesticides, and interactions between them on honey bee colony performance and survival, a 2-year longitudinal study was performed in corn and soybean growing areas of Iowa. Varroa infestation and pesticide content in stored pollen were measured from 3 apiaries across a gradient of corn and soybean production areas and compared to measurements of colony health and survival. Colonies were not treated for Varroa the first year, but were treated the second year, leading to reduced Varroa infestation that was associated with larger honey bee populations, increased honey production, and higher colony survival. Pesticide detections were highest in areas with high-intensity corn and soybean production treated with conventional methods. Pesticide detections were positively associated with honey bee population size in May 2015 in the intermediate conventional (IC) and intermediate organic (IO) apiaries. Varroa populations across all apiaries in October 2015 were negatively correlated with miticide and chlorpyrifos detections. Miticide detections across all apiaries and neonicotinoid detections in the IC apiary in May 2015 were higher in colonies that survived. In July 2015, colony survival was positively associated with total pesticide detections in all apiaries and chlorpyrifos exposure in the IC and high conventional (HC) apiaries. This research suggests that Varroa are a major cause of reduced colony performance and increased colony losses, and honey bees are resilient upon low to moderate pesticide detections.

Effects of flight activity and age on oxidative damage in the honey bee, Apis mellifera.

Frequent and highly aerobic behaviors likely contribute to naturally occurring stress, accelerate senescence and limit lifespan. To understand how the physiological and cellular mechanisms that determine the onset and duration of senescence are shaped by behavioral development and behavioral duration, we exploited the tractability of the honey bee (Apis mellifera) model system. First, we determined whether a cause-effect relationship exists between honey bee flight and oxidative stress by comparing oxidative damage accrued from intense flight bouts to damage accrued from d-galactose ingestion, which induces oxidative stress and limits lifespan in other insects. Second, we experimentally manipulated the duration of honey bee flight across a range of ages to determine the effects on reactive oxygen species (ROS) accumulation and associated enzymatic antioxidant protective mechanisms. In bees fed d-galactose, lipid peroxidation (assessed by measuring malondialdehyde levels) was higher than in bees fed sucrose and age-matched bees with a high and low number of flight experiences collected from a colony. Bees with high amounts of flight experience exhibited elevated 8-hydroxy-2'-deoxyguanosine, a marker of oxidative DNA damage, relative to bees with less flight experience. Bees with high amounts of flight experience also showed increased levels of pro-oxidants (superoxide and hydrogen peroxide) and decreased or unchanged levels of antioxidants (superoxide dismutase and catalase). These data implicate an imbalance of pro- to anti-oxidants in flight-associated oxidative stress, and reveal how behavior can damage a cell and consequently limit lifespan.

Inhibitory effect of indole analogs against Paenibacillus larvae, the causal agent of American foulbrood disease.

Paenibacillus larvae, a Gram-positive bacterium, causes American foulbrood (AFB) in honey bee larvae (Apis mellifera Linnaeus [Hymenoptera: Apidae]). P. larvae spores exit dormancy in the gut of bee larvae, the germinated cells proliferate, and ultimately bacteremia kills the host. Hence, spore germination is a required step for establishing AFB disease. We previously found that P. larvae spores germinate in response to l-tyrosine plus uric acid in vitro. Additionally, we determined that indole and phenol blocked spore germination. In this work, we evaluated the antagonistic effect of 35 indole and phenol analogs and identified strong inhibitors of P. larvae spore germination in vitro. We further tested the most promising candidate, 5-chloroindole, and found that it significantly reduced bacterial proliferation. Finally, feeding artificial worker jelly containing anti-germination compounds to AFB-exposed larvae significantly decreased AFB infection in laboratory-reared honey bee larvae. Together, these results suggest that inhibitors of P. larvae spore germination could provide another method to control AFB.

Pteridine levels and head weights are correlated with age and colony task in the honey bee, Apis mellifera.

Background. The age of an insect strongly influences many aspects of behavior and reproduction. The interaction of age and behavior is epitomized in the temporal polyethism of honey bees in which young adult bees perform nurse and maintenance duties within the colony, while older bees forage for nectar and pollen. Task transition is dynamic and driven by colony needs. However, an abundance of precocious foragers or overage nurses may have detrimental effects on the colony. Additionally, honey bee age affects insecticide sensitivity. Therefore, determining the age of a set of individual honey bees would be an important measurement of colony health. Pteridines are purine-based pigment molecules found in many insect body parts. Pteridine levels correlate well with age, and wild caught insects may be accurately aged by measuring pteridine levels. The relationship between pteridines and age varies with a number of internal and external factors among many species. Thus far, no studies have investigated the relationship of pteridines with age in honey bees. Methods. We established single-cohort colonies to obtain age-matched nurse and forager bees. Bees of known ages were also sampled from colonies with normal demographics. Nurses and foragers were collected every 3-5 days for up to 42 days. Heads were removed and weighed before pteridines were purified and analyzed using previously established fluorometric methods. Results. Our analysis showed that pteridine levels significantly increased with age in a linear manner in both single cohort colonies and colonies with normal demography. Pteridine levels were higher in foragers than nurses of the same age in bees from single cohort colonies. Head weight significantly increased with age until approximately 28-days of age and then declined for both nurse and forager bees in single cohort colonies. A similar pattern of head weight in bees from colonies with normal demography was observed but head weight was highest in 8-day old nurse bees and there was no relationship of head weight with age of foragers. Discussion. Although the relationship between pteridine levels and age was significant, variation in the data yielded a +4-day range in age estimation. This allows an unambiguous method to determine whether a bee may be a young nurse or old forager in colonies with altered demographics as in the case of single cohort colonies. Pteridine levels in bees do not correlate with age as well as in other insects. However, most studies used insects reared under tightly controlled laboratory conditions, while we used free-living bees. The dynamics of head weight change with age is likely to be due to growth and atrophy of the hypopharyngeal glands. Taken together, these methods represent a useful tool for assessing the age of an insect. Future studies utilizing these methods will provide a more holistic view of colony health.

Genetics, Synergists, and Age Affect Insecticide Sensitivity of the Honey Bee, Apis mellifera.

The number of honey bee colonies in the United States has declined to half of its peak level in the 1940s, and colonies lost over the winter have reached levels that are becoming economically unstable. While the causes of these losses are numerous and the interaction between them is very complex, the role of insecticides has garnered much attention. As a result, there is a need to better understand the risk of insecticides to bees, leading to more studies on both toxicity and exposure. While much research has been conducted on insecticides and bees, there have been very limited studies to elucidate the role that bee genotype and age has on the toxicity of these insecticides. The goal of this study was to determine if there are differences in insecticide sensitivity between honey bees of different genetic backgrounds (Carniolan, Italian, and Russian stocks) and assess if insecticide sensitivity varies with age. We found that Italian bees were the most sensitive of these stocks to insecticides, but variation was largely dependent on the class of insecticide tested. There were almost no differences in organophosphate bioassays between honey bee stocks (<1-fold), moderate differences in pyrethroid bioassays (1.5 to 3-fold), and dramatic differences in neonicotinoid bioassays (3.4 to 33.3-fold). Synergism bioassays with piperonyl butoxide, amitraz, and coumaphos showed increased phenothrin sensitivity in all stocks and also demonstrated further physiological differences between stocks. In addition, as bees aged, the sensitivity to phenothrin significantly decreased, but the sensitivity to naled significantly increased. These results demonstrate the variation arising from the genetic background and physiological transitions in honey bees as they age. This information can be used to determine risk assessment, as well as establishing baseline data for future comparisons to explain the variation in toxicity differences for honey bees reported in the literature.

Effects of Flight on Gene Expression and Aging in the Honey Bee Brain and Flight Muscle.

Honey bees move through a series of in-hive tasks (e.g., "nursing") to outside tasks (e.g., "foraging") that are coincident with physiological changes and higher levels of metabolic activity. Social context can cause worker bees to speed up or slow down this process, and foragers may revert back to their earlier in-hive tasks accompanied by reversion to earlier physiological states. To investigate the effects of flight, behavioral state and age on gene expression, we used whole-genome microarrays and real-time PCR. Brain tissue and flight muscle exhibited different patterns of expression during behavioral transitions, with expression patterns in the brain reflecting both age and behavior, and expression patterns in flight muscle being primarily determined by age. Our data suggest that the transition from behaviors requiring little to no flight (nursing) to those requiring prolonged flight bouts (foraging), rather than the amount of previous flight per se, has a major effect on gene expression. Following behavioral reversion there was a partial reversion in gene expression but some aspects of forager expression patterns, such as those for genes involved in immune function, remained. Combined with our real-time PCR data, these data suggest an epigenetic control and energy balance role in honey bee functional senescence.