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In behavioural trials, mated H. virescens females spent a signi®-
cantly greater proportion of time (80% of the observed hour) in an
areawith only undamaged plants than in an areawith both damaged
and undamaged plants (Fig. 2A, a). Moths exhibited a tendency to
¯y south when ®rst released; however, if the damaged plants were
placed to the south, the moths would often turn and ¯y in the
opposite direction. The same preference was displayed in oviposition.
Females selected only non-infested plants for oviposition
(Fig. 2A, b; F1,10 = 427, P , 0.0001) and also avoided uninfested
plants close to infested plants.
Because the preference of H. virescens for uninfested plants might
conceivably be explained by visual cues associated with plant
damage, we repeated the behavioural assays using synthetic volatile
blends. Synthetic blends of the major volatile compounds that were
produced in signi®cant amounts and emitted by the plants during
the dark period were formulated on rubber septa19,20 to release
volatiles in approximately the same proportions and amounts as
released by herbivore-damaged plants. In behavioural assays,
H. virescens demonstrated a signi®cant preference (F1,10 = 134, P , 0.0001) for untreated tobacco plants over those with septa
releasing the synthetic blends (Fig. 2B, b). In control trials, the
response to plants with septa containing only solvent (hexane) was
statistically indistinguishable (F1,10 = 1.91, P . 0.05) from that to
untreated plants (Fig. 2B, a). Thus, female H. virescens are able to
identify infested plants on the basis of chemical cues consisting of
volatile compounds emitted by the plants at night.
Some of the volatiles in the nocturnal volatile pro®le were also
present in the diurnal pro®le. However, others were released only at
night. To determine the importance of volatiles produced at night,
relative to those produced during the day, we repeated the behavioural
assays using the diurnal blends. Although these daytime
volatiles produced avoidance behaviour (Fig. 2B, c; F1,10 = 55, P ,
0.001), they were signi®cantly less repellent than the nocturnal
volatiles (Fig. 2B, b), indicating that the moths are speci®cally
repelled by the night-time volatile blends. To further examine the
extent to which the repellence of the night-time volatiles is due to
the presence of compounds produced exclusively at night, the
experiment was repeated using a synthetic blend containing only
these compounds. The presence of these exclusively nocturnal
compounds alone was suf®cient to explain the moth repellence
effect (Fig. 2B, d; F1,10 = 145, P , 0.0001).
It is well established that induced plant volatiles function as
signals between plants and the natural enemies of insect herbivores.
The discovery that the herbivores themselves exploit the information
present in night-time volatile blends to avoid oviposition on
previously damaged plants reveals a new dimension of chemically mediated plant±insect interactions and raises a number of issues.
The ®tness advantages to herbivores of avoiding oviposition on
induced plants are obvious, as such plants are likely to host not only
larvae that represent potential competitors for the moth's offspring
but also potentially a population of natural enemies attracted by the
volatile blend21. Moreover, the associated induction of direct
defence mechanisms means that infested plants are likely to contain
chemical toxins and to be of lower nutritional value than uninfested
plants22,23. It is less clear whether plants bene®t signi®cantly from the
release of nocturnal volatiles or whether such release is merely a
physiological by-product associated with diurnal volatile production.
The protection enjoyed by undamaged plants that reside near
induced plants is interesting, but it is not certain whether this
phenomenon would have been signi®cant in the ancestral environment
or represents only an effect of high-density agricultural
cultivation. If plants do bene®t from advertising their status to
herbivores, it raises the question of whether herbivore-induced
signals ®rst evolved as parasitoid attractants or as herbivore repellents;
alternatively, the dual functions of herbivore-induced plant
signals may have evolved simultaneously.