Theresa L. Pitts-Singer

The long summer: Pre-wintering temperatures affect metabolic expenditure and winter survival in a solitary bee

The impact of climate change on insect populations depends on specific life cycle traits and physiological adaptations. The solitary bee Osmia lignaria winters as a pre-emergent adult, and requires a period of cold temperature for winter diapause completion. It is a univoltine species, and diapause induction does not depend on photoperiod. To understand the potential effects of longer summers on O. lignaria populations, we exposed individuals to three treatments simulating early, mid and late winter arrivals, and measured respiration rates, metabolic expenditure, weight loss, fat body depletion, lipid levels and winter mortality. The early-winter treatment disrupted diapause development, but had no apparent negative effects on fitness. In contrast, late-winter bees had a greater energetic expenditure (1.5-fold), weight (1.4-fold) and lipid (2-fold) loss, greater fat body depletion, and a 19% increase in mortality compared to mid-winter bees. We also monitored adult eclosion and arrival of winter temperatures under natural conditions in four years. We found a positive correlation between mean degree-day accumulation during pre-wintering (a measure of asynchrony between adult eclosion and winter arrival) and yearly winter mortality. Individually, bees experiencing greater degree-day accumulations exhibited reduced post-winter longevity. Timing of adult eclosion in O. lignaria is dependent on the duration of the prepupal period, which occurs in mid-summer, is also diapause-mediated, and is longer in populations from southerly latitudes. In a global warming scenario, we expect long summer diapause phenotypes to replace short summer diapause phenotypes, effectively maintaining short pre-wintering periods in spite of delayed winter arrivals.

Olfactory cues and nest recognition in the solitary bee Osmia lignaria

Abstract.  The use of olfactory cues for nest recognition by the solitary bee Osmia lignaria is studied in a greenhouse environment. Glass tubes are provided as nesting cavities to allow the in‐nest behaviour of bees to be observed. In addition, each glass tube is cut into three sections for experimental manipulation and for subsequent chemical analysis. Nesting females drag their abdomen along the tube before exiting, spiral inside the tube, and sometimes deposit fluid droplets from the tip of the abdomen. For the manipulation, the outer section, the middle section, or both sections are removed and replaced with similar clean glass tube sections, and the behaviour exhibited by test females is recorded upon arrival in front of the nesting site and inside the nesting tubes. The resulting hesitation behaviour displayed by females after treatments appears to indicate the loss of some olfactory cues used for nest recognition inside the entire nest. Chemical analysis of the depositions inside the nesting tube, as well as analysis of the cuticular lipids of the nesting bees, reveals the presence of free fatty acids, hydrocarbons and wax esters.

Nesting site density and distribution affect Osmia lignaria (Hymenoptera: Megachilidae) reproductive success and almond yield in a commercial orchard

The potential of commercially managed, native blue orchard bees, Osmia lignaria, to augment honey bees in orchard pollination depends on various factors, particularly how to enhance O. lignaria retention while optimising even pollination throughout orchards by varying their stocking density and nesting site distribution. In 2011, we investigated how artificial nest box density and the number of cavities within nest boxes influenced O. lignaria retention and reproduction in a 61 ha almond orchard pollinated by a mixture of O. lignaria and Apis mellifera in the southern Central Valley of California. We assessed how localised O. lignaria nesting affected total nut yield. Retention of O. lignaria females was significantly greater in orchard areas with high density nest boxes compared with areas with low density nest boxes. Females preferred to nest in high density (low cavity) nest boxes compared to low density (high cavity) nest boxes. All measures of O. lignaria reproductive success were greater in orchard areas with high density nest boxes than areas with low density nest boxes. Localised O. lignaria foraging and nesting activity influenced nut yield by producing more nuts in orchard areas with high density nest boxes compared with low density nest boxes, although differences in nut yield just failed to meet statistical significance. Results of this study show that the density and distribution of nest boxes for nesting O. lignaria females can strongly influence the reproductive success of an alternative, managed bee pollinator in a large production orchard.

Effects of the insect growth regulator, novaluron on immature alfalfa leafcutting bees, Megachile rotundata

Abstract Alfalfa leafcutting bees, Megachile rotundata F. (Hymenoptera: Megachilidae), are the most common pollinators of alfalfa in the Pacific Northwest. Reports from users of M. rotundata in Idaho, Utah and Colorado have indicated exceptionally poor bee return from fields treated with novaluron to control Lygus spp. Our goal was to evaluate novaluron toxicity to immature M. rotundata using two different possible mechanisms of exposure. One goal was to assess immature mortality via treating nectar-pollen provisions and adults with novaluron. Immature M. rotundata mortality in all novaluron provision dosing treatments was significantly higher than the water or blank controls, providing evidence that novaluron is toxic to progeny in nest cells. The mean cumulative frequency showed that more eggs and 1st–2nd instars died compared to older instars. Female M. rotundata nested similarly in field cages during the field cage experiment; however, there was greater immature mortality in cages where females were fed sugar-water + novaluron compared to sugar-water only. Although females provided adequate provisions, there was a low percentage of egg hatch and larval development when females ingested novaluron before mating and nesting. Novaluron was also present in egg provision of bees collecting resources from novaluron-sprayed plants. At least 84% of progeny died when the females were allowed to mate and nest 24 hours after a novaluron application. Novaluron could be contributing to poor bee return in alfalfa grown for seed. Timely insecticide applications to suppress Lygus spp. is an important consideration to improve ongoing bee health.

Do Weather Conditions Correlate with Findings in Failed, Provision-Filled Nest Cells of Megachile rotundata (Hymenoptera: Megachilidae) in Western North America?

Abstract Cavity-nesting alfalfa leafcutting bees, Megachile rotundata (F.) (Hymenoptera: Megachilidae), are excellent pollinators of alfalfa, Medicago savita L., for seed production. In commercial settings, artificial cavities are placed in field domiciles for nesting and, thereby, bee populations are sustained for future use. For this study, cells from leafcutting bee nests were collected in late summer from commercial seed fields. Over 3 yr (2003–2005), 39 samples in total of ≈1,000 cells each were taken from several northwestern U.S. states and from Manitoba, Canada. X-radiography of 500 cells from each sample was used to identify “pollen balls” (i.e., cells in which the pollen–nectar provision remained, but the egg or larva, if present, was not detectable on an x-radiograph). Most U.S. samples seemed to have higher proportions of pollen ball cells than Manitoba samples. Pollen ball cells were dissected to determine the moisture condition of the mass provision and true contents of each cell. Most pollen ball cells from Manitoba samples contained fungus, the frequency of which was positively correlated with cool, wet weather. In the United States, most pollen ball cells had moist provisions, and many of them lacked young brood. Correlation analysis revealed that pollen ball cells occurred in greater proportions in fields with more hot days (above 38°C). Broodless pollen ball cells occurred in greater proportions under cool conditions, but dead small larvae (second–third instars) seemed to occur in greater proportions under hot conditions. Pollen ball cells with unhatched eggs and first instars (in the chorion) occurred in lesser proportions under hot conditions.

Olfactory Response of Megachilid Bees, Osmia lignaria, Megachile rotundata, and M. pugnata, to Individual Cues from Old Nest Cavities

Abstract The megachilid bees Osmia lignaria Say, Megachile rotundata (F.), and M. pugnata Say were tested for attraction to various components associated with their old nest cavities, or chemical extracts of these components, using a Y-tube olfactory response bioassay. Female bees of these species are known to nest in or near old nest cavities, implying that remnant nest components are important cues for bees looking for nest cavities. Significant results show that female bees were attracted to components that may provide species-specific cues or indicate conspecific nesting activity. Specifically, O. lignaria females showed attraction only to the female cocoon. M. rotundata females were attracted to intact nest cells, the fecal material on the outside of a cocoon, leaf pieces used as nest cell lining, and the extract of leaf pieces. M. pugnata females were attracted to the whole nest cell, the paper straw nesting material with attached cocoon, and feces.

Examination of “pollen balls” in nests of the alfalfa leafcutting bee, Megachile rotundata

SUMMARY Nests of Megachile rotundata, the alfalfa leafcutting bee, were removed from alfalfa fields in the early autumn. X-radiography was used to analyse nests and revealed that 20–50% of the bee cells contained so-called ‘pollen balls’, a term commonly used in the USA to indicate cells that are provisioned but lack healthy eggs or developing larvae. Dissection of these cells showed that most contained pollen-nectar provisions with collapsed eggs or early, pre-defecating larvae, and a moderate proportion of them had a provision onto which no egg had been laid. Other cells contained incomplete or no provisions along with older dead larvae, beetle larvae and their faeces, or saprophytic fungi (as hyphae). Most provisions were at least somewhat moist, with 25% being very dry. A significant difference in the moisture condition and in the proportions of cell content categories was found among the bee populations examined. These results help direct future studies into understanding why bee cells are provisioned with nectar and pollen but fail to produce mature larvae, and why there are differences in the types of failure between commercial populations. X-ray analysis is inadequate for determining the true moisture condition and content of pollen ball cells. The use of the term pollen ball should be reserved for categorizing data from X-rays of bee cells, but because of its ambiguous nature should not be used to implicate bee mortality.

Cuticular lipids of female solitary bees, Osmia lignaria Say and Megachile rotundata (F.) (Hymenoptera: Megachilidae).

The cuticular lipids of the cavity-nesting adult female solitary bees, Osmia lignaria Say and Megachile rotundata (F.) (Hymenoptera: Megachilidae), were analyzed by gas chromatography (GC) and combined GC-mass spectrometry. The cuticular lipids of these female bees are mainly consisted of hydrocarbons. For O. lignaria, nearly 64% of the cuticular lipids were C(25)-C(31) mono-alkenes. For M. rotundata, 48% of the cuticular lipids were C(23)-C(33) alkanes with nearly the same quantities of the same chain-length mono-alkenes (45%). For the mono-alkenes of O. lignaria, 14 mono-alkene constituents were identified, with two of these, 9-heptacosene and 7-nonacosene, comprising 67% of the total alkene distribution. For M. rotundata females, the mixtures of mono-alkenes were more complex with 26 constituents identified and quantified. For the M. rotundata mono-alkenes, 57% of the total composition consisted of the three alkenes, 7-pentacosene, 9-pentacosene and 7-heptacosene. For both bee species, small quantities of C(40)-C(48) wax esters were also characterized with the major components possessing a C(18) mono-unsaturated fatty acid (9-octadecenoate) moiety esterified to even-carbon number (C(22-30)) fatty alcohols. The possible role of these cuticular lipids as nest recognition chemicals is discussed in light of nesting behavior of managed crop pollinators.

Effects of Fungicide and Adjuvant Sprays on Nesting Behavior in Two Managed Solitary Bees, Osmia lignaria and Megachile rotundata.

There is a growing body of empirical evidence showing that wild and managed bees are negatively impacted by various pesticides that are applied in agroecosystems around the world. The lethal and sublethal effects of two widely used fungicides and one adjuvant were assessed in cage studies in California on blue orchard bees, Osmia lignaria, and in cage studies in Utah on alfalfa leafcutting bees, Megachile rotundata. The fungicides tested were Rovral 4F (iprodione) and Pristine (mixture of pyraclostrobin + boscalid), and the adjuvant tested was N-90, a non-ionic wetting agent (90% polyethoxylated nonylphenol) added to certain tank mixtures of fungicides to improve the distribution and contact of sprays to plants. In separate trials, we erected screened cages and released 20 paint-marked females plus 30–50 males per cage to document the behavior of nesting bees under treated and control conditions. For all females in each cage, we recorded pollen-collecting trip times, nest substrate-collecting trip times (i.e., mud for O. lignaria and cut leaf pieces for M. rotundata), cell production rate, and the number of attempts each female made to enter her own or to enter other nest entrances upon returning from a foraging trip. No lethal effects of treatments were observed on adults, nor were there effects on time spent foraging for pollen and nest substrates and on cell production rate. However, Rovral 4F, Pristine, and N-90 disrupted the nest recognition abilities of O. lignaria females. Pristine, N-90, and Pristine + N-90 disrupted nest recognition ability of M. rotundata females. Electroantennogram responses of antennae of O. lignaria females maintained in the laboratory did not differ significantly between the fungicide-exposed and control bees. Our results provide the first empirical evidence that two commonly used fungicides and a non-ionic adjuvant can disrupt nest recognition in two managed solitary bee species.

Prewinter Management Affects Megachile rotundata (Hymenoptera: Megachilidae) Prepupal Physiology and Adult Emergence and Survival

ABSTRACT The alfalfa leafcutting bee, Megachile rotundata F. (Hymenoptera: Megachilidae), is widely used as a pollinator for production of alfalfa, Medicago sativa L., seed, and populations of these bees can be maintained by alfalfa seed growers or can be purchased from mostly Canadian bee providers. M. rotundata raised in Canada have higher survival rates during the incubation that occurs after winter storage than do bees produced in the northwestern United States, but no reason has been found for this difference. We investigated whether storing immature M. rotundata for various time periods at a warm temperature (16°C) before winter or allowing them to remain unmanaged at ambient temperatures affects physiological aspects of prepupae during the winter as well as the survival and longevity of adult bees after spring or summer incubation. Our results show that the timing of the onset of winter storage and incubation does affect prepupal weights, prepupal lipid and water contents, adult emergence, and adult female longevity. Winter storage of prepupae in November or December with a late June incubation resulted in heavier adults that emerged more readily than bees incubated in late May. However, adult females incubated in May thrived longer than June-incubated bees if fed a honey-water diet. Thus, some prewinter management regimes for M. rotundata commercial stocks may be more effective than others for achieving optimal adult emergence synchrony, as well as adult survival and longevity for pollination of a summer crop.