References

428 V.Ninkovic,R.Glinwood,J.Pettersson
28.3.1
Allelobiosis and Aphid Response
Aphids belong to widely distributed group of insects, of which numerous
species are serious pests that damage plants mainly by sucking phloem
sap, but also by transmitting plant viruses. Aphids make considerable use of
chemical information in host plant location and selection, and are sensitive
to changes in the quality and physiological status of their host plant (Pickett
et al. 1992). Aphids feed by inserting a long flexible mouthpart, the stylet,
directly into the phloem. Together, these factors make aphids an excellent
model herbivore to detect changes in plants following allelobiosis.
28.3.1.1
Interspecific Allelobiosis and Aphid Host Plant Acceptance
When barley plants of certain cultivars were exposed to allelobiotic chemicals
from common weeds (using the exposure system described earlier),
they became significantly less acceptable to the aphid Rhopalosiphum padi.
This occurred when barley was exposed to root exudates from the couch
grass Elytrigia repens (Glinwood et al. 2003) and to volatiles from the thistles
Cirsium arvense and C. vulgare (Glinwood et al. 2004). R. padi showed
no behavioural response to volatiles from Cirsium spp. directly, and a range
of compounds previously identified in E. repens root exudates did not negatively
affect R. padi feedingonanartificialmedium.Thus,exposureto
allelobiosis from both weeds induced changes in barley plants that made
them less acceptable for aphid feeding.
28.3.1.2
Intraspecific Allelobiosis and Aphid Host Plant Acceptance
When barley plants of one cultivar were exposed to volatiles from plants of
a different cultivar (using the exposure system described earlier), they became
significantly less acceptable to R. padi.Thisoccurredonlyincertain
pairwise combinations of four barley cultivars. This reduction of acceptability
to the aphid also occurred when certain cultivars were exposed to
volatiles from the same cultivar, i.e. self-exposure (Pettersson et al. 1999).
Whenthepairwisecombinationswereplantedtogetherinalternaterows
in the field, certain combinations again led to reduced aphid acceptance of
particular cultivars, compared with pure stands of that cultivar (Ninkovic
et al. 2002) (Fig. 28.4). Whereas in the laboratory system plants interact only
via volatiles, planting in the field allows for interaction via both volatiles
and root exudates, as well as the effects of competition and environmental
factors.