The mate-seeking strategies of four braconid parasitoids

159 Pages·2017·3.28 MB·English

by  Xu Hao

The mate-seeking strategies of four braconid parasitoids
(Hymenoptera: Braconidae)

HAO XU
Thesis direction:
Prof. Ted Turlings
Thesis committee:
Prof. Joachim Ruther (University of Regensburg, GE)
Prof. Sergio Rasmann (University of Neuchatel, CH)
Dr. Gaylord Desurmont (European Biological Control Laboratory,
USDA, FR)

Defended on 16th June, 2017
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Summary
Parasitic wasps are widely used as biological control agents. To optimize their applications in
agriculture, we need to understand how they locate hosts and mate. Parasitic wasps have
about 50,000 described species, but less than 30 species have so far been tested in terms of
their use of pheromones. In this thesis, I used a combination of bioassays, chemical analyses,
and electrophysiological measurements to study the sexual communication in four braconid
parasitoids: Cotesia glomerata (L.), Cotesia marginiventris (Cresson), Microplitis rufiventris
Kokujev and Microplitis mediator (Haliday). Virgin females of both Cotesia species release
sex pheromones to attract conspecific males, which are not attractive to the other species. In
M. rufiventris, virgin males are attractive to females, whereas males and females of M.
mediator exhibit attraction to both sexes. The sex pheromones of the gregarious parasitoid C.
glomerata and the congeneric solitary species C. marginiventris comprise both specific
components and non-specific components. Some non-specific compounds (such as heptanal
and nonanal) are produced by all body parts (heads, thoraxes and abdomens), indicating they
are probably constituents of cuticular lipids (CLs). Some of these non-specific compounds are
components of sex pheromones of the two Cotesia species, but only work synergistically with
other components of their sex pheromone. The pheromone specificity of two Cotesia species
appears to be determined by two specific compounds, which are released only in very small
amounts, and which we, so far, failed to identify. Interestingly, the nonspecific CLs, which
originally are probably part of the cuticular barrier to avoid desiccation and infection by
entomopathgens, may have evolved pheromonal functions linked to the specific biology of the
two Cotesia species. Heptanal is an anti-aphrodisiac in the gregarious species C. glomerata,
probably reducing natal mate competition among sibling males, but in the solitary species C.
marginiventris, heptanal is a sex pheromone constituent that synergistically enhances the
attractiveness of other sex pheromone components. To my knowledge, this thesis presents the
first study showing that CLs can evolve into distinct pheromonal functions. The sex ratio
(percentage of males) of the gregarious parasitoid C. glomerata ranges from 25-67%
depending on how ovipositing females perceive the quality and size of a host patch . In this
species, mating on natal patches (the hosts from which the wasps emerge) is probably strongly
influenced by pheromones: males normally emerge a bit earlier than sibling females, and
males that emerge after that are arrested by emerging virgin females with sex pheromones, but
repelled by other males and mated females, which produce the anti-aphrodisiac heptanal. By
using a combination of attractive sex pheromones and the anti-aphrodisiac, the proportion of
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males mating on the emergence sites probably varies according to the sex ratio (i.e. the level
of male-male competition). Since the pheromones of the four braconid parasitoid species
seem to work in a relatively short range, locating mate may be much more of a challenge for
the studied solitary parasitoid species, as well as for those individuals of gregarious C.
glomerata that leave their natal patch and try to find mates in other patches. Interestingly,
virgin parasitoids of both sexes of all four braconid parasitoids were found to be strongly
attracted by herbivore-induced plant volatiles (HIPVs), implying that host-damaged plants
probably serve as rendezvous sites for mate-seeking individuals, in addition to being sites
where the females find hosts for their offspring. Similar strategies have been reported for
herbivorous insects and pollinators: host plant volatiles stimulate these insects to produce (or
release) pheromones, and/or synergistically increased the attractiveness of pheromones to
mate-seeking conspecifics. Based on the chemical properties (volatility) of typical insect
pheromones and those of relevant plant volatiles, as well as recent theoretical and
experimental advances in our understanding of the odour plumes that they form, I propose
that insect pheromones and plant volatiles serve complimentary functions in mate location. I
postulate that in many insects the use of plant volatiles has evolved into an efficient foraging
strategy to not only find food (or host), but also mates.

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Table of Contents
CHAPTER 1 Introduction and thesis outline .......................................................................................... 9
CHAPTER 2 Exceptional use of sex pheromones by parasitoids of the genus Cotesia: males are
strongly attracted to virgin females, but are no longer attracted to or even repelled by mated females 21
CHAPTER 3 The use of attractive and repellent sex pheromones in a gregarious parasitoid .............. 41
CHAPTER 4 The dynamic use of cuticular lipids as pheromonal cues in two Cotesia parasitoids ..... 71
CHAPTER 5 Combined use of herbivore-induced plant volatiles and sex pheromones for mate
location in braconid parasitoids ............................................................................................................. 97
CHAPTER 6 General discussion: Plant volatiles as mate finding cues for insects ............................. 121
CONCLUSIONS and OUTLOOKS .................................................................................................... 153
ACKNOWLEDGEMENTS ................................................................................................................ 157
CURRICULUM VITAE ..................................................................................................................... 159

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CHAPTER 1 Introduction and thesis outline

Hao XU

Laboratory of Fundamental and Applied Research in Chemical Ecology (FARCE), Institute of
Biology, University of Neuchâtel, CH-2000 Neuchâtel, Switzerland.

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