Early findings show promise but low pest pressure means more work is required to validate results from a trial using a trap crop as part of a pest management programme to control green vegetable bug (GVB) in sweet corn.
This past summer the A Lighter Touch-Vegetables New Zealand project trialled the use of the trap crop at the Pukekohe Research and Demonstration Farm as part of an integrated pest management (IPM) approach to controlling GVB, a significant crop pest in sweet corn. The project is based on previous research conducted in New Zealand and overseas.
It involved selecting species of plants that flower and set seed that the pest likes to feed on as the trap crop. The trap crop is planted around the outside of the sweet corn, with the goal of aligning the flowering and seeding time of the trap crop with the sweet corn crop.
GVB migrates from the surrounding area into the crop, and encounters the flowering and seeding trap crop first, which provides the pest with a food source they find attractive and will feed on. A non-planted area is maintained as a barrier between the trap crop and the sweet corn, encouraging the pest to stay in the trap crop and continue feeding.
The trap crop surrounding the sweet corn in late March, with the mustard and sunflowers both flowering. Credit Olivia Prouse
Project technical lead Olivia Prouse monitored the crop weekly, using a structured scouting method to assess pest movement between the trap crop and adjacent sweet corn crop. A one metre quadrant was monitored in the trap crop, paired with a two-plant wide by six-plant deep section of sweet corn directly opposite.
Across the course of the season, Olivia found adult GVB and some juveniles in the trap crop, but only one in the sweet corn crop. However, numbers of GVB found were so low, it is planned to repeat the trap crop trial next season to determine whether the trap crop is successful in controlling the GVB and keeping it out of the sweet corn crop, even when pest numbers are high.
Olivia says another factor potentially impacting on GVB pressure was the proximity of the native perennial planting at the farm, located next to the trap crop and the sweet corn. The combination of trap cropping and adjacent native vegetation offered an opportunity to observe how ecological features interact within the cropping system to influence pest dynamics.
This planting is part of the ALT-Vegetables NZ-Onions NZ biodiversity project at the farm, which aims to enhance natural predator populations to assist with pest management. Monitoring of the native planting by Plant and Food Research last winter identified the presence in the native plantings of the small wasp, Trissolcus basalis, which specifically parasitises eggs of GVB and other stinkbug species.
While the success of the trap crop is yet to be determined, another aspect of the project this past season was to identify best trap crop species to use for GVB, and aligning the trap crop flowering with the sweet corn flowering. The aim was for the trap crop to have a staggered flowering so when the sweet corn crop flowers and the cobs are developing, the trap crop remains an attractive food source to the pest.
Olivia says this aspect of the project was very successful, with the selection of species providing continuous flowering from pollination in the sweet corn crop and cob development, right through to harvest.
Project technical lead Olivia Prouse pictured in the sweet corn crop, with the trap crop in the foreground. Credit Howe Young.
The plant species used in the trap crop were pea, sorghum, sunflower, sunflower ‘skyscraper’, soybean and white salad mustard, all of which were selected based on their flowers being an attractive food source for GVB. One early flowering species, soybean, will be left out of the seed mix for next season because it got shaded out under the bulk of the other plant species and didn’t grow as well.
It was the first time sweet corn had been grown at the demonstration farm, and ironically, although GVB was the focus of the project, it was other pests – fall army worm and tomato fruit worm – that caused Olivia and farm manager Howe Young the biggest problems. The harvested crop was sold and while they achieved a good yield, the crop quality was impacted by a late infestation of both pests within the withholding period of effective control options.
Next season’s sweet corn trap crop trial is planned for a late spring planting, compared to this past season’s very late planting date of January.
GVB can cause significant damage in fresh market and processed sweet corn crops. The insect invades crops when cobs are developing, puncturing corn kernels through the wrapper leaves which results in discolouration of the kernel. The discolouration is enhanced in the processing of corn and can lead to whole crops being rejected. In fresh market corn, whole cobs may be rejected if secondary rotting occurs.
The only products registered for the control of GVB in New Zealand were broad-spectrum pesticides which have since been phased out of use by regulators. Finding an alternative control for GVB is a priority for the A Lighter Touch programme as it is a high priority crop protection control gap in sweet corn. Previous trials evaluated biological and synthetic chemistry options, but the results were inconclusive.
The vegetable industry is looking for new solutions for GVB management that will support an IPM approach to crop protection, and trap cropping is a practice that has the potential to be part of that solution.
Continued evaluation of this approach will help inform how biodiversity tools can be practically embedded in crop protection programmes.
