Understanding the Guava Moth Pest
A pest threatening the feijoa industry in being studied in Northland
The guava moth is a pest that is affecting the fledging feijoa export market in Northland and has the potential to threaten a large range of commercial crops as it moves down the country.
Guava moth (Coscinoptycha improbana) is a small black and white speckled moth. It lays its eggs on the fruit surface and the caterpillar burrows into the fruit. The caterpillar leaves rotting, brown patches, excreta and mould inside the fruit, making it inedible and it causes early fruit drop.
The moth is thought to have blown across the Tasman from Australia in the 1990’s and was first observed in New Zealand in 1997 on citrus at Ahipara. It has become well established in Northland and has been seen in Auckland and even Waikato.
Jenny Dymock is a Northland-based entomologist. She has been studying the moth for many years. She says the guava moth can make itself at home on apples, pears, nashi, peaches, quinces, plums, feijoa, macadamia, loquat, guava, and all citrus (grapefruit, mandarins, lemons, oranges), although they are currently not a problem in commercial citrus orchards.
Jenny says that climate modelling shows guava moth could colonise all of the North Island and northern part of the South Island (Marlborough and Nelson). She says by November 2016 the moth had spread as far south as Te Awamutu and possibly to Tirau in Waikato.
Jenny has undertaken mating disruption trials in Kerikeri on feijoa and macadamia orchards, using the Asian peach fruit moth, Carposina sasakii, a close relative of the guava moth, but not currently found in New Zealand. She imported pheromone dispensers from Japan. The trial showed a 50% reduction in nut infestation in the first year in a macadamia orchard and Jenny believes she may have got better results if she had been funded to continue the trial.
Jenny says one problem with the guava moth (and the reason for its success as a pest) is that it has such a wide host range. It can continue its life cycle throughout the whole year. Jenny says it hangs on over winter on citrus, then launches a big infestation on feral loquat fruit, which in turn sets the population up for shifting to summer fruit hosts. Towards autumn, the moth moves on to the feijoa. At this stage there is no indication that kiwifruit is a host plant.
Jenny says fine mesh covering on the fruit stops females ovi-positing but that approach is not feasible on a commercial basis, unless the growing system is changed to a system like grapes, which would allow the crops to be covered in nets.
Guava moth larvae pupate in debris under fruit trees, so one approach to controlling infestations is to maintain good orchard hygiene by removing windfall fruit and other leaves and sticks.
In 2012 Jenny got a small amount of funding from the Sustainable Farming Fund and industry to look for parasitism of guava moth in Australia. She found six (undescribed) parasitoid wasps of Ichneumonid and Braconid species.
Although guava moth is a native to Australia there’s been little work done on control of the moth because it isn’t a problem in its native country. Any work into the future with guava moth biocontrol would have to be funded by New Zealand. Jenny says guava moth is rare in Australia but it is found in Norfolk Island. Gene analysis on the New Zealand moth shows that it is the same species as the Australian moth.
There are no chemicals registered for use on guava moth. Jenny says the best long term sustainable option for the industry is biocontrol, but that is a longer-term option and not particularly reassuring for growers already dealing with the issue. The feijoa industry is pushing for further funding to find a predator or parasitoid that can deal with the problem. The industry says work also needs to be done to find out how widespread the problem actually is.
Monitoring can be done via a synthetic pheromone trap which has been developed by Plant and Food but that is only useful as a monitoring tool.
Neem oil applied every 7 days to ripening feijoa fruit in 2015 at the recommended rate of 7m/L reduced infestation of feijoa by guava moth by 50% but there were very few replicates. Jenny says light traps (solar powered with LED batteries) placed in loquat trees in spring 2015 showed little sign of success.
Spinetoram (Yates “Success”) applied to feijoa in Feb/March 2016 as recommended for codling moth in peaches reduced infestation by guava by 50%.
Growers Tracy Bain and Ian Ruddle have been producing feijoas for about 10 years. Their property has 1,800 trees and in 2016 they picked approximately 25 tonnes.
Tracy explains the moth attacks fruit in a similar way to codling moth but the difference between the two moths is that the guava moth has a continuous life cycle and is almost impossible to control with insecticides because of the short period the caterpillar is on the surface of the fruit. Early caterpillar damage is hard to find – much of it only becomes apparent once the fully-grown caterpillar chews its way out. Infestation is usually at the bottom end of the fruit. Early fruit drop can be a first indicator of the problem.
Myth – Guava moths start affecting fruit at the flower stage.
Fact – Female guava moths lay eggs on fruit as it starts to swell and ripen. Infestation by guava moth larvae causes fruit and nuts to drop prematurely.
Fact – Not all fruit are attacked by guava moth all of the time, every year. Some years it will be plums, another perhaps feijoa or citrus.
Fact – First ripening fruit is more likely to be infested by guava moth – often close to 100%. Later maturing fruit on a tree escapes infestation by guava moth.
Fact – It is not known how far guava moths fly.
Fact – No insecticides are registered for use against guava moth.
Myth – Pheromone traps hung in fruit trees reduce guava moth infestation.
Fact – Pheromones are “perfumes” produced by female moths to attract male moths and do not control guava moth.
Myth – Light traps (solar powered or otherwise) reduce guava moth infestation.
Fact – Light traps or any traps containing attractants (such as vegemite, yeast or ammonia) do not reduce guava moth infestation. Most of the moths trapped are not guava moths. One trial showed that only one guava moth was caught per week in a light trap and guava moth infestation of the fruit was not reduced.
Fact – Removing rotting infested fruit from around the base of trees lowers numbers of guava moths.
Fact – Removing loquat trees or loquat fruit from trees in spring reduces guava moth numbers. A single large loquat tree in your neighbourhood can produce 2000 guava moths ready to infest your Christmas plums and peaches.
Fact – Wrapping fruit on branches in fine mesh material (such as curtain netting) as they are ripening, prevents guava moth females laying eggs on the fruit.
Fact – No guava moth predators or parasitoids have been found attacking guava moth in New Zealand. Guava moths are rare in their home country, Australia, as they are kept under control by predators/parasitoids or excluded from fruit and nuts by other insects such as fruit fly (it is a similar situation as possums in New Zealand compared to Australia).