Overview
- Lucerne seed wasp is a widespread and ongoing production risk, causing an estimated average industry loss of $5.4 million each year.
- Later-maturing crops generally face greater risk. Where practical, earlier crop closure to promote earlier flowering and harvest may help reduce exposure.
- Regular DNA testing, combined with paddock, seasonal and management records, can help growers identify high-risk paddocks and understand the factors contributing to losses.
Lucerne seed wasp (LSW) is a serious pest in lucerne crops causing damage that is difficult to see in the paddock. Its larvae feed inside developing seed, meaning losses may not become apparent until harvest, cleaning or seed delivery, when it is too late to intervene. Each wasp represents a lost seed, with infested seeds removed by cleaning sheds along with other impurities.
Research conducted over six harvest seasons, from 2018 to 2023, used DNA-based testing to investigate LSW and its two principal parasitoid wasps in South Australian lucerne seed crops. Most samples came from the south-east around Keith, with a smaller number from the Mid North. The testing was incorporated into the existing collection of seed samples from loads delivered to participating cleaning sheds for routine golden dodder testing. This provided an efficient way to detect and quantify pests that would otherwise remain largely hidden within the seed.
In total, 4,097 seed delivery samples were tested, with 2,015 linked to individual paddocks and locations. The DNA tests were calibrated to estimate the number of wasps per gram of seed. By analysing these results alongside information on delivery timing, irrigation method, the weight of uncleaned seed and, where available, clean-out rates, the researchers were able to explore how pest prevalence and losses varied between paddocks, regions, seasons and harvest periods.
LSW was detected in every year and across all seed crops tested, confirming that it is an ongoing production risk rather than an occasional outbreak pest. Based on wasp concentrations in seed delivered for cleaning and annual farmgate seed prices, estimated industry losses totalled $32.4 million across the six harvests. This equated to an average of approximately $5.4 million a year, with annual losses ranging from $1.5 million in 2020 to $13 million in 2018. Actual losses may have been higher because the estimates did not include infested seed that was lost before or during harvest.
The monitoring also showed that pest pressure was not evenly distributed. Around 35% of estimated losses occurred in only 10% of paddocks. This suggests that paddock characteristics, seasonal conditions or management practices may strongly influence LSW prevalence. It also highlights the value of paddock-level monitoring, as industry or regional averages may conceal substantial differences between individual crops.
Timing was one of the clearest patterns identified. LSW levels and overall wasp numbers generally increased as the season progressed, with later seed deliveries associated with higher infestation. As lucerne seed crops typically matured between January and April, the findings indicate that later-maturing crops may experience greater exposure to increasing LSW populations.
Where practical, commencing the seed-production phase in early November to promote earlier flowering and harvest may help reduce this risk. Growers delaying until December to obtain additional grazing or hay may face greater LSW exposure. However, earlier flowering should be viewed as a risk-management option rather than a guaranteed control, and the potential reduction in pest losses needs to be weighed against the value of additional fodder production.
DNA testing also detected the parasitoid wasps Idiomacromerus perplexus and Pteromalus sequester. These beneficial insects kill LSW larvae and may contribute to suppressing populations in following seasons, although they do not recover seed already damaged by the pest. Differences in parasitoid populations, climate, crop isolation and farm management may help explain why some crops experienced lower infestation than others, including some harvested later in the season.
For growers, the most important message is that monitoring provides the foundation for effective pest management. A single test reveals what occurred in one crop, but consistent monitoring across several seasons can establish a baseline for each paddock. When DNA results are considered alongside management and seasonal records, growers can begin to understand how their decisions and local conditions affect LSW prevalence. Useful information includes crop closure, flowering and actual harvest dates, rainfall and temperature conditions, crop history, irrigation method, and changes in management.
