The honeybee, brood pheromone is a blend of 10 fatty acid esters (methyl and ethyl linoleate, linolenate, oleate, palmitate, and stearate) produced in the larval salivary glands (LeConte et al., 1990, 2006). Brood pheromone has been shown to increase protein content in the brood food-producing glands of nurse bees, thus increasing the quality of the diet fed to larvae and the queen (Mohammedi et al., 1996; Pankiw et al., 2004; Peters et al., 2010). It also stimulates queen egg laying rate (Sagili and Pankiw, 2009).
Honeybee adult workers show plasticity in age-based and hormonally regulated division of labor by adjusting their behavioral development pattern in response to shifts in colony conditions (Huang and Robinson, 1992). When synthetic brood pheromone was dispensed by offering minute amounts on a glass plate suspended between the combs of a hive, it had a remarkable impact on foraging by worker bees, including: increased ratio of pollen to nonpollen foragers (Pankiw and Page, 2001); more pollen foragers by up to 150% (Pankiw, 2004; Pankiw and Page, 2001; Pankiw and Rubink, 2002); more pollen forager trips per unit time (Pankiw, 2007); and greater weight of pollen returned to the hive (Pankiw, 2004; Pankiw et al., 2004).
In seed crops, adequate pollination is the key for high seed quality and yield. Many hybrid seed crops are not very attractive to honeybees and they may be easily lured to alternative flowering plants that are more attractive (Delaplane and Mayer 2000). In spite of introduction of high number of honeybees, significantly lower number of bees was found foraging on hybrid carrot seed crop (Howlett, 2012). Improved production practices and new varieties may increase the yield of seed crops (Rao and Stephen, 2009), but there is a dearth of studies on managing pollinators as a potential method of improving yield.
Several tactics have been employed over the years to enhance pollination efficiency of bees, predominantly in crops that are not very attractive to bees because they might either produce low quantities and quality of nectar and pollen or environmental conditions are suboptimal during their bloom period. Bee attractants ranging from sugar syrup to bee pheromones such as Nasanov and queen mandibular gland pheromone have been tested to enhance bee visitation and crop yields with meager success (Delaplane and Mayer, 2000) or no success (Ellis and Delaplane, 2009; Mayer et al., 1989; Roper et al., 1990; Schultheis et al., 1994). Bee visitation, seed yield, and germination percent could be increased by pheromone-based bee attractants in niger [Guizotia abyssinica (Sivaram et al., 2013)]. Using queen mandibular pheromone (QMP) sprays, yield was increased in cranberry [Vaccinium oxycoccos (Currie et al., 1992a)], blueberry [Vaccinium corymbosum (Currie et al., 1992a)], and pear [Pyrus communis (Currie et al., 1992b)], but not apple [Malus ×domestica (Currie et al., 1992b)].
Central Oregon accounts for ≈85% of hybrid carrot seed crop grown in the United States (Hart and Butler, 2004). This region offers warm day temperatures and cooler nights that are ideal for hybrid carrot seed production. Hybrid carrot seed production is based on the seed-to-seed method or the root-to-seed method. The seed-to-seed method is most common in central Oregon where parent lines are planted in August and the crop is harvested the next year during September. An isolation distance of 1 mile is recommended to avoid cross-pollination with other hybrid carrot seed varieties and wild carrot if present in the production. In August, parent lines are planted in rows on 30-inch centers. Four rows of female plants are alternated with two rows of male plants with a blank row separating male and female rows. There is not much known regarding volume and concentration of nectar or amounts of pollen produced by these hybrid carrot varieties grown in central Oregon. Honeybees are extremely important pollinators for carrot seed crop in this region, and the fields on average are typically provided with three honeybee colonies per acre for pollination.
SuperBoost [Contech Enterprises, Delta, BC, Canada (Fig. 1)], a commercial product based on the honeybee brood pheromone, was used in this study. It comprises a small pheromone-permeable plastic pouch containing 180 mg of a synthetic blend that precisely mimics the natural pheromone composition (Pankiw et al., 2010); the pouch is suspended in a rigid plastic holder between the frames at the level of the brood comb (Lait et al., 2012). Tertiary-butylhydroquinone, a food-grade antioxidant, is added as 0.05% of the total composition to stabilize the methyl and ethyl esters of linolenic, linoleic, and oleic acid.
When tested experimentally, synthetic brood pheromone has been shown to increase the ratio of pollen to nonpollen foragers (Pankiw et al., 2011), stimulate workers to bring heavier pollen loads back to the hive (Pankiw et al., 2011), enhance the vigor of colonies during spring build-up (Moeri et al., 2011) and fall feeding (Sagili and Breece, 2012), improve overwintering survival (Lait et al., 2012), and increase the amount of honey harvested (Borden et al., 2013; Foster et al., 2011; Lait et al., 2012). One synthetic brood pheromone device lasts for over 5 weeks (Pankiw et al., 2011).
Some field studies have shown brood pheromone to increase foraging activity of honeybees, but to our knowledge, there is a dearth of studies that have examined if this increased foraging activity actually leads to increase in crop yield. The only study that exists on this subject investigated the effect of synthetic brood pheromone on watermelon (Citrullus lanatus) yield in a greenhouse (Guerra Sanz and Roldán Serrano, 2008). Here, we examined whether brood pheromone treatment of colonies would increase foraging activity in honeybee colonies and thus increase yield of a horticultural crop (hybrid carrot seed). The specific objectives of this study were to 1) examine foraging activity (flower visitations in the fields, number of foragers entering colonies in 5-min intervals) of bees in fields where honeybee colonies received brood pheromone and fields with colonies that did not receive brood pheromone (controls) and 2) evaluate and compare crop yield between hybrid carrot seed fields in which colonies received brood pheromone treatment and control fields with colonies that did not receive brood pheromone.
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