Parasitoid Induced Digging Behavior in Bumblebee Workers

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Parasitoid Induced Digging Behavior in Bumblebee
Workers
Christine B. Muller Anim. Behav. 1994 48
961-966
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Bombus terrestris
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Canopid fly life cycle

• Female fly attacks a foraging bumblebee worker
  and oviposits an egg in the abdomen.
• Larva develops inside the host and kills it in
  10-12 days.
• Larva pupates inside the carcass of the dead
  host and must overwinter wherever the host dies.

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• Parasite Manipulation Parasitoid changes
  behavior of worker so the host dies in a
  protected site.
• Host Suicide Host dies so as to increase risk
  of death to parasitoid pupa.
• Pathology no pattern to where hosts dies and
  no effect on either host inclusive fitness or
  parasitoid success.

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• Experiment 1
• Death Sites of Parasitized Hosts

• Workers collected in field kept in large
  containers with 15 cm soil layer and debris.
• Noted if worker died on surface, under leaves or
  buried themselves in soil.

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Experiment 2 Effect of Host Death Site on
Hibernation Success of Fly

• Parasitized hosts placed in two hibernation
  conditions on surface or buried. Containers
  placed outside and allowed to overwinter.
• When flies emerged the following March, were
  killed and measured for weight, size and wing
  deformities.

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Effect of overwintering above vs. below ground
on Conopid fly

• Fresh weight (mg) (F1,52 6.45 P 0.014)
• Dry weight (mg) (F1,52 21.11 P lt 0.001)
• Thorax width (mm) (F1,52 10.23 P 0.002)
• Wing length (mm) (F 1,47 3.60 P 0.064)

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Conopid flies that emerged with
Malformed Normal Overwintered Wings Wings
Above ground 8 (33) 16 Below ground 2
(6) 29 (X2 6.57
df 1 P 0.014)
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Experiment 3 Effect of Host Death Site on
Predation and Hyperparasitism

• Each host body (parasitized or not) was attached
  by a thread to a fixed spot and then buried under
  10 cm soil or left exposed on surface in the
  woods.
• Each body examined every day for 13 days and
  recorded if vanished or destroyed.
• Surviving carcasses brought into lab and
  dissected to check for hyperparasitoids.

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Fate of Bumblebee Corpses
Not Destroyed Destroyed Above
ground 6 (12) 44 (N 50) Below
ground 15 (36) 27 (N 42)
(X2 5.91 df 1
P 0.015)
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Pupae with Hyperparasitoids
Above ground 3 of 5 Below ground 0
of 8
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Conclusion Altered behavior of hosts supports
Parasite Manipulation Hypothesis

• Benefits Benefits Benefits
• Parasitoid Host Neither
• Digging behavior
• of host
• -- lowers predation YES NO NO
• -- lowers hyperpara. YES NO NO
• Flies from buried
• hosts
• -- weigh more YES NO NO
• -- are bigger YES NO NO
• -- fewer deformities YES NO NO

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Hypotheses for Altered Behavior of Parasitized
Hosts

• Parasite manipulation
• Host suicide
• Pathology

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Altered behavior of Bumblebees Parasitized by
Conopid flies

• Schmid-Hemple and Muller (1991) parasitized
  workers spend more time outside of nest and may
  abandon colony. Benefits the parasitoid, not the
  host.
• Poulin (1992) Benefits the hosts inclusive
  fitness, by acting as target for other flies
  and reducing energy drain on colony.
• Muller and Schmid-Hemple (1992) No kin benefits
  to host bumblebees dont forage in kin groups
  and could still contribute by foraging.

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Bodies on Soil Surface
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Bodies below Soil Surface
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Possible Mechanism for Altered
BehaviorDecreased JH Levels

• Young mated queens dig to hibernate in soil in
  autumn and have low JH levels.
• Bumblebees infected with nematode, Sphaerularia
  bombi exhibit digging behavior and have low JH
  levels.
• Parasitoid may therefore promote digging behavior
  by influencing JH levels of host.