References

430 V.Ninkovic,R.Glinwood,J.Pettersson
28.3.1.3
Intraspecific Allelobiosis and Aphid Olfactory Response
Plant odour plays an important role for aphids in host plant location and
selection (Pickett et al. 1992), and changes in volatile profiles can indicate
plant physiological status. In olfactometer experiments, R. padi was significantly
less attracted to the combined odour of two different barley cultivars
that were allowed to interact via volatiles than to the combined odour from
the same two cultivars that were isolated from each other (Pettersson et
al. 2003). Odour from a plant of a single cultivar that had been previously
exposedtovolatilesfromaplantoftheotherwasalsolessattractivethan
odour from an unexposed plant.
Thisindicatesthatexposuretovolatilesfromadifferentcultivarmay
induce a systemic change in the plant that can lead to a modified volatile
profile. This may be one factor responsible for reduced aphid plant acceptance
of exposed plants.
28.3.2
Allelobiosis and Ladybird Searching Behaviour
The seven-spotted ladybird, Coccinella septempunctata, isanimportant
aphid predator, but its impact as a control agent is variable. The searching
behaviour of the seven-spotted ladybird has been studied from many perspectives
(Dixon 2000), and food-searching behaviour of adults is influenced
by volatiles emitted by barley plants infested with aphids (Ninkovic
et al. 2001).
It seems to be a general phenomenon that increased botanical diversity
reduces the incidence of pests, and enhances the impact of their natural
enemies. So far the effects of aerial allelobiosis between undamaged plants
sharing an environment have not been considered as a factor that can
contribute to this phenomenon. In a field study, the abundance of adult
C. septempunctata was greater in barley plots containing high naturally
occurring densities of the common weeds C. arvense and E. repens than in
other plots containing only barley (Ninkovic and Pettersson 2003).
In a subsequent laboratory study, adult C. septempunctata showed significantly
stronger attraction to mixed odours of barley and each of the
two weeds, than to barley alone. Ladybirds responded differently to barley
plants that had been previously exposed to volatiles from the two weeds.
ThebarleyplantsthathadbeenexposedtoE. repens lost their attraction
when E. repens was removed, whereas barley plants that had been exposed
to C. arvense remained attractive even after C. arvense was removed. This
indicates that volatiles from these weeds can induce effects in barley plants
that affect habitat-searching behaviour by ladybirds.