Estimating the populations of three species of Volucella

in an English woodland

Roger MorrisStuart Ball

Volucella species studied

V. inflata

V. bombylans

V. pellucens

Study site – Old Sulehay Forest, BCNP Wildlife Trust

Reserve

• 35 hectares of ancient semi-natural woodland• Managed as hazel coppice with oak/ash standards before 1940s• Main ride is well managed and open

Sampling sites in the wood

20 visitsfrom 7 June to 27 July 2003

Marking

Numbers marked and recaptured

Capture

V. bombylans

V. inflata V. pellucens

Total

1 73 158 574 805

2 22 21 87 130

3 2 3 22 27

4 3 3

Total 97 182 686 985

160 out of 985 marked flies were recaptured at least once = 16.25%

Some practical lessons

• The consistency of paint is important - too thin and the insect gets covered (and may be fatally damaged)

• Species with hairy thoraxes are much harder to mark clearly

• Constant effort is needed to ensure that new recruits to the population are marked before they displace existing members or disperse themselves

V. bombylans population estimates

0

20

40

60

80

100

120

140

160

180

200

Est

imat

ed p

opul

atio

n wit

h 95%

con

fide

nce

inte

rval

φ = 0.714 (0.244)

φ = estimate of daily

survival (standard error)

V. inflata population estimates

-500

0

500

1000

1500

2000

2500

Est

imate

d p

opula

tion w

ith 9

5%

confi

dence

inte

rval

φ = 0.853 (0.027)

V. pellucens population estimates

-400

-200

0

200

400

600

800

1000

1200

1400

Esti

mat

ed p

opul

atio

n wi

th 9

5% c

onfi

denc

e in

terv

al

= 0.765 (0.013)

Summary of population estimates

Estimated population size with 95%

confidence interval

V. bombylans 200 (142, 339)

V. inflata 656 (468, 1097)

V. pellucens 1963 (1669, 2384)

Estimate of the total population of each speciesusing Schnabel’s method, Krebs (1989)

Some emerging questions?

• How big is the population of V. bombylans ?

• The wood is only 35 hectares! It is difficult to believe:• There are sufficient sap runs large

enough to support the larvae of 650 V. inflata

• There are sufficient social wasps’ nests to support 2,000 V. pellucens

Longevity

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10 11 12 14 15 16 17 18 19 20 22 24 36

Days af ter fi rst capture

Num

ber

of

reca

ptur

es

bombylans infl ata pellucens

Distance moved by recaptured individuals of V.

bombylans

Female Male

Recaptured at the same station

5 50% 8 32%

Moved within a ride

4 40% 13 52%

Moved between rides

1 10% 4 16%

Total recaptures 10 25

Average distance moved (m)

84 122

0%

20%

40%

60%

80%

100%

female male

Moved between rides

Moved within ride

Same station

Behaviour of V. bombylans

• Mating strategy well known: males perch on prominent foliage and dart out at passing insects

• Our results suggest males are quite mobile. Therefore likely male does not return to the same perch each day

• Males moved more than females

Distance moved by recaptured individuals of V.

inflata

Female Male

Recaptured at the same station

4 36% 17 65%

Moved within a ride

6 55% 6 23%

Moved between rides

1 9% 3 12%

Total recaptures 11 24

Average distance moved (m)

136 64

0%

20%

40%

60%

80%

100%

female male

Moved between rides

Moved within ride

Same station

Behaviour of V. inflata

• Both sexes mainly captured around flowers

• Males make rapid flights around nectar sources (bramble & shrubs)

• Only one observation of copulation: male flew high round dogwood (Cornus sanguinea) and briefly coupled with female in flight

• Males moved much less than females• Some evidence that males held a territory

Distance moved by recaptured individuals of V.

pellucens

Female Male

Recaptured at the same station

4 36% 249 78%

Moved within a ride

5 45% 45 15%

Moved between rides

2 18% 18 7%

Total recaptures 11 318

Average distance moved (m)

164 55

0%

20%

40%

60%

80%

100%

female male

Moved between rides

Moved within ride

Same station

Details of movements of male

V. pellucens

At the same station

Moved within ride

Moved between

rides

7-14 June

17 7% 14 30% 12 55%

23-29 June

28 11% 23 49% 5 23%

7-13 July

65 26% 7 15% 5 23%

17-27 July

139 56% 3 6% 0 0%

Consecutive captures of the same individual

0%

20%

40%

60%

80%

100%

7-14June

23-29June

7-13July

17-27July

Moved between rides

Moved within ride

Same station

Behaviour of male V. pellucens

Visiting flowers

Resting on foliage

Hovering

7-14 June

77 63% 4 3% 41 34%

23-29 June

9 5% 1 1% 161 94%

7-13 July

44 18% 2 1% 205 82%

17-27 July

20 8% 2 1% 219 91%0%

20%

40%

60%

80%

100%

7-14 June 23-29June

7-13 July 17-27 July

HoveringResting on foliageVisiting flowers

Behaviour of V. pellucens

• Mating strategy well known: males hover and defend an air space, dart after passing insects

• Male hovering extremely sensitive to sunshine:• When the sun goes in they rapidly perch in

trees• When it comes out again, equally rapidly

resume hovering

• Copulation observed just once. Coupling appears to be very brief and in flight, like V. inflata

Behaviour of V. pellucens (2)

• Both sexes visit flowers early during emergence and are fairly mobile

• Later on males almost exclusively caught hovering and become very immobile

• Some evidence that males hold a territory, sometimes over several days

• As season progressed, it was much more likely an individual male would be found at the same hovering post

• Suggests fierce competition for hovering posts early on leads to rapid turnover

Potential for further study

• Our knowledge of V. bombylans population size is incomplete

• Detailed survey for potential breeding sites is needed to better understand the population dynamics of V. inflata

• Questions remain over the degree to which V. pellucens moves from breeding sites to suitable territories for mate-acquisition

ANY QUESTIONS?