Field and landscape management to support beneficial arthropods for IPM on vegetable farms

Field and landscape management of crop pests, by biological control or by direct effects of vegetation on pests, offer great opportunities to vegetable growers. An initial survey of 491 fields of brassica vegetables, sweetcorn, carrot, lettuce, French bean and lettuce established that pest and beneficial arthropod densities are not uniform within each crop field. Rather, densities are strongly affected by the immediately adjacent land use. Robust effects were evident over the six crop species, multiple geographical regions and several seasons. Overall numbers of pests were suppressed in crops adjacent to riparian areas, roads and waterbodies but elevated adjacent to crops and weedy areas. Natural enemies (‘beneficials’) are likely to have been responsible for some of these effects because overall numbers were elevated in areas of crops adjacent to riparian areas whilst roads (lined by undisturbed vegetation and often with trees) significantly increased parasitic Hymenoptera, predatory beetles, spiders and brown lacewings. Pest suppression did not, however, consistently result from the elevated numbers of beneficials adjacent to weedy areas, crops and shelterbelts. Further, pest suppression was not necessarily dependent on beneficials since pests were suppressed adjacent to waterbodies despite that land use having only negative effects on beneficials, likely reflecting a barrier effect to movement of all insects into adjacent crops.

Results suggest that within-field insect assemblages have strong spatial structure that is strongly affected by
immediately adjacent land use. From a practical perspective, this work is important in showing that, irrespective of any underlying effect of the wider landscape, farmers can exert strong effects on pest management via crop placement in relation to existing land uses, control of weeds, and the preservation and rehabilitation of riparian areas. Partitioning this large data set according to each of the six crop types surveyed allowed the production of a series of practical, evidence-based recommendations for growers that guide crop placement in relation to other land uses on adjoining the farm and for weed control or promotion of riparian vegetation. From a research perspective, the results showed powerfully that there is scope to influence relative densities of pests and beneficials in vegetable crops under Australian conditions. Importantly, however, there is a need to develop approaches based on plants that growers can rapidly establish rather than be reliant on slower -to-establish woody vegetation features such as shelterbelts. This conclusion underpinned the second major phase of the project in which on-farm trials were undertaken. Three annual plants (alyssum, buckwheat and cornflower) and one biennial (yellow rocket) were selected based on a literature review and assessment of practicalities. Trials of in-crop flower strips in New South Wales, Victoria, Queensland and South Australia brassica crops showed that annual plant strips promoted abundance of beneficials such as parasitic wasps and predatory beetles with numbers elevated for up to
20 m into the crop.

Pest numbers and numbers of pest-damaged crop plants were reduced, and parasitism of
diamondback moth doubled. Benefit: cost ratios were as high as 8:1 in cases where the flower strips were
accommodated in uncultivated areas such as sprinkler rows. The biennial plant, yellow rocket, proved to be highly attractive to diamondback moth so has potential as a trap crop, relieving the primary crop of pest pressure, but proved difficult to establish from seed, with the planting of seedlings recommended. In sweetcorn, drought conditions and low water allocations led to work being restricted to three sites in Queensland.

Findings have been communicated in a series of industry–focused magazine and TV features, workshop and farm walk (‘field day’) activities and this has led to significant levels of interest by growers.
Fact sheets have been produced to guide farmers interested in adoption of these practices.