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Caterpillar-induced nocturnal plant volatiles repel conspecific females
Plants respond to insect herbivory by synthesizing and releasing
complex blends of volatile compounds, which provide important
host-location cues for insects that are natural enemies of herbivores
1±3. The effects of these volatile blends on herbivore behaviour
have been investigated to only a limited extent4,5, in part
because of the assumption that herbivore-induced volatile emissions
occur mainly during the light phase of the photoperiod6,7.
Because many mothsÐwhose larvae are some of the most important
insect herbivoresÐare nocturnal, herbivore-induced plant
volatiles have not hitherto been considered to be temporally
available as host-location cues for ovipositing females. Here we
present chemical and behavioural assays showing that tobacco
plants (Nicotiana tabacum) release herbivore-induced volatiles
during both night and day. Moreover, several volatile compounds
are released exclusively at night and are highly repellent to female
moths (Heliothis virescens). The demonstration that tobacco
plants release temporally different volatile blends and that lepidopteran
herbivores use induced plant signals released during the
dark phase to choose sites for oviposition adds a new dimension to
our understanding of the role of chemical cues in mediating
tritrophic interactions.
Feeding by insect herbivores induces plants to release chemical
signals that serve as important foraging cues for parasitoids and
predators, and thus enhance the plants' defence1±3,8±10. Synthesis and
release of these chemical signals is an active physiological process
triggered by substances in the oral secretion of herbivores11,12. The
recent discovery that plant volatiles can transmit herbivore-speci®c
information that allows natural enemies to identify particular
herbivore species demonstrated that chemically mediated plant±
insect interactions are more sophisticated and complex than was
previously appreciated13. However, the role of chemical signals in
plant±herbivore interactions remains largely unexplored. Some
researchers have examined the effects of constitutive plant volatiles
and herbivore-induced daytime volatiles on conspeci®c herbivores4,14
±17 including some lepidopterans18, but the effect of herbivore-
induced plant volatiles on moths that are active at night has
been neglected. The fact that several major terpene components of
herbivore-induced plant volatiles have high emissions during the
periods of maximal photosynthesis6,7 may explain why little attention
has been paid to the importance of these volatiles to female
moths searching for oviposition sites at night. To our knowledge,
this study represents the ®rst demonstration that plants emit
herbivore-induced volatile blends that exhibit systematic temporal
variation, that some volatile compounds are released exclusively at night, and that female moths exploit these speci®c night-time
signals to avoid oviposition on previously damaged plants.
Gas chromatographic analysis of volatiles collected in two-hour
intervals continuously for seven days revealed consistent differences
in the composition of volatile blends released by H. virescensinfested
tobacco plants (n = 6) during the light and dark phases
of the photoperiod (Fig. 1a). Visual and auditory observations
con®rmed that larvae fed during both the light and dark phases.
Seven major compounds were consistently released during both
light and dark phases, but usually in lesser amounts during the dark
phase (Fig. 1a). In addition, ®ve compounds ((Z)-3-hexenyl butyrate,
(Z)-3-hexenyl isobutyrate, (Z)-3-hexenyl acetate, (Z)-3-hexenyl
tiglate, and one unidenti®ed compound) were produced only
during the dark phase. OthersÐ(E)-2-hexenal and three unidenti-
®ed compoundsÐwere produced in signi®cantly larger amounts
during the dark than the light period. Thus, the qualitative and
quantitative composition of volatile blends emitted by tobacco
plants in response to feeding by H. virescens larvae can differ
signi®cantly between night and day.
Repetition of our analysis using two other species of lepidopteran
larvae (n = 6 per species), Manduca sexta and Helicoverpa zea,
provided further evidence of induced volatile release from tobacco
plants during the dark period (Fig. 1b). Although the volatile pro®les revealed quantitative differences in plant response to the three herbivores, all caterpillar species induced release of the same
volatile compounds from tobacco during the night (Fig. 1). To
determine whether plants continue nocturnal volatile production in
the absence of continuous feeding by the caterpillars, H. virescens
were removed from the plants at 15:00 after 48 hours of feeding. In
this case, plants still emitted volatiles during the dark phase
although in smaller amounts.