Biosecurity risks from weeds in the seed for sowing industry : A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Lincoln University

Abstract

The international trade of crop seed supports global food security and human health, but it also acts as a major pathway for the unintentional introduction of weeds, including invasive species and herbicide resistant biotypes. Many globally established weeds originated as contaminants in agricultural seed lots. Ongoing management of this pathway is essential to protect biosecurity, productivity, and ecosystems, yet published studies on weed seed contamination remain scarce. This thesis addresses that gap using inspection and purity records from three sources: seed lots grown domestically in New Zealand, seed lots imported into New Zealand, and seed lots imported into Canada. The first component examined contamination trends in New Zealand forage seed lots from 1912 to 2019, focusing on perennial ryegrass (Lolium perenne) and white clover (Trifolium repens), two important pasture species in New Zealand. Overall contamination declined three to sixfold, suggesting that herbicides, seed certification, and improved practices have been effective. However, some annual weeds increased or remained prevalent, highlighting emerging risks. Vulpia bromoides and Chenopodium album were the most frequent recent contaminants, while Sherardia arvensis and Poa annua increased significantly, and Rumex acetosella Signiant declined. Perennial ryegrass seed lots contained significantly more grass weed species than white clover. The second component analysed border interceptions of 41,610 seed lots imported into New Zealand from 2014 to 2018, representing 1,420 crop species from over 90 countries. Seed lot contamination was rare, occurring in 1.9% of all imports. However, contamination varied by crop type and was highest in forage crops at 12.6% and lowest in arable crops at 0.5%. Medicago and Trifolium were the most contaminated crop genera, and Chenopodium album was the most common weed. Species classified as quarantine weeds in New Zealand were rare at 0.06%, with Sorghum halepense found only in vegetable seed lots. Larger seed lots were significantly more likely to be contaminated and to contain quarantine weeds, supporting ISTA seed lot size limits. These results suggest that New Zealand’s strict biosecurity policies have been largely effective. The third component examined seed lot contamination in Canada using CFIA inspection data for imports from the USA from 2007 to 2019. Among 2,080 seed lots, 262 contaminant species were reported. Of these, 70% were introduced in Canada, 23% were native, and 7% had not previously been recorded in Canada. Species classified as noxious weeds in Canada comprised 12% of contaminant species, declined over time, and were reported significantly less often than non-noxious species. Species classified as entry prohibited in Canada were rare and limited to four records of Cuscuta spp. Overall, fourteen species not previously recorded in Canada were reported, notably the frequently detected Trifolium vesiculosum, along with Galium parisiense, Torilis nodosa, and Trifolium hirtum, all established in climatically similar regions of the USA. Other weeds, such as Apera spica venti, are limited to one Canadian province but have potential to spread. Weeds with a known history of herbicide resistance in the USA but not in Canada increased significantly over time, including Poa annua and Sorghum halepense. Some imported crops, including Poa annua and Bromus tectorum, are also regulated as weeds in Canada, raising regulatory concerns. Additional concerns include the permissibility of contaminants absent from Canada in seed lots and the dual classification of native species such as Cuscuta campestris as entry prohibited. Together, these three components provide an integrated view of weed seed contamination in the seed for sowing system, highlighting trends, crop specific risks, high risk contaminants, and the role of regulation. The findings emphasise the need for long term monitoring, effective inspection, and adaptive policy to mitigate risks to agriculture and ecosystems while supporting trade.