The tomato potato psyllid (TPP) has caused enormous problems for potato growers in New Zealand since it was discovered in 2006. Since then, the potato industry has been waging a battle to control this insect pest.
The tomato potato psyllid vectors the bacterium Candidatus Liberibacter solanacearum (CLso). CLso is the putative agent of zebra chip (ZC) disease in potato tubers. Zebra chip disease makes potatoes unsuitable for processing, causes disease in seed tubers, and affects the taste as well as their internal appearance. Zebra chip disease causes darkening of the potato chip when fried.
TPP is a small insect, approximately the size of an adult winged aphid. Adults and nymphs feed on the phloem in a plant and do so by puncturing the leaf, similar to the way a mosquito bites people, so feeding damage is basically invisible.
Both adults and nymphs can carry CLso. However, not every TPP carries and therefore transmits CLso; only a small percentage of the population tests positive for the bacterium. There is no indication on the outside of the adult and nymph of whether it is infected with CLso or not. Adults have to be screened with molecular techniques (polymerase chain reaction or PCR), to ascertain whether they carry the bacteria.
The result of TPP infection
Once TPP has infected the plant with the bacterium there is no way back; the plant is infected and some degree of damage will be done. CLso cannot be prevented from reaching the tubers even if a pesticide spray is applied. Once a potato plant is infected with CLso, it takes about 4 weeks for the foliar symptoms to become visible and to find visible tuber symptoms. The symptoms are a response of the plant to being infected with CLso, they are not the bacteria themselves.
In New Zealand, TPP populations are peaking in potato crops at around January/February.
Industry driven research on TPP management, coordinated and funded by Potatoes NZ, have been conducted for several years in the country. During an annual Field Walk (usually in February), growers visit several ongoing research trials at different locations, including trials involving TPP. These trial visits are coordinated by Potatoes NZ staff members.
In this year’s Field Walk, growers will learn that Potatoes NZ tested action thresholds in previous field trials based on sticky trap counts and accumulated degree days to commence insecticide programmes, and incorporation of agricultural oils. In the Potatoes NZ funded ‘Future-proof spray programme Canterbury’ trial, Potatoes NZ test different spray programmes using integrated pest management tools.
Insecticide resistance management and product label have been taken into account when developing these spray programmes with industry. The spray programmes consist of:
1. A positive control – a standard Canterbury spray programme,
2. Alternating the Canterbury programme with Excel oil from emergence,
3. Canterbury programme but only start when degree day threshold is reached,
4. Alternating the Canterbury programme with Excel oil but only start when degree day threshold is reached,
5. A future proof spray programme without neonicotinoids and organophosphates, and
6. A negative control that will not be sprayed.
TPP adults are monitored using 5 yellow sticky traps in the crop, which are collected and replaced on a weekly basis from crop emergence. A sticky trap is placed on each side of the crop, about 5 m into the crop. The fifth trap is placed in the centre of the crop.
At harvest total yield (kg) per plot, weight and number of marketable and reject tubers, dry and wet weight of 30 tuber sub-sample of marketable tubers to calculate Specific Gravity, and Zebra chip
incidence and severity of 30 marketable tubers per plot will be taken for frying to assess ZC.
Tamarixia triozae trial
A trial closely related to TPP involves Tamarixia triozae – a tiny parasitic wasp (around 2 mm long). Tamarixia is a natural enemy of TPP and can be used as a biological control option. It lays its eggs under the psyllid nymph. The eggs hatch and eat the psyllid nymph. Tamarixia is found in the USA and Mexico as a naturally occurring parasitoid of TPP.
The application to release Tamarixia by Horticulture New Zealand’s Vegetable and Research Innovation Board on behalf of industry groups including Potatoes New Zealand was approved by the Environmental Protection Authority (EPA) in June 2016.
Establishing self-sustaining populations
A Sustainable Farming Fund (SFF) project aims to establish self-sustaining populations of Tamarixia. Growers who take part in the Field Walk this year will be informed on the progress with this project. To date:
• Around 2800 adults of the small wasp were released between Nov 2016 and Feb 2017, across two sites in Canterbury, three sites in Hawke’s Bay and one site in the Auckland region. The challenge has been to find release sites where year-round populations of the psyllid are present and that are not exposed to insecticides. The research team has therefore focused on sites in Canterbury and Hawke’s Bay where the perennial TPP host plant, African boxthorn, is present. The Auckland site was a tomato grower’s property.
• A post-release survey that began in December 2018, has recovered the tiny wasp from both sites in Canterbury and two sites in Hawke’s Bay.
• Plant & Food Research supplied T. triozae to Bioforce, a company that produces biological control agents, and the company is now selling the wasp as a commercial product.
• PFR also supplied Lincoln University, who are successfully rearing the wasp to supply further releases as part of the SFF project.
• Further releases, as part of the SFF project, have been made by growers and other industry personnel keen to establish the wasp on their farms since December 2018.
This article was published in the February issue of Global Potato News magazine. Information contained in this article was graciously provided to Global Potato News by staff members of Potatoes NZ, as well as Dr Jessica Dohmen-Vereijssen (The New Zealand Institute for Plant & Food Research Limited). We wish to thank them all for their contributions and wish them the best with their work on this important matter going forward.