Phenotypic variation in male and worker encapsulation response in the bumblebee Bombus terrestris

Abstract.  1. Life-history traits such as immunity are often characterised by the presence of large phenotypic variation, but it often remains unclear how and why this variation is maintained by selection.
2. Here an annual social insect, the bumblebee Bombus terrestris , was used to study variation in encapsulation response of males and workers. Bumblebees are a suitable system to study offspring immunity because they are host to a broad variety of different parasites. Bumblebee males, in particular, have a long lifespan compared with other social insect males and their immunity should therefore be an important element for colony reproductive success.
3. Encapsulation response, which was used here as a measurement for the generalised immune defence capacity of an individual, was found to be a highly variable trait. High levels of worker response correlated with low levels of colony parasitism rates.
4. Encapsulation response was found to be (a) lower in males compared with sister workers, and (b) lower in late-produced cohorts compared with early ones.
5. In colonies with delayed sexual reproduction, males had a lower encapsulation response. Thus, investments into immunity seemed reduced in later male cohorts and those eclosing later in the season, perhaps because males had a shorter expected remaining time to acquire matings. The results presented add further evidence that immune defence is a key variable defining colony fitness in social insects.     

Microsatellite loci for Bombus spp.

We report the details and characteristics of a total of 44 novel microsatelllite loci for Bombus spp. Most of them are highly polymorphic in Bombus terrestris, and a high degree of polymorphism is also found where these primers have been tested in 10 other bumblebee species. These markers will therefore be useful for the genetic study of this group.     

Correlates of parasite load in bumblebees in an Alpine habitat

Essentially, all animals face parasites, but little data are available on the rate of parasitism in wild animals, particularly in insects. Here, we report observations of more than 400 bumblebee workers collected at an Alpine site, including the parasites observed (Crithidia bombi, Nosema bombi, conopid parasitoid fly larvae and tracheal mites), as well as date of collection, bumblebee species and body variables (size, fat content, egg development and antibacterial activity). Among the 14 bumblebee species collected, C. bombi and tracheal mites reached a prevalence of approximately 10 and 6%, respectively, while conopids and N. bombi were almost absent. Correlations among the measured parameters suggest that larger workers are more likely to develop eggs and contain more tracheal mites. Across the season, we found a decrease in fat content but an increase in C. bombi and mite prevalence. Mites’ fitness was higher in fatter bees and lower in bees with more tracheal mites. Antibacterial activity was found in approximately 10% of the workers, suggesting at least sporadic infection with bacteria.     

Correlates of reproductive success among field colonies of Bombus lucorum: the importance of growth and parasites

Abstract.

• 1 In natural populations, colonies of bumble bees vary in many important life history traits, such as colony size and age at maturity, or the number and sex of reproductives produced. We investigated how the presence of parasites in field populations of the bumble bee Bombus lucorum L. relates to variation in life history traits and reproductive performance. A total of thirty-six colonies was placed in accessible nest sites in the field and monitored at regular intervals throughout a season.
• 2 Among the life history correlates, early nest foundation was strongly associated with large maximum colony size, old age and large size at maturity, and this in turn with successful production of males and queens, as well as with the number of sexuals produced. Overall, reproductive success was highly skewed with only five colonies producing all the queens. Sixteen colonies failed to reproduce altogether.
• 3 The social parasite Psithyrus was abundant early in the Bombus colony cycle and preferentially invaded host nests with many first brood workers and thus disproportionately large size, i.e. those colonies that would otherwise be more likely to reproduce or produce (daughter) queens rather than males. To prevent nest loss, Psithyrus had to be removed soon after invasion. Therefore, the effects reported here can only be crude estimates.
• 4 Parasitoid conopid flies are likely to cause heavy worker mortality when sexuals are reared by the colonies. Their inferred effect was a reduction in biomass that could be invested in sexuals as well as a shift in the sex ratio at the population level resulting from failure to produce queens. We suggest to group the inferred correlates into ‘early events’ surrounding colony initiation and social parasitism, and ‘late events’ surrounding attained colony size in summer and parasitism by conopid flies. Our evidence thus provides a heuristic approach to understand the factors that affect reproductive success of Bombus colonies.

     

How do bees choose flight direction while foraging?

ABSTRACT. Various authors have reported that nectar-collecting bees usually maintain an overall degree of directionality in their successive foraging flights. I here ask what kind of cues honeybees (Apis mellifera ) use to decide on departure direction from one inflorescence to the next. In a horizontally placed flower array, individual workers were experimentally rotated while imbibing sugar solution from an artificial flower mounted on a turntable. Rotation itself did not affect the subsequent departure direction. However, rotating the bees by 180° resulted in choices that were rotated by the same amount, as compared to the control treatment (i.e. no rotation). I suggest that flight directionality is determined in bees from the spatial orientation of their body before departure (e.g. by moving to the nearest flower they face).     

Larval development of two parasitic flies (Conopidae) in the common host Bombus pascuorum

Abstract.

• 1 Whenever parasitism by more than one female occurs, larvae of parasitoids not only have to resist host defence but also face competition with other (unrelated) larvae. Competition is particularly important in solitary parasitoids where only one larva is able to complete its development. Such a situation is found in Conopidae (Diptera) parasitizing adult bumble bees where larvae of two species of conopid flies, Sicus ferrugineus L. and Physocephala rufipes F. often compete within the common host Bombus pascuorum Scopoli. This study analysed the larval development of the two species and asks how competition among larvae may be regulated.
• 2 Parasitized workers of B.pascuorum were caught in the field and kept according to different experimental schedules in the laboratory. This provided stage-structured data for the temporal course of development of the parasitic larvae. For the analysis, a simulation model was constructed that estimated the duration of all parasitic stages (Manly, 1990, first method). In both species the egg stage was found to be approximately 2 days, first instar 3 days, second instar 4 days, and third instar 3 days. The total development time is an estimated 10.8 days from oviposition in S.ferrugineus and 11.4 days in P.rufipes. S.ferrugineus develops faster in the beginning, probably because of its larger egg size, whereas P.rufipes pupates at larger size. First-instar larvae of both species possess strong, pointed mandibles.
• 3 The success of conopid larvae seems only marginally affected by host defence, for a single larva per host almost always completes development. Under competition, however, mortality rate increases substantially, and most larvae die in their first instar. Moreover, they show signs of melanization. The estimates for developmental times and the patterns found in this study suggest that conopid larvae seem capable of physical attacks, particularly during the first instar, when elimination of competitors is most common, and that S.ferrugineus has a time advantage because of its faster early development. Because most studies have previously been carried out with hymenopteran parasitoids, this study provides new information about the other large group of parasitoid insects, the Diptera, and demonstrates convergent patterns.

     

Molecular Divergence Defines Two Distinct Lineages of Crithidia bombi (Trypanosomatidae), Parasites of Bumblebees

ABSTRACT. This study provides, for the first time, sequence data for the protozoan flagellates Crithidia bombi and Crithidia mellificae (Kinetoplastea: Trypanosomatidae). We amplified the partial sequences of the small subunit ribosomal RNA (SSU rRNA), glycosomal glyceraldehyde phosphate dehydrogenase (gGAPDH), cytochrome b (Cyt b), and the complete internal transcribed spacer region 1 (ITS1) of the ribosomal RNA gene region for 66 clones of C. bombi from Switzerland and Alaska. Furthermore, we sequenced the same stretch of SSU rRNA and gGAPDH for one isolate of C. mellificae from Switzerland. All four molecular markers classified the C. bombi samples into two distinct lineages A and B. Both lineages were found in the two sampling locations. Variation within lineages was small or non‐existing. Sequence differences between lineages were 1.64% for SSU rRNA, 4.36% for gGAPDH, and 12.02% for Cyt b. The ITS1‐sequences of lineages A and B have diverged so much that no alignment was possible. With regard to ITS1, we additionally found fragment length polymorphism (variation in microsatellite repeat numbers) as well as nucleotide diversity within each lineage. Furthermore, the sequences of SSU rRNA and gGAPDH of C. mellificae were different from both lineages of C. bombi. The separation of lineages A and B, based on sequence differences and phylogenetic reconstruction, is so pronounced as to characterize two species of “C. bombi.” We propose to retain C. bombi for the more common lineage A and suggest the name Crithidia expoeki n. sp. for lineage B.     

Parasitic flies (Conopidae, Diptera) may be important stress factors for the ergonomics of their bumblebee hosts

Bumblebees harbour a wide range of parasitic organisms that attack all stages of their life cycles (reviews in Postner, 1951; Pouvreau, 1973, 1974; Alford, 1975; Kistner, 1982). Among them, conopid flies (Conopidae, Diptera) are particularly interesting because they attack foraging bumblebees which are handling flowers, or even on the wing (Frison, 1926; Cumber, 1949; Postner, 1951; Howell, 1967; Askew, 1971). A single egg is attached to (Frison, 1926; Plath, 1934; Cumber, 1949) or inserted into (DeMeijre, 1904; Howell, 1967) the host's abdomen, where the larva hatches and feeds on haemolymph and internal organs. Within 6–10 days the larva passes through three recognizable stages (Pouvreau, 1974) before the fly pupates in situ within the abdomen. The host bee dies shortly before the parasite pupates (Postner, 1951; Smith, 1966) and the parasite overwinters in its puparium; the adult fly then emerges in early summer (Frison, 1926; Townsend, 1935; Cumber, 1949; Postner, 1951). Conopid flies as parasites of bumblebees are known from all major habitats where the hosts occur (e.g. Kröber, 1939; Smith, 1966). However, the effect of parasitism on distribution and abundance of bumblebees is not known. In this preliminary note we have estimated degrees of infestation and concomitant reduction of life span in affected workers. The results are compared with literature reports on infestation levels in Europe.