Potato tuber moth in growers’ sights

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. 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.

Further trials could also look at how far-reaching pesticide resistance is in populations, with Pukekohe currently affected and some possible resistance showing up in Waikato and Manawatu potato crops as well. PFR’s Graham Walker who led earlier research on PTM at Pukekohe, said sanitation and cultural controls were essential along with using selective pesticides, with some insecticides used earlier in the growing season then rotating to different insecticides at a later stage. Targeting the moths is ineffective because of their impenetrable wing scales, and as they can cover 100 metres a night they could easily spread from crop to crop. Potato growers also looked at a Masters and Sons block nearby where a mix of different species have been planted on three-metre-wide headlands to attract a range of insects. Buckwheat, linseed, clover, phacelia and Smart radish were used. Shane said PTM seemed particularly keen on the radish, which was used because its deep root broke up the soil. Once trapping figures were collected it could be decided whether the mix could be improved and other pests such as the tomato potato psyllid (TPP) could also be attracted away from the potato crop.

Large hoverfly larvae can eat around 40 TPP or aphids a day if there’s no other food source

Frances said trial work previously run with a potato grower using planted phacelia borders beside potato crops reflected documented research that the primary purpose of such a border crop is to attract beneficial insects into it. Different distances into the crop were scouted to see how far and how many beneficial insects moved into it. While there was no difference noticed when it came to lacewing numbers, a large increase was seen in hoverflies.

“Large hoverfly larvae can eat around 40 TPP or aphids a day if there’s no other food source,” she said.

It would be useful to see how this increase in beneficial insects translates into a cleaner crop at processing time. “I would love to validate that beneficial insects are making a difference,” she said. “I believe they are.” Iain Kirkwood also asked potato growers to keep a look out for two other pests as well. The red tomato spider mite was found near Auckland Airport last year and a later survey found it at nine different sites throughout the city on weed species. More recently one had been found in Tauranga, but it was unclear if this was from the same incursion or a new one. If growers think the mite is on any of their crops they should take a sample and send it to the Ministry for Primary Industries (MPI). The same advice applies if growers find any signs of potato mop top virus, which has not been seen on the 30% of seed potato crops sampled this year.

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