Friday, February 3, 2023

Moth damage to potatoes is on the rise

Estimated reading time: 3 minutes

The incidence of moth damage to potatoes has increased in the main production areas. This damage can be attributed to two moth species, the potato tuber moth (Phthorimaea operculella) which causes both foliar and potato tuber damage, as well as the tomato leafminer (Phthorimaea absoluta, previously Tuta absoluta) which attacks the foliage.

 Both pest species attack crops in the Solanaceae plant family (including tomatoes, potatoes and tobacco) and can also be observed on weeds in the same plant family, such as night sage. During field visits to the Limpopo production areas, both species were present in abundance.

Potatoes SA initiated a research project to evaluate the impact of the moth complex on potatoes with regard to the efficacy of control strategies. This study, conducted by Prof Hannalene du Plessis and her team at North-West University, tested the efficacy of four widely used insecticides representing four different modes of action (MoA) groups, as defined by CropLife’s Insecticide Resistance Action Committee (IRAC).

Read more about hybrid potatoes in South Africa here.

The sensitivity of potato tuber moth to these chemicals was evaluated under laboratory conditions using internationally approved methods (the tomato leafminer was not included in this study). The following insecticide groups were tested:

  • Organophosphates (IRAC 1B): Azinphos-methyl.
  • Pyrethroids (IRAC 3A): Lambda-cyhalothrin.
  • Chitin biosynthesis inhibitor (IRAC 15): Lufenuron.
  • Sodium channel blocker (IRAC 22A): Indoxacarb.

The results from this study showed shifts in the sensitivity of potato tuber moths collected from the Sandveld and Limpopo production areas to three of the four MoA groups tested – organophosphates, pyrethroids and the chitin biosynthesis inhibitors. Indoxacarb still showed the expected efficacy from registered rates. The diamides (IRAC Group 28), which are also widely used against the pest, were not included in the assessment.

Tips to mitigate moth damage

Potatoes SA and CropLife SA are concerned about these developments and propose the following control strategies to mitigate damage by both the potato tuber moth and the tomato leafminer:

  • Ensure proper coverage of the crop when applying insecticides. This can be achieved by using well-serviced equipment and adequate water volumes for spray mixtures. Adjuvants can be used where registered.
  • Do not apply insecticides as drenches or as drip applications if not registered as such.
  • Scout regularly and monitor pest population levels throughout the production season. Introduce protection measures and corrective action when spikes are noticed.
  • Rotate MoAs based on IRAC guidelines.
  • Individual producers should align spray programmes in production areas to avoid consecutive insect generations being exposed to the same MoA group.
  • Only use insecticides that are registered for use against the potato tuber moth and tomato leafminer complexes and strictly abide by label instructions.
  • Practice good sanitation by removing infestation sources such as old production fields and potato heaps.
  • Avoid cracks in the soil by scheduling irrigation or mechanically closing cracks to prevent potato tuber moths from reaching the tubers.
  • Introduce good ridging practices.

Read more about how potatoes can curb hunger in South Africa here.

Potatoes SA will continue to work with CropLife SA crop advisors to create awareness among producers to mitigate damage from potato tuber moth and tomato leafminer, by promoting responsible production practices. The full results from the study conducted by Prof Du Plessis are available in the November/December 2022 edition of CHIPS. Click here to access the flipbook.  

For more information, contact your Potatoes SA regional advisor or Dirk Uys of Potatoes SA at dirk@potatoes.co.za. – Press release, CropLife SA and Potatoes SA