Canola (Brassica napus L.), yellow mustard (Sinapis alba L.) and intergeneric crosses of S. alba×B.
napus were assessed for resistance (antixenosis) to the cabbage seedpod weevil (Ceutorhynchus
assimilis Paykull). Pod trichomes did not appear to be a major factor in the resistance of S. alba to
weevils. The number of feeding punctures and eggs per pod in S. alba was not significantly different
in pods with trichomes than in those where the trichomes had been removed. Choice and no-choice
laboratory tests examining feeding punctures and eggs laid per pod suggested that resistance in S. alba
is not conferred in the intergeneric cross, S. alba×B. napus. Similar data on feeding and weevil
oviposition were found in field test plots. However, despite many eggs being laid in S. alba×B. napus
hybrid plants, fewer cabbage seedpod weevil larvae developed to exit the intergeneric hybrid pods.
Glucosinolate analyses of leaves, pods and seeds showed that S. alba plants have a high concentration
of p-hydroxybenzyl glucosinolate in all three plant parts, but B. napus has no p-hydroxybenzyl.
Interestingly the intergeneric hybrid examined in this study had 62% and 60% of p-hydroxybenzyl
concentration in the leaves and seeds, respectively, than was found in the S. alba parent. However,
pod tissues contained very little (3%) compared with the S. alba parent. It is possible, therefore, that
the adult cabbage seedpod weevil feeds on the pods of the intergeneric hybrid and lays eggs in the pod,
because of the low concentration of p-hydroxybenzyl glucosinolate, but the larvae then fail to develop
as they feed on the seeds containing high concentrations of p-hydroxybenzyl glucosinolate. It should
be noted also that this hybrid produced pods that were more similar in physical shape to canola pods
and that this may also be a factor determining cabbage seedpod weevil feeding and subsequent egg
laying. In addition, both B. napus and the intergeneric hybrid produced 3-butenyl and 4-pentenyl
glucosinolates in their pods, and degradation products (3-butenyl, and 4-pentenyl isothiocyanates)
from these glucosinolate types, are known to be stimulatory kairomones that attract cabbage seedpod
weevil. Further studies are being conducted to examine these factors in more detail.
Biology and harmfulness of Brassica pod midge (Dasineura brassicae Winn.) in winter oilseed rape