Brassica Pod Midge -
Dasineura brassicae
The Brassica pod midge (Dasineura brassicae) is one of many
insect pests that targets oilseed rape.
They lay their eggs in the developing silique (often via holes formed by the pod weevil) (Nijveldt, 1973), the larvae then
feed on the pericarp and subsequently damage seed development. A further consequence
of infestation is that the larvae stimulate atypical pod development that cause the
silique to senesce, followed by premature 'shatter'. D. brassicae is a widespread pest,
although it is generally not considered an economical problem as it mainly confined to the
field margins (Gould, 1975).
Does D. brassicae
Induce Dehiscence?
Meakin & Roberts (1991) investigated the anatomical and biochemical changes that occur
during the induced, premature pod dehiscence of D. brassicae infestation, so as to
identify the mechanisms involved in order to provide further understanding of the natural
process of dehiscence. The aim was to use the information to eventually genetically manipulate
pod development to either delay (ie. shatter after harvest) or prevent dehiscence.
Unfortunately they revealed that the regulation of midge-induced and
maturation-induced shatter is different.
Midge larvae infestation leads to the weakening of the valve attachment, leaving
the pods susceptible to shatter after exposure to physical stress. There
are similarities between midge-induced and natural shatter events, as both exhibit complete and
intact valve shedding from the false septum that ensures larvae escape and seed dispersal
respectively.
They also both show dehiscence zone (DZ) specific separation due to the loss of cellular
cohesion as a result of elevated enzyme activity (cellulase & polygalacturonase [PG]), which
correlates with the elevated activities present during
abscission (Sexton & Roberts, 1982). The stages at which the two mechanisms differ is actually
in the shatter events because during normal shatter the cells in the DZ senesce and lose functional
capacity, whereas the DZ cells of infected pods exhibit cell wall degradation but without apparent
loss of cytoplasmic function (Meakin & Roberts, 1991).
Midge-induced shatter should therefore be regarded as an abscission-like event that requires
physical stress to separate the weakened valve attachment. The exact method of dehiscence/abscission
stimulation is still unknown, however examples of induced leaf abscission of insect damaged
plants have been reported (James & Pritchard, 1988), including the example of pectinase
(eg. PG) secretion by aphids that causes cell wall degradation.
| Pest | Importance on winter rape | Importance on spring rape | Stage of crop attacked | Description of pest | Damage
|
|---|
| Brassica pod midge | 3 | 3 | Late flowering and pod set | Adult:small delicate fly.
Larva:tiny, white, legless. Usually about 10 per pod. | Adults are not damaging.
Larvae feed on pod wall leading to premature shattering.
|
Key:
| - Not present. | 1 Of little importance.
|
| 2 Occasionally important. | 3 Liable to cause damage if present.
|
Life Cycle
