Attraction of the Larval Parasitoid Spintherus dubius (Hymenoptera: Pteromalidae) to Feces Volatiles from the Adult Apion Weevil Host
J Insect Behav
Attraction of the Larval Parasitoid Spintherus dubius (Hymenoptera: Pteromalidae) to Feces Volatiles from the Adult Apion Weevil Host
N. Faraone 0
G. P. Svensson 0
O. Anderbrant 0
0 Department of Biology, Lund University , Sölvegatan 37, SE-223 62 Lund , Sweden
The behavioral response of the larval parasitoid Spintherus dubius (Hymenoptera: Pteromalidae) to volatile compounds derived from its Apion weevil hosts was investigated in two-choice bioassays. Odor source candidates were the larval and adult stages of weevils, clover flowers, and feces from adult weevils. Despite S. dubius being a larval parasitoid, the odor of weevil larvae isolated from the clover flowers was not attractive to female parasitoids. Surprisingly, S. dubius females were instead attracted by the odor from the feces of adult weevils. The female parasitoids were similarly attracted to the feces produced by the two main hosts, the red clover weevil (A. trifolii) and the white clover weevil (A. fulvipes). Chemical analysis of the volatile composition of feces produced by the two hosts revealed qualitatively similar odor profiles, correlating with the observed attraction by the parasitoid towards both odor sources. Some of the identified volatile compounds are commonly present in clover plant headspace fractions and may function as a kairomone to facilitate orientation by S. dubius to Apion-infested clover flowers. Larval and adult weevils were not attractive for parasitoid females, whereas, for the white clover weevil-plant association, infested flowers were highly attractive. These data show the use by the clover weevil parasitoid of an alternative source of olfactory information for locating its host.
Larval parasitoid; oviposition cues; kairomone; host feces; Apion trifolii; Apion fulvipes
Introduction
Insect herbivores as well as their natural enemies rely heavily on olfactory cues for
locating food and oviposition sites, even at great distances. In the case of parasitoids,
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volatiles from the insect host itself
(Cournoyer and Boivin 2004; Chuche et al. 2006;
Obeysekara and Legrand 2014)
, from the host habitat (Rutledge 1996), from sources
indirectly related to the host
(Steidle and Schöller 1997; Sullivan and Berisford 2004)
or their combination with plant volatiles released during herbivore attack
(Obeysekara
et al. 2014; Wäschke et al. 2014)
may contribute to host localization. Moreover
parasitoids may use different chemical volatiles emitted from different sources in
various stages of the host location process
(Rutledge 1996)
.
Spintherus dubius (Nees, 1834), a small hymenopteran wasp (2 mm long), is the
most common parasitoid species found in clover fields
(Kruess and Tscharntke 1994;
Lundin 2013)
, and the main hosts are weevils of the genus Apion, which are important
pests in clover seed production (Hansen and Boelt 2008). In Sweden, S. dubius mainly
attacks A. fulvipes (Geoffry, 1785) feeding on white clover, Trifolium repens L., and
A. trifolii (L., 1768) feeding on red clover, T. pratense L. (
Gønget 1997
). These weevils
overwinter below dead leaves or grass, and in the spring overwintered adults emerge
and locate host plants where they feed and mate
(Freeman 1967)
. Mature weevils start
to mate during the early flowering phenology of the clover host, and females oviposit in
the developing clover head. The weevil larvae feed on the clover seeds and they stay
inside the floret/flower head during all their development
(Notini 1935; Kruess and
Tscharntke 1994; Nyabuga et al. 2015)
. They pupate inside the flower, and only when
they reach their adult stage, they leave the flowers and emerge as young weevils.
After mating, female S. dubius are observed walking around on the clover flowers,
drumming the florets with the antennae (i.e. antennation) as for searching cues for
localizing the host (personal observation) (Fig. 1). Once located, females deposit an egg
inside the weevil larva (medium to late instar), and the parasitoid larva feeds on the host
larval body causing its death as a final result. The host location ability seems being
highly developed in the female parasitoid that is able to detect the presence of a weevil
larva, even when surrounded by a very complex odor background
(Kigathi et al. 2009;
de Rijk et al. 2016)
.
Whether S. dubius uses olfactory cues in the host location and recognition processes
has not been investigated. In the present study we aimed to identify the source of
olfactory cues that the female parasitoid uses for locating its Apion host in infested
clover flowers. The selection of the odor stimuli was made based on the most potential
candidate sources for host location cues and odors that characterized the common
background bouquet of a clover field. Potential odor sources were volatiles from clover
flowers, from larvae and adult weevils, and from feces of adult weevils, and we used
them in bioassays to evaluate the response of S. dubius females to diffe (...truncated)