Parasitoid turns host into bodyguard – part01

23 martie 2009

Parasitoid Increases Survival of Its Pupae by Inducing Hosts to Fight Predators

Amir H. Grosman1, Arne Janssen 1*, Elaine F. de Brito2, Eduardo G. Cordeiro2, Felipe Colares2, Juliana Oliveira Fonseca2, Eraldo R. Lima2, Angelo Pallini2, Maurice W. Sabelis1

Bookmark: aff11
Institute for Biodiversity and Ecosystem Dynamics, Section Population
Biology, University of Amsterdam, Amsterdam, The Netherlands, Bookmark: aff22 Department of Animal Biology, Section Agricultural Entomology, Federal University of Viosa, Minas Gerais, Brazil

Abstract

Many
true parasites and parasitoids modify the behaviour of their host, and
these changes are thought to be to the benefit of the parasites.
However, field tests of this hypothesis are scarce, and it is often
unclear whether the host or the parasite profits from the behavioural
changes, or even if parasitism is a cause or consequence of the
behaviour. We show that braconid parasitoids (Glyptapanteles sp.) induce their caterpillar host (Thyrinteina leucocerae)
to behave as a bodyguard of the parasitoid pupae. After parasitoid
larvae exit from the host to pupate, the host stops feeding, remains
close to the pupae, knocks off predators with violent head-swings, and
dies before reaching adulthood. Unparasitized caterpillars do not show
these behaviours. In the field, the presence of bodyguard hosts
resulted in a two-fold reduction in mortality of parasitoid pupae.
Hence, the behaviour appears to be parasitoid-induced and confers
benefits exclusively to the parasitoid.

Introduction

Diseases, parasites and parasitoids can induce spectacular changes in the behaviour of their host [1][11]. Some of these changes, such as behavioural fevering [12] and exposure to cold temperatures [13], are thought to benefit the host, but others have been suggested to result in increased transmission of parasites [1], [3], [4], [14][17] or increased survival of parasitoids [18][22]. One of the most famous examples is the parasitic trematode Dicrocoelium dendriticum,
which induces its intermediate host, ants, to move up onto blades of
grass during the night and early morning, and firmly attach themselves
to the substrate with their mandibles [3].
This is believed to enhance parasite transmission due to increased
ingestion of infected ants by grazing sheep, the final host [23].
In contrast, uninfected ants return to their nests during the night and
the cooler parts of the day. Other examples of such spectacular
behavioural changes include parasitoid larvae (Hymenoepimecis sp.) that induce their spider host (Plesiometa argyra) to construct a special cocoon web in which the larvae pupate [7], rodents infected by Toxoplasma that lose their innate aversion to odours of cats, the parasite’s final host [9],
and hairworms that induce their terrestrial arthropod hosts to commit
suicide by jumping into water, after which the hairworms desert the
host to spend their adult stage in their natural habitat [6], [8].

Although many of
these examples are consistent with host manipulation, concern has been
voiced over this interpretation of the existing evidence [1], [2], [4].
For example, supporting evidence for increased transmission of
parasites comes mainly from laboratory studies and consists of
correlations between behavioural changes and a higher risk of predation
of intermediate hosts by the final host [4], [5].
Obviously, fitness consequences for the host and parasite should be
evaluated under field conditions, where the host-parasite complex may
also suffer increased predation from organisms that are not hosts of
the parasite [1], [4], [14], [24].

The key problem with
field experiments is the difficulty in assessing whether a behavioural
change is adaptive for the parasite, adaptive for the host, or actually
represents a non-adaptive and/or accidental pathological side-effect
resulting from infection of the host [1], [4], [10], [18], [25]. Moreover, it is possible that parasites more readily infect or parasitize hosts that behave differently to conspecifics [4], [25]. In the latter case, the observed behaviour would not be a consequence, but rather a cause, of parasitism.

In contrast to the case of true parasites [1], [2], [4],
behavioural changes in parasitoid hosts are hypothesized to result in
increased parasitoid survival through decreased host predation [19][22], [26],
because parasitoids typically die with the host. Although such
behavioural manipulation of hosts by parasitoids has been reported
frequently [19][22], [27], [28], field evidence for the advantages of the behavioural change for parasitoids is even scarcer than for true parasites [10], [18], [21], and is also constrained by the possibility that parasitoids selected hosts with aberrant behaviour [4].

In this study we
present evidence for behavioural changes in a host that are beneficial
to its parasitoid under field conditions. We studied the consequences
of behavioural manipulation of the geometrid moth Thyrinteina leucocerae by its parasitoid wasp (Glyptapanteles
sp., Braconidae) on parasitoid survival in the field in Brazil. Adult
female parasitoids oviposit in first- and second-instar caterpillars of
the moth, which feed on foliage of various trees of the Myrtaceae
family, such as guava and eucalyptus. Parasitized caterpillars continue
developing and feeding until the 4th or 5th
instar, when up to c. 80 full-grown parasitoid larvae egress from the
host to pupate (A.H. Grosman and A. Janssen, pers. obs.). The larvae
spin cocoons on a twig or leaf close to the caterpillar and pupate (Fig. 1).
Subsequently, the host undergoes a series of behavioural changes,
including cessation of feeding and moving. The most profound change in
behaviour, however, is a strong increase of violent head-swings upon
disturbance, in an apparent attempt to hit the agent of disturbance
(A.H. Grosman and A. Janssen, pers. obs.). It has been suggested that
such head-swings could serve as a defence of the parasitoid pupae
against predation or hyperparasitism [20], [22],
but evidence is lacking. We therefore quantified the effects of these
behavioural changes on interactions with predators in the laboratory,
as well as on survival of the parasitoid pupae in the field.
 
 
 

Figure 1. A caterpillar of the geometrid moth Thyrinteina leucocerae with pupae of the Braconid parasitoid wasp Glyptapanteles sp.

Full-grown
larvae of the parasitoid egress from the caterpillar and spin cocoons
close by their host. The host remains alive, stops feeding and moving,
spins silk over the pupae, and responds to disturbance with violent
head-swings (supporting information). The caterpillar dies soon after
the adult parasitoids emerge from the pupae. Photograph by Prof. Jos
Lino-Neto.

doi:10.1371/journal.pone.0002276.g001

10x to this great website
part 02 soon to come up

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