The phytosanitary risks posed by seeds for sowing trade networks

Abstract

When successful, the operation of local and international networks of crop seed distribution or “seed systems” ensures farmer access to seed and impacts rural livelihoods and food security. Farmers are both consumers and producers in seed systems and benefit from access to global markets. However, phytosanitary measures and seed purity tests are also needed to maintain seed quality and prevent the spread of costly weeds, pests and diseases, in some countries regulatory controls have been in place since the 1800s. Nevertheless, seed contaminants are internationally implicated in between 7% and 37% of the invasive plant species and many of the agricultural pests and diseases. We assess biosecurity risk across international seed trade networks of forage crops using models of contaminant spread that integrate network connectivity and trade volume. To stochastically model hypothetical contaminants through global seed trade networks, realistic dispersal probabilities were estimated from quarantine weed seed detections and incursions from border security interception data in New Zealand. For our test case we use contaminants linked to the global trade of ryegrass and clover seed. Between 2014 and 2018 only four quarantine weed species (222 species and several genera are on the quarantine schedule) warranting risk mitigation were detected at the border. Quarantine weeds were rare considering that average import volumes were over 190 tonnes for ryegrass and clover, but 105 unregulated contaminant species were allowed in. Ryegrass and clover seed imports each led to one post-border weed incursion response over 20 years. Trade reports revealed complex global seed trade networks spanning >134 (ryegrass) and >110 (clover) countries. Simulations showed contaminants could disperse to as many as 50 (clover) or 80 (ryegrass) countries within 10 time-steps. Risk assessed via network models differed 18% (ryegrass) or 48% (clover) of the time compared to risk assessed on trade volumes. We conclude that biosecurity risk is driven by network position, the number of trading connections and trade volume. Risk mitigation measures could involve the use of more comprehensive lists of regulated species, comprehensive inspection protocols, or the addition of field surveillance at farms where seed is planted.

Factors impacting the detection of weed seed contaminants in seed lots

“…In the case of New Zealand, the border verification protocol, inspecting all seed lots at a higher rate, mostly addresses the problem. Nevertheless, there have been at least two post‐border detections (incursions) of the regulated species Alopecurus myosuroides in New Zealand after it established in fields sown with imported perennial ryegrass and linseed seed from the UK in 2007 and France in 2021, respectively 10,49 . These post‐border incursions are being successfully managed, highlighting the value of post‐border interventions.…”

“…We chose these two crop species because they provide a large proportion of temperate grazing forage in New Zealand 25 . Also the importation of basic seed of these two species for multiplication and reexport is a common profitable enterprise for New Zealand farmers 10 . New Zealand is the largest global clover seed exporter, providing 16% of the global supply and is the fourth largest ryegrass seed exporter, providing 11% of the global supply 26,27 .…”

“…At the field‐level, management of pests, weeds and diseases can be mitigated by seed treatments or seed cleaning. The effectiveness of such measures is confirmed by seed inspection, including analytical purity tests, and seed certification, carried out by seed laboratories linked to regulatory authorities or commercial seed companies 10 . At a national/international level, the detection of regulated or problem seed contaminants can be mitigated through removal of seed lots from certification, destruction, cleaning or reshipping.…”

“…The evidence for pests other than weeds is that imperfect prevention efforts are cost effective 11,12 . Detections of new seeds of concern at the border prior to clearance in New Zealand, are rare, given the high inspection rates, with seed cleaning and inspections occurring offshore prior to shipment, and on arrival in New Zealand 2,10 . Post‐border incursions ( i.e ., detection of newly imported regulated weeds growing in fields New Zealand) are also rare 2,10 .…”

“…Seeds are a key agricultural input and output and are a part of a complex global trade network. 1 , 10 However, as a global commodity, contaminated seed for sowing can provide a pathway for the introduction and establishment of pests, weeds and diseases with the potential to cause unwanted impacts across field, farm, regional or national scales. 1 , 2 , 3 These unintended introductions contribute to the homogenization of world floras and are a threat to global food security.…”