Reduced Spray Program – Canterbury Psyllid Management Trials

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Completed: Final Report available on request

Lead Organisation Plant and Food Research Lincoln  Project leader Jessica Dohmen-Vereijssen

Objective – This trial was designed to test a future-proof insecticide programme and further develop reduced insecticide programmes for the management of tomato potato. This study aimed to 1. Develop a future-proof reduced insecticide programme for TPP management for potato growers in Canterbury, and 2. Further develop reduced insecticide programmes using Degree Day (DD) accumulation and agricultural oils in a standard Canterbury insecticide programme.

Milestones –

The first TPP adults were trapped on yellow sticky traps on 6 December 2018, only 8–10 days after 100% crop emergence. On plants, TPP eggs, and small and medium nymphs were found at the first assessment date on 17 December 2018. Counts for all TPP life stages varied between the treatments but this was primarily because of the higher counts for the unsprayed Control treatment than for other treatments, plus the absence of all nymph classes for the standard Canterbury insecticide programme (SE).

Overall, neither marketable weights nor numbers varied significantly among the treatments, and neither did unmarketable weights or numbers. Mean ZC score did not vary significantly with treatment at harvest and was highest for the unsprayed Control treatment and lowest for the future proof insecticide programme (F). Overall, ZC incidence did not vary significantly between treatments, and neither did the percentage of crisps with slight and medium ZC discolouration. However, the highest percentage of crisps with slight and medium ZC discolouration were for the unsprayed Control treatment, and the lowest for treatment F.

The relative profit for each treatment with penalties over ZC scores higher than 2 varied from 70.9 for C to 103.5% for F, when SE ‘profit’ was set at 100%, a similar trend was observed for ZC>4. The 3.5% increase in profit for F compared to SE comes mainly from a low percentage of tubers with ZC, as the yield was lower than for SE. The option of no TPP management did not pay off in this trial this year, with the lowest yields and the highest ZC percentage, resulting in the lowest relative profit.

In contrast to this study, in previous trials, alternation with Excel® oil from emergence was the best treatment. In this trial, with a different base insecticide programme, the alternating with Excel oil from either emergence (treatment AE) or 716 DD (treatment AD) produced little benefit, with an AD being slightly worse than AE insecticide programme.

It was interesting to note that the 716 DD (reached on 28 December 2018) did not coincide with 3 TPP per trap per week (reached between 8 and 10 December), where normally for the South Island these two variables vary by 2–13 days. Using the adjusted African boxthorn degree days (600 instead of 716DD), which was reached 9 December 2018, would have been closer to the trap threshold. Applying treatment AD 2–3 weeks earlier may have made a difference to the performance of this insecticide programme.

It is suggested to repeat the insecticide programmes for at least another one to two seasons to obtain a good feel for the effectiveness of these programmes and their resilience, and adjust where needed. In addition, a future-proof insecticide programme using either a trap or DD threshold, or including alternation with an agricultural oil would be a logical next step to make TPP management more sustainable.

Our 2019 field walks booklet outlined this work succinctly.