Brassicas: comparison of treatments to control cabbage root fly

Project summary:

For many years the cabbage root fly (Delia radicum) has been controlled on transplanted brassica crops through the application of an organophosphorus insecticide (chlorpyrifos – Dursban) to the modules prior to transplanting.  However, the future of this treatment is now uncertain.  Within the last decade, an alternative treatment (spinosad – Tracer) has become available to growers, but whilst Dursban has been available, Tracer has not been used widely.  One of the reasons for its limited use is the perception that Tracer is not such an effective treatment.  Now that future use of Dursban is likely to be time-limited, it is important to establish whether there are limitations in the performance of Tracer.

Tracer drench treatments have been evaluated extensively in HDC projects on control of cabbage root fly and in general, when modules are transplanted immediately after treatment, there is little difference (on average) in the levels of control achieved with Tracer and Dursban.  However, until 2013, the performance of Tracer under sub-optimal conditions had not been evaluated, particularly in relation to delays in planting, where the modules have been treated but planting is delayed, often due to adverse weather conditions.   An HDC-funded project in 2013 (FV 416) showed that module drench treatments of Tracer were as effective as Dursban WG at protecting the root zone of transplanted cauliflowers from attack by cabbage root fly larvae.  The efficacy of Tracer was only marginally diminished when planting was delayed for 2 weeks following treatment or by heavy watering of the modules pre-planting. Residue studies suggested that Tracer was at least as persistent as Dursban WG when treated modules were exposed to a series of heavy watering events and stored at maximum moisture capacity.  Overall the project indicated that in most circumstances Tracer is likely to be an acceptable alternative to Dursban WG.

The aim of this new proposed project is to evaluate further the performance of module drench treatments to control cabbage root fly.  This includes a novel product applied as a drench that has been evaluated in the SCEPTRE project and shown to be effective at controlling cabbage root fly.  The project objectives are to 1) assess the performance of pre-planting module drench treatments with Tracer, Dursban and the novel product under ‘normal’ and ‘sub-optimal’ conditions i.e. when planting is delayed post-treatment and 2) compare the performance of post-planting drench treatments (with the same products), and a novel granule treatment, with the pre-planting treatments.  A bio-insecticide applied pre-planting will also be assessed.

The proposed project will also 3) assess the application efficiency in a commercial nursery.  With the likely removal of chlorpyrifos as a modular drench application efficiency (both in terms of mean dose and module-module variability) could become more significant.

Benefits to industry:

Cabbage root fly is a serious pest of brassica crops and most crops are exposed to egg-laying by cabbage root fly females.  It is likely that growers will soon have to rely on Tracer for cabbage root fly control and they need to be aware of any constraints in terms of the relationship between planting time post-treatment and the reliability of cabbage root fly control.  The results will be delivered through the Final Report, an article in HDC News, the HDC Brassica Newsletter and a presentation at an industry meeting. 

Aims and objectives:

Project aim(s):

 The aim of the project is to evaluate the performance of module drench treatments to control cabbage root fly.

 Project objective(s):

1.  Assess the performance of pre-planting module drench treatments with Tracer, Dursban and a novel product (evaluated in the SCEPTRE project) under ‘normal’ and ‘sub-optimal’ conditions i.e. when planting delayed post-treatment.
2. Compare the performance of post-planting drench treatments (with the same products), and a novel granule treatment, with the pre-planting treatments.
3. Assess the application efficiency of module drench treatments in a commercial nursery.  With the likely removal of chlorpyrifos as a modular drench application efficiency (both in terms of mean dose and module-module variability) could become more significant.