YOUR INDUSTRY
POTATO TUBER MOTH IN GROWERS’ SIGHTS Words by Glenys Christian
Pukekohe potato growers check out the mulching trial
The next step in Pukekohe potato growers’ battle against potato tuber moth Phthorimaea operculella (PTM) is about to get underway with a technical panel formed and field trials being discussed. At a Potatoes New Zealand (PNZ) potato walk at Pukekawa in mid-March around 25 growers and rural professionals were able to check out a one-hectare trial using straw mulch to suppress PTM, as well as hear from scientists about their best pesticide options. Iain Kirkwood of PNZ estimated that up to 40% of some potato crops in the Pukekohe region were lost due to PTM last summer. Shane Smith, the chief executive officer of Inta-Ag, showed trial work it has carried out with straw mulch used on a crop of potatoes sown in mid-October. Ten tonnes of barley straw, produced by the grower Murray Aarts, was laid in a thick blanket on the potato crop at emergence to retain moisture and stop PTM larvae getting to potato tubers so easily. Only now were patches of bare soil beginning to be seen. While plenty of PTM had been found in traps set on the property, there was little damage to the crop, which could also be attributed to regular monthly rainfall. Results from the trial will be known later in the year.
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Plant & Food Research (PFR) entomologist Frances MacDonald, who is working on the PNZ Potato Tuber Moth project, tested nine commonly used insecticidal sprays in laboratory assays using direct spraying and residue testing. The PNZ project is to determine the tolerance of PTM to a selection of chemical controls at the recommended application rate. Efficacy was variable, with one commonly used synthetic pyrethroid, (lambda-cyhalothrin/Karate Zeon®) tested at more than twice the label rate, with a 29% of PTM larvae survival rate after direct spraying and 6% with residue testing. “We found with this field collected population that two tested synthetic pyrethroids were failing to control PTM in a lab setting” she said. “There is the risk that with this practise resistance may develop within a population.” While some insecticides were still working, growers run the risk that they might not work so well in the future. One diamide tested (Cyantraniliprole/Benevia®) had been very effective in lab trials on the larval life stage of PTM, with a slower kill due to its mode of action, but she suggested that more testing is required in a field situation targeting larvae. “PTM adults are very hard to kill because they shelter during the day on the underside of potato leaves at the bottom of the plant” she explained.
