Tiny nematode parasites raise big biosecurity worry

Nematodes - like this one pictured in white clover roots - are hardy enough to survive in dry soil for at least three years. Photo / Lee Aalders

Kiwi scientists have shown how one tiny, worm-like creature - able to travel here on the shoes of airline passengers - is hardy enough to survive in dry soil for at least three years.

A new study by AgResearch scientists offers fresh insights into how biosecurity officials can best detect nematodes before they cross borders and potentially reduce yields of important crops and pasture.

Nematodes can be extremely hardy - and not all are problematic, with some being used as biological controls to counter other harmful pests.

But damaging ones, called plant parasitic nematodes (PPN), include species that attack plants and affect their growth and survival.

In the study, funded by AgResearch through the trans-Tasman Better Border Biosecurity collaboration, soil collected from a native forest and an organic orchard was stored separately in cupboards at room temperature for 36 months.

Samples were then taken at regular intervals to see if any nematodes could be recovered from the soil and, if they could, whether they were able to infect plant hosts.

The researchers used different methods to detect nematodes - including a water misting technique to draw them out of the soil, and a baiting method - where they grew white clover and ryegrass plants in pots containing a soil sample.

"One of the PPN we looked at was the root lesion nematode," AgResearch nematologist Lee Aalders said.

"What we found was that lesion nematodes were able to successfully invade the roots of ryegrass even after 36 months.

"They were also able to produce offspring at 13 months.

"Interestingly, no PPN were recovered from soil stored beyond the 13th month using the three-day misting technique."

This meant that, given the right conditions, PPN in soil, which is carried on sea freight, footwear or used machinery, and protected from sun or extreme heat, will survive if they end up near a suitable host plant.

This was a result that may not be detected using an extraction test such as misting.

For quarantine officials around the world, this result was an important find, as it reinforced the risk associated with soil that, even though it may look sterile, unwanted nematodes may be present and undetected until paired with a suitable host plant.

"In the context of biosecurity, we think that the development of a generic test for plant parasitic nematodes - based around a molecular based bioassay - would enhance the probability of detection of PPNs and, therefore, prevent unwanted incursions beyond the border."

Earlier this year, another AgResearch study into the survival rates of various transported soil organisms concluded that biosecurity risks from soil organisms are to increase with declining transport duration and increasing protection from environmental extremes.