Midge Tolerance Testing

Practicalities of Refuge and Wheat Midge Tolerance Testing

By Kim Kenward, Research & Development Manager, 20/20 Seed Labs

?The basic principles behind seed refuge use are straightforward:

• Genetic resistance in plants is the best preventative and least expensive option growers have in their arsenal to protect against yield loss.?
• Pests will find a way to overcome preventive measures, chemical or genetic, taken to protect crops. They adapt faster the more exposure they have to the preventative measure.?

Sm1 is the only genetic resistance available for wheat midge. If you haven’t already done so, please take time to check out the Wheat Midge Stewardship website (https://midgetolerantwheat.ca/) for excellent information on why it is so important to protect this particular resource.?

Providing refuge is good science and a proven strategy to maximize how long plant genetic resistance remains effective. Resistant and non-resistant plants are intermingled in the field to give the target pest a choice: Stay on the resistant plant and deal with the consequences of the resistance OR move on to a convenient nearby, equally luscious-looking plant that doesn’t fightback.?

The choice turns out to be an easy one. I saw it work in feeding trials with Bt-inoculated plants.? Caterpillars, even if placed directly on a Bt-treated leaf, would take one bite and immediately move off to feast on a water-treated one.? Once finished, they would wait for new food rather than try the Bt-treated leaf again.?

Refuges are created by mixing seed varieties with and without genetic resistance. The choice of varieties to blend is important, so is decided prior to release of a resistant variety.? End use parameters are matched, of course. From a refuge perspective though, blended varieties should have identical or as close to identical as possible agronomic characters for emergence, growth rate, and maturity.?

10% is the magic number for the amount of refuge you want in a varietal blend. ??Non-resistant plants will occur at a high enough frequency for the pest to choose them but at a low enough presence to minimize loss in the event of a heavy infestation. Different classes of certified seed specify ranges around 10% that are allowed to be used.?These ranges reflect the different numbers of seed required to be tested and the statistical probability that the amount of refuge found in a given subsample was taken from a larger seed lot accurately mixed at 10%.??

At 20/20 Seed Labs Inc., we don’t directly test wheat midge tolerant varietal blends for the Sm1 resistance gene.?We test for markers that differentiate the two varieties known to be present in a given blend and measure the amount of refuge against the tolerant background. This is essentially a varietal purity test that assumes only two varieties are present.?

Our in-house R&D team creates the tests used for each blend.?Genetic fingerprints are defined for each variety to identify markers that differ between the two varieties present in a blend.? Candidate markers are tested against known mixes to check for ease and accuracy of the refuge quantitation. ?Blind proficiency tests are done as proctored by the breeder or distributor, who will then notify CSGA of their permission for us to test those blends.?To date, we have been approved to test 12 different varietal blends that collectively represented 81% of the 2023 season acreage seeded to wheat midge tolerant wheat.?We add more regularly as new blends come out.

Reported estimates of refuge content reflect the percentage number of seed NOT the percentage weight. Differences in seed size and weight have been observed between varieties within blends, so Thousand Kernel Weight (TKW) should be considered for each individual seed lot used in mixes or re-mixes. We also advise that seed lots used for re-mixing should demonstrate similar germination and vigour profiles.?? ?

Ultimately, it is a combined effort of the breeder, distributor, test lab, and grower to get the right number of live plants in a field to make the refuge work effectively.? ???