Potato moth
Description
The adult moth is brownish – grey, measuring 12 - 16 mm with outspread wings. Small dark markings are scattered on the forewings and a fringe of fine hairs line its pale cream hind wings. Eggs are small and pearly white initially, turning yellow later. The colour of larvae depends on their food source. If foliage is their food source, they appear green, while, if tubers are their food source, they tend to be pink – grey in colour. Fully grown larvae are 12 mm long and have a dark head. Pupae are 8 mm long and dark brown.
Adults commence laying eggs two days after emerging from pupation. Eggs are laid singly onto the underside of leaves, while on tubers they are laid in groups around surface scars or the eyes. Over the course of two weeks up to 100 eggs can be laid, hatching within 5 days during summer. This extends to two weeks in winter. Larvae are active straight after hatching and commence mining into the plant or tubers. They go through four stages of growth, which takes about 14 days in summer, during winter this takes much longer. Once mature larvae leave the plant, they spin cocoons on plant debris or clods on the ground. Adults emerge within a week in summer or up to a month during cooler months. One life cycle usually takes about 4 weeks in summer and there can be 6 - 8 generations in a year.
Larvae mine the leaves, stem, fruit and tubers. Leaf mining from larvae can cause plant tips to die off and under heavy infestations this can result in premature plant death. Larvae easily infest tubers when eggs are laid directly on them, or when larvae move down from the plant tops once the plant material begins to senesce. Soil surface cracking can also exacerbate tuber infestation by making it easier for larvae & adults to infest developing tubers in the soil. Tunnels formed by larvae in the tubers become filled with frass, which enable the entry of decay forming organisms, directly affecting the crops marketability. In tomatoes, larvae enter fruit through the calyx, resulting in damaged produce which is unmarketable. Larvae may also enter stems via pruning scars. In young plants, stem mining can cause significant damage. Potato moths are mostly active during warm conditions from spring to autumn, particularly in coastal areas.
Control
Biological and cultural control is extremely important in PM management. Chemical control should be used only when all other methods have been exhausted. Minimising the use of broad spectrum insecticides allows beneficial insects such as parasitic wasps to maintain pressure on PM populations. In some cases, parasitism levels of up to 80 per cent have been recorded. Other beneficial insects such as damsel bugs, spiders, earwigs, ladybird beetle and red and blue beetle also benefit from reduced insecticide use. Planting flowering plants on the side of a field will help encourage parasitoid species.
To reduce the risk of larvae infesting tubers maintain good soil coverage, particularly on heavy soils which can be prone to cracking. Do not plant successive crops of potatoes or tomatoes and avoid planting close to other susceptible crops. Destroy all volunteer host plants and weeds prior to sowing. Use healthy seed where possible and plant tubers at least 15 cm deep. Destroy any infested produce at harvest. Good hilling later in the season helps keep tubers covered and can significantly reduce the chance of tuber infestation. If crops are ruined, plough in debris as soon as possible. Ensure produce is removed from fields promptly after harvesting to prevent infestation. Do not store produce in warm temperatures as infestations can still occur. To prevent this, store produce below 9°C.
Regular monitoring enables well informed decisions to be made. Inspect leaves and stems for evidence of mining weekly, starting from crop emergence. Pheromone traps can also be used to monitor PM populations in the field and in storage.
When all other options have been utilised, an insecticide may be required. Belt® is a fast-acting insecticide with good residual properties, which is also soft on beneficial insects used to control chewing insects. Belt is registered for the control of PM in fruiting vegetables such as capsicum, eggplant, peppers and tomato as well as root and tuber vegetables including potato.
References
Hamilton, J (1985), ‘Potato moth’, NSW Agriculture, Agfacts, Potato moth (nsw.gov.au)
HerbiGuide (Cited 2021), ‘Potato Moth’, HerbiGuide, Potato Moth (herbiguide.com.au)
Jackson, G (2015), ‘Potato tuber moth’, Pacific Pests, Pathogens and Weeds – Fact Sheets, Fact sheet - Potato tuber moth (298) (lucidcentral.org)
Queensland Government (2017), ‘Potato moth’, Department of Agriculture and Fisheries, Potato moth | Department of Agriculture and Fisheries, Queensland (daf.qld.gov.au)
