Know what invertebrates are associated with weed seed control systems

Key messages

• Paddocks in the south are more likely to be refuges for crop pests
• Consider using pitfall traps to locate pests in paddocks

Aims

To better understand how HWSC systems influence invertebrate populations in paddocks.

Introduction

There are three techniques commonly employed in harvest weed seed control (HWSC) systems, chaff dumping, chaff lining and chaff tram-lining. The highest adoption of HWSC is in the GRDC western region with an estimated 67% of all farmers undertaking at least one HWSC strategy in 2014.

Chaff dumping is the collection of the chaff fraction using a cart towed behind the harvester. The chaff in the cart is then dumped, usually in piles in the paddock. The chaff is then either burnt, grazed or left to decompose.

Chaff lining, involves funnelling the chaff fraction of crop residue (containing weed seeds) into a confined row directly behind the harvester using a narrow chute. The chaff and weed seeds are then left to rot down over time. To promote rotting, the chaff lines need to be placed in the same location year after year by running the harvester on a controlled traffic system (CTF) system.

Chaff tramlining is a similar concept to chaff lining, but the chaff fraction is diverted through a chaff deck onto permanent wheel tracks in a CTF system. Wheel traffic creates a hostile environment that inhibits weed seed germination.

There has been a recent system change with more growers opting for leaving chaff in-situ to rot, whether in dumps or in lines rather than burning. This investigations aims to better invertebrate species associated with HWSC systems, which to date are poorly understood.

Method

The methodology for surveying for invertebrates associated with chaff was tested in 20 paddocks before wide scale surveys of HWSC systems commenced. Two methods for invertebrate assessments being pitfall traps and direct surveys of chaff were undertaken in the Geraldton, Esperance, Kwinana West, Kwinana East, and Albany Port zones.

A total of 87 paddocks were surveyed from 2019-2020. An effort was made to identify a similar number of HWSC systems per port zone for this study. However, some HWSC systems are under-represented in certain zones. For instance, chaff tram lining is more common in the Esperance port zone than in the Kwinana port zones (Figure 1).

Two times of sampling occurred: prior to planting and post planting when crops were at the seedling stage.

All invertebrates at each sample time and from each survey method were collected and then identified to species. Each species was divided into functional groups of beneficials (i.e. species that predate on pests or harvest weed seeds) or pests (i.e. species that are known to feed on germinating crops).

Pitfall trapping

Pitfall traps were placed at least 50 metres from any fence line or vegetation.

Pitfall traps consisted of 250 mL containers, dug into the ground so the top lip was level with the surface. Pitfall traps were placed in two rows and kept open for 7 days. Each row consisted of 10 pitfall traps placed at least 10 metres apart. One row was placed adjacent to chaff (near chaff), at a distance of 5 cm from chaff; the second row (far from chaff) was placed parallel to the first at a distance of 20 metres away. In paddocks with chaff lines or tram-lines, pitfall traps were located at least 3 m from a chaff line (Figure 2), where as in paddocks with chaff dumps, the second row was located 20 m from chaff.

Due to paddock variation in the length of chaff dumps, most chaff dumps were less than 100 metres in length. In this case, as many pitfall traps as possible were placed 10 metres apart 5 cm from the chaff on one side of a chaff dump; and then travelling, ideally, in the same direction until another chaff dump was encountered at which point the remaining pitfall traps were placed in a row 10 metres apart, 5 cm from chaff. For each pitfall trap adjacent to chaff, a pitfall trap at least 20 m from a chaff dump was located (Figure 3).

Direct surveys of pests associated with chaff

Adjacent to each trap, a 0.1m² quadrat was placed on the chaff or if the pitfall trap was located in standing stubble then the quadrat was placed on the eastern side of the trap.

All chaff and/or stubble was removed from the quadrats and all invertebrates collected, and assessed.

Results and discussion

Overall, low numbers of invertebrates were found in chaff. Less than 1% of all surveys found a single invertebrate. Of those that were found, 80% were in association with chaff than in standing stubble.

Unlike the direct sampling method, pitfall traps captured invertebrates in every paddock, however, the diversity of invertebrates captured varied between port zones. Pitfall traps located in the Albany port zone captured on average 70% more pests, and pitfall traps located in the Kwinana East port zone captured 70% more weed seed harvesting ants than pitfall traps in other port zones (Figures 4, 5).

If the HWSC system is taken into consideration, pitfall traps adjacent to chaff dumps captured the most (40% more) natural enemies compared to pitfall traps located adjacent to chaff from lines or tramlines. The most abundant natural enemy was weed seed harvesting ants, comprising 90% of the average pitfall trap catch. However, after seeding, there was a 90% reduction in natural enemies caught in pitfall traps (Figure 6).

Pests such as slaters, snails, pest beetles, and European earwigs were 70% more likely to be found in pitfall traps next to chaff from lines or dumps, whereas desiantha weevil was more likely (40% more) to be captured in pitfall traps located in standing stubble. After seeding, pitfall trap catches of desiantha weevil, European earwig, and snails increased, suggesting that these pests are more likely to actively move after seeding has occurred (Figure 7).

Conclusion

Both pests and natural enemies were found in association with chaff, and the species composition depended on the location of the paddock. Paddocks in the Albany port zone were more likely to have higher densities of pests, whereas paddocks located in the Kwinana East port zone were more likely to have beneficial insects, like weed seed harvesting ants.

Pests such as European earwigs, slaters, and snails use chaff as a refuge. Control measures in a paddock could be targeted to areas of the paddock with chaff.

These surveys highlighted that actively looking for invertebrates in paddocks is difficult, but pitfall traps are a useful tool to determine if pests are present.

Acknowledgments

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

The following groups assisted with surveys and data collection. Tis project would not have been possible without their invaluable assistance: Minginew Irwin Group, The Facey Group, Anastazi Agronomy, Stirlings to Coast Farmers, Fitzgerald Biosphere Group, South East Premium Wheat Growers Association, Southern Dirt, Liebe Group, Corrigin Farm Improvement Group.

Contact details

Svetlana Micic
Department of Primary Industries and Regional Development
444 Alban Hwy ALBANY WA 6330
Ph: 08 9892 8591
Email: svetlana.micic@dpird.wa.gov.au