Defensive behaviour in the laboratory – part 03

25 martie 2009

In contrast,
all unparasitized caterpillars continued feeding and moving, any
difference in locomotion before and after the time at which egression
would have taken place (had they been parasitized) was not significant
(Wilcoxon Matched Pairs test: V = 44, P = 0.23). The difference in the
number of parasitized and unparasitized caterpillars moving after the
time of parasitoid egression was highly significant (Fisher’s exact
test: p<0.0001), as was the difference in distance travelled (Fig. 2, KW = 24.0, d.f. = 3, P<0.001).

There were no
significant differences in distance travelled comparing either
parasitized (Kruskal Wallis test: KW = 0, d.f. = 1,
Bonferroni-corrected P = 1) or unparasitized (KW = 0.011, d.f. = 1,
Bonferroni-corrected P = 0.92) caterpillars with and without pupae.
This indicates that the presence of parasitoid pupae does not induce a
change in host behaviour.

Defensive behaviour in the laboratory

When
detecting a predator that was introduced on the twig, 17 out of 19
parasitized caterpillars lashed out at the bug with repeated violent
head-swings (see Movie S1).
Only one of 20 unparasitized caterpillars showed this behaviour,
whereas the others hardly responded to the presence of the predator,
even when it was walking on the host (see Movie S2).
The difference in the number of parasitized and unparasitized
caterpillars that showed head-swings was highly significant (Fisher’s
exact test: P<0.0001). Prior to parasitoid egression, parasitized
caterpillars also do not respond to disturbance with head-swings (A.H.
Grosman and A. Janssen , pers. obs.). Parasitized caterpillars showed a
significantly higher number of head-swings towards the predator than
unparasitized caterpillars (Fig. 3A, GLM with quasi-Poisson errors, F1,37
= 57.6, P<0.001). In more than half of the encounters of a predator
with a parasitized caterpillar, the repeated head-swings caused the
predators either to give up and leave the twig or to be knocked off (Fig. 3B),
and the predators succeeded in contacting the pupae in only 35% of the
interactions. Predators were never knocked off by unparasitized
caterpillars, and gave up in only 15% of the cases (Fig. 3B, difference between parasitized and unparasitized caterpillars: Fisher’s exact test, P = 0.008).

 

 

Figure 3. Effect of parasitism on host-predator interactions in the laboratory.

A
predator was introduced on a twig, 24 cm away from a parasitized or
unparasitized caterpillar, without disturbing the caterpillar. A. Upon
being encountered by a predator, parasitized caterpillars (black bars:
mean+s.e.m.) swung their heads more frequently than unparasitized
(white bars: mean+s.e.m.) caterpillars (***: GLM with quasi-Poisson
errors, F1,37 = 57.6, P<0.001). B. The proportion of
predators that gave up or were knocked off the twig was higher for
parasitized compared with unparasitized hosts (**: Fisher’s Exact Test,
P = 0.008). Numbers of replicates are given in brackets.

doi:10.1371/journal.pone.0002276.g003

Effect of host on parasitoid pupa mortality in the field

In the field, parasitoid pupae were readily attacked by various ant species, predatory bugs such as Supputius spp.,
and four species of hyperparasitoid wasps. Significantly more pupae
were damaged or disappeared from batches of pupae that were exposed to
predators and parasitoids than from unexposed batches in sleeve cages
(average mortality per batch: unexposed = 4.2%1, exposed: 26.6%3.2,
GLM, F1,132 = 10.5, P<0.005). We scored predation in the
exposed batches as the proportion of pupae per batch that had
disappeared or was damaged.

Removal of the caterpillars resulted in a two-fold increase in mortality of batches of parasitoid pupae (Fig. 4A, GLM, F1,116 = 8.25, P<0.005). Contrary to what has been suggested [19], this was mainly due to differences in predation (Fig. 4A, F1,116
= 8.85, P<0.005) and not hyperparasitism, which accounted for only
3.1 (0.8) % mortality and did not differ between treatments (Fig. 4A. F1,116
= 0.09, P = 0.76). Caterpillars disappeared from 25% of the (exposed)
batches of parasitoid pupae in the field. This is likely to be due to
predation because parasitized caterpillars hardly move once parasitoid
larvae egress (Fig. 2),
and caterpillars inside sleeve cages did not disappear. The mortality
in batches of parasitoid pupae from which the caterpillars disappeared
was as high as that in batches from which caterpillars were
experimentally removed (Fig. 4, F1,66 = 0.27, P = 0.60), and much higher than in batches from which the caterpillar survived the period of field exposure (Fig. 4, F1,65
= 23.9, P<0.0001). We do not know whether death of these pupae
occurred before or after the disappearance of the caterpillar, or was
actually causally related to it. Possibly, some predators were
attracted by the caterpillar and subsequently also fed on the
parasitoid pupae. If this were the case, this suggests that there may
also be costs involved with the behavioural changes in the caterpillar:
behavioural changes might attract some predators against which the
caterpillar cannot defend the parasitoid pupae. Nevertheless, the
overall effect of caterpillar presence on survival of parasitoid pupae
was positive (Fig. 4A).
 
 

Figure 4. Effect of removing the guarding host on field mortality of parasitoid pupae.

Twigs
with known numbers of parasitoid pupae were attached to a leaf of a
guava tree (each batch to a different tree) mimicking the natural
situation. The guarding caterpillar was removed at random from 43% of
the batches of parasitoid pupae. A. Total mortality, expressed as mean
proportion of pupae per batch eaten by predators (white bars:
mean?s.e.m.) or hyperparasitized (black bars: mean+s.e.m.). The mean
proportion of pupae lost per batch (presumably eaten by predators) was
significantly lower in the presence of the host (+ host) than when the
caterpillar was absent (- host) (total: ***: GLM with quasi-binomial
errors, F1,116 = 8.25, P<0.005, predation: F1,116
= 8.85, P<0.005). Levels of hyperparasitism per batch were not
significantly different in the presence or absence of the host (F1,116
= 0.09, P = 0.76). B. Of the batches of pupae with host (+ host in A),
total mortality and predation with a live host was lower than when the
host was missing at the end of the period of field exposure (total: **:
F1,65 = 23.9, P<0.0001, predation: F1,65 = 32.7, P<0.0001), but hyperparasitism did not differ significantly between treatments (F1,65 = 2.78, P = 0.10). Numbers of replicates are given in brackets.

doi:10.1371/journal.pone.0002276.g004

 
article possible thanks to this team

part 04 – final discussion is soon to come up

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