Estimated reading time: 7 minutes
The potato tuber moth (Phthorimaea operculella) challenge is not new to South African potato producers and has been around for 130 years, causing losses of up to 30%. Alongside the tomato leaf miner moth (Phthorimaea absoluta), also known as Tuta, which primarily affects leaves, both pests have posed challenges in the past season. The potato tuber moth damages the tubers, while Tuta causes foliar damage. Dr Dietrich Visser highlighted the differences between these two moth types in the March/April 2020 edition of CHIPS. The increased incidence of these pests is aggravated by various circumstances, which are discussed here.
Resistance to insecticides
Poor control is often attributed to the crop protection programme being applied. Potatoes SA commissioned a study along with North-West University’s Prof Hannelene du Plessis to assess the sensitivity of the most used insecticides. This study, published in the January/February 2023 issue of CHIPS, showed a sensitivity shift against at least three groups of insecticides.
The development of resistance to insecticides is a common occurrence in many agricultural pests, but it must be highlighted that this is not the only reason for poor control. The following factors must be considered:
- Late applications when populations are already high.
- Low water volume, poor water quality, and inadequate deposition of insecticides may reduce efficacy.
- Correct timing of application to target the correct development stage.
- Targeting the pest with the correct product at the correct life stage of the insect.
If a high degree of resistance is found, the active ingredient’s mode of action (MoA) should be replaced with a compound from an alternative MoA. The Insecticide Resistance Action Committee (IRAC) provides valuable guidance in this regard, and it is worth visiting www.irac-online.org/pests/tuta-absoluta/ for more information.
For Lepidoptera pests, various MoAs are available, including:
- Growth regulators: Target insect development by impacting cuticle development, providing a slower process of control.
- Respiration inhibitors: Inhibit the transportation of electrons and act faster.
- Nerve and muscle action: These are older actives that work fast and have shown signs of a shift in efficiency due to resistance developing.
- Unclassified MoAs: Act on various target points.
- Midgut targets: Derived from microbial toxins such as Bacillus thuringiensis (Bt), better known in crops such as maize, soya bean and cotton.
Table 1: Sensitivity shifts against insecticides.
| Insecticide | Group |
|---|---|
| Organophosphates represented by Azinphos-methyl | IRAC Group1B |
| Pyrethroids represented by Lambda-cyhalothrin | IRAC Group 3A |
| Benzylureas represented by Lufenuron | IRAC Group 15 |
Figure 1: Different target sites for controlling potato tuber moth.
Weather
Under normal conditions, the potato tuber moth completes its life cycle in four to five weeks. However, at higher temperatures (above 26 to 30ºC) the cycle can be as short as three weeks. Growers should be prepared and implement cultivation practices that will reduce moth numbers, such as ridging and irrigation, as this minimises the development of cracks.
Sources of infestation
Potential sources of infestation, such as storage or waste heaps where potatoes are dumped, need to be identified.
Pheromone traps can serve as an early warning before moths infest new fields. If possible, new fields should be planted as far away as possible from previously infested fields. Any waste heaps should be covered with a soil layer of at least 50 cm, to prevent adult larvae from escaping to the soil surface and making pupae.
During soil preparation, moths are not present in the fields. The first moth generation develops slowly and is often unnoticed. However, subsequent generations will overlap stages, including moths laying eggs, larvae, and pupae which increase exponentially and peak during the foliage dieback stage.
It is during this time that tuber infestations increase.
The first instar larvae of the potato moth cause damage because they are very small (< 0.2 mm in diameter) and can move down to tubers through microscopic cracks that develop after irrigation or rain followed by a dry spell. These cracks lead directly to the tubers, depending on the soil type. The shallower the tubers are underground, the easier the infestation by moth larvae.
Efforts should be made to prevent potato moth larvae from reaching the potato tubers by reducing or sealing cracks, even after foliage dieback. This will limit the damage to the tubers. Harvest as soon as possible if moths are present in fields after foliage dieback.
Table 2: Mode of action for different IRAC groups.
| IRAC Group | Class | Active ingredient | Mode of action |
|---|---|---|---|
| 1A | Carbamate | Methomyl | Acetylcholinesterase (AChE) inhibitors with nerve action |
| 1B | Organophosphates | Acephate, azinphos-methyl, methamidophos, methidathion, profenofos | AChE inhibitors with nerve action |
| 3A | Pyrethroids | Alpha-cypermethrin, beta-cyfluthrin, bifenthrin, deltamethrin, esfenvalerate, gamma-cyhalothrin, lambda-cyhalothrin | Sodium channel modulators with nerve action |
| 4A + 3A | Neonicotinoids and pyrethroids | Acetamiprid + bifenthrin | Nicotinic acetylcholine receptor (nAChR) competitive modulators with nerve action |
| 4A + 15 | Neonicotinoids and benzoylureas | Acetamiprid + novaluron | nAChR with nerve action as well as growth regulation |
| 5 | Spinosyns | Spinetoram, spinosad | nAChR allosteric modulators with a nerve action |
| 13 | Pyroles | Chlorfenapyr | Uncouplers of oxidative phosphorylation via disruption of the proton gradient impacting energy metabolism |
| 15 | Benzoylureas | Diflubenzuron, lufenuron, novaluron | Inhibitors of chitin biosynthesis affecting growth regulation |
| 22A | Oxadiazines | Indoxacarb | Voltage-dependent sodium channel blockers result in nerve action |
| 28 | Diamides | Chlorantraniliprole, cyantraniliprole, Flubendiamide | Ryanodine receptor modulators with muscle action |
| 28 + 3A | Diamides + 3A | Chlorantraniliprole and lambda-cyhalothrin | Ryanodine receptor and sodium channel modulators with a nerve and muscle action |
| UN | Pyridalyl | Pyridalyl dichloropropene derivative | Unknown MoA |
| UNF | Fungus | Beauveria bassiana | Unknown MoA |
| Pheromone | Pheromones | (E,Z)-4,7 and (E,Z)-4,7,10-tridecatrienyl acetate and permethrin | Lure with nerve action |
Figure 2: Percentage of parasitism of potato tuber moth larvae in unsprayed potato fields in the different potato production areas between 1982 and 2007. The figure represents the combined values for the two parasitoids, Copidosoma koehleri and Apanteles subandinus. (Study by Dr Diedrich Visser, ARC)
Beneficial insects
The establishment of an international partnership programme has allowed for comparison with other countries, such as Australia, which has faced the same challenge and managed to address the problem by reviewing its control strategy. During a Potatoes SA webinar, Dr Paul Horne of IPM Technologies showed that reducing the use of broad-spectrum insecticides increases the population of natural enemies of the potato tuber moth, which prevents infestations and reduces the need for extensive spraying. Fungicides should also be monitored, as these may impact these beneficial insects.
Studies by Dr Diedrich Visser, a researcher at the Agricultural Research Council, have identified beneficial insects that have been observed locally, including:
- The polyembryonic wasp.
- Copidosoma koehleri.
- Apanteles sabandinus.
Unfortunately, they are also very sensitive to the pyrethroids and organophosphates, and softer options should therefore be considered.
In conclusion
Given that the tuber moth has been present in South Africa for at least 13 decades, one might expect it to be well understood. However, controlling this pest requires a multifaceted approach. Some strategies at our disposal include:
- Incorporation of integrated pest management (IPM) practices.
- Scouting to ensure that populations are controlled timeously.
- Eliminate sources of infestation. • Avoid soil infestations by managing soil cracks.
- Use the correct product at the right stage of the insect development.
- Ensure the correct timing of insecticides.
The challenge is that these are not foolproof. Potatoes SA is seeking growers willing to try new strategies to control this pest, with various regional workgroups able to offer platforms to test new concepts.
For more detailed guidance, refer to the document provided by IRAC at www.irac-online.org/documents/potato-tuber-mothinsecticides-their-moa-and-sensible-use-to-prevent-or-delay-resistance-development/?ext=pd. – Dirk Uys, research and innovation manager, Potatoes SA
For more information, contact Dirk Uys at dirk@potatoes.co.za or visit www.potatoes.co.za.