Protect your feijoas from guava moth

Five years ago, guava moths flapped into Diti Hill-Denees garden in Leigh, north of Auckland, and laid their eggs among her fruit trees.
That first year was a shock, Hill-Denee says. The lemons and the feijoas were riddled with tiny caterpillars. It was a mess. I wasnt expecting it to arrive in my garden, and I was so disappointed, because I love feijoas.

She has a large, old feijoa tree that produces around 400 fruit a season, and that year, very few were larvae-free. She resorted to cutting edible bits out of each feijoa, and throwing the rest away.

Diti Hill-Denee with her solar trap.

Guava moths scientific name Coscinoptycha improbana come from Australia, where they are found from Queensland to Tasmania. Their native host plant is the magenta lilly pilly, a anggota of the Myrtaceae plant family that includes phutukawa, eucalyptus and feijoas.

* Candy-floss grapes & pineapple-tasting strawberries: new overseas fruit varieties
* 8 tips to get fruit trees through winter
* Organic pest & disease control for your favourite fruits
* An alien invader could spell the end of home feijoa trees

Guava moths in the lab.

In Australia, theyre not considered a pest to agriculture, suggesting they are kept at low numbers by natural predators or competition with other fruit feeders. It also means very little research has ever been done on them.

Guava moths were first reported in New Zealand in 1997, on citrus fruits in Ahipara in the Far North (they were probably blown over in a storm.) Since then, the tiny moths have been working their way south and here, guava moth definitely is a pest.

By 2000, the caterpillars were infesting a wide range of plants in Northland feijoas, citrus, nashi pears, macadamias, loquats, guava, plum and peach (but, strangely, not lilly pilly, despite the introduced trees being relatively abundant in Northland.) By 2004 the moths had reached Whangarei Heads, and they were widespread in Auckland by around 2015.

The latest scientific surveys indicate they have now established populations in Miranda, Thames and Coromandel Peninsula.

Some hope that the cold will prevent them from spreading much further south, but Andrew Twidle from Plant & Food Research says thats probably wishful thinking. I dont think temperature is going to limit it, he says. Its native range is down to Tasmania. I think it will make it everywhere feijoa can grow.

So what can we do to stop it? Twidle is part of the Plant & Food Research team, led by Auckland-based entomologist Asha Chhagan, which has spent the past four years trying to answer that question, investigating a wide range of possible control techniques.

Asha Chhagan.

They looked for natural enemies in New Zealand, but didnt find any. They tried mass-trapping with the existing pheromone traps, but even when they put out 300 traps per hectare, and caught a lot of male moths, it didnt definitively reduce fruit damage.

To really have an impact, youd need to kill the females before they lay any eggs or stop the males from finding them. As part of his doctoral research, Twidle a chemist has been trying to do just that. There are so many conversations going on in the environment, and we are only just scratching the surface of whats going on, he says. Im looking at those chemical conversations and seeing what tools we can develop.

Twidle created a pheromone analogue a fake pheromone meant to block up the males antennae and make it harder for them to follow the females pheromone plume. (Developing this involved Twidle holding down a live moth and recording individual neuron responses on its antenna with an electrode so sharp the tip is invisible to the naked eye.)

The pheromone trap commonly used by gardeners.

At the same time, hes been trying to develop a trap for female guava moths. To attract males, you need

to mimic a female. But for females, you need to create something thats more attractive than the feijoa fruit. That is a more complex task feijoas give off somewhere between 50 and 100 chemical compounds.

Twidle used a machine called a gas chromatograph to separate out the scents produced by the feijoa and by the magenta lilly pilly, and found some compounds that were common to both.

So its the feijoas bad luck that it happens to smell like the magenta lilly pilly? Its a perfect storm, he says. As well as smelling like a good host, it is a good host the moth can develop on it.

In the lab, Twidle sliced off a female guava moths antenna, attached it via electrodes to a computer, and wafted the individual compounds over it to see which ones the moths might be responding to. Then he synthesised some of those enticing compounds.

The pheromone trap lets you know when moth activity is high.

Last season, Asha Chhagans team tested the fake pheromones and the female attractants in some Auckland feijoa orchards, but the results were inconclusive. They have caught the odd female using Twidles compounds, but its not effective enough yet to be a realistic tool for gardeners or growers.

The basics of the guava moth life cycle are fairly well understood. Moths lay their eggs on the surface

of fruit. Within 24 hours of hatching, the larvae burrow inside and start eating. Once mature, they crawl

out, drop to the ground on a silken thread, and make a cocoon out of soil and spit. Adult moths emerge, find each other using pheromones, and mate. The whole cycle takes around eight weeks.

Yet there is still much we dont know about the moths biology. For example, how many generations

they have per year. This complicates control, because unlike some other pests, guava moth larvae can emerge at any time. Weve got multiple generations overlapping, says Chhagan. It would be interesting to see whether certain life stages were more abundant at different times of year, and how that relates to different climates.

We also dont know how far they can fly whether a neighbours infested tree at the end of your street could infect yours. (Its important to remember that its not just flight when it comes to the spread, Chhagan says, its also people taking infected fruit with them the human factor is involved in the distribution as well.)

Damaged fruit.

Some of these questions could potentially be answered in the lab, but Chhagans team have been unable

to get guava moths to produce fertile eggs in captivity. We havent been able to pinpoint exactly what factor were missing. Weve bred so many different insects, she says, but this one has definitely been a challenge.

Funding research into guava moth has been difficult. In 2001, the then Ministry of Agriculture and Fisheries decided not to initiate any official control action against guava moths or conduct any research into them, seemingly arguing it was pointless as the moths were already well-established and could easily re-introduce by wind from Australia.

Feijoa and citrus growers scraped together funding for a research effort in the early 2000s, and a coalition of regional and district councils, MPI, feijoa and macadamia growers have funded Chhagans team. But the money is about to run out.

Chhagan has just enough to repeat the work that had to be cancelled during lockdown, which unfortunately coincided with the height of the feijoa season.

Guava moth doesnt affect New Zealands big-business produce such as apples, grapes or kiwifruit, and its hard for the small numbers of feijoa and macadamia growers to raise funds, especially when their profits are down in part because of guava moth, and the disruptions caused by Covid-19.

Since that first terrible year, Diti Hill-Denee has trialled a range of interventions to get her guava moth problem under control.

Pick every single feijoa off the ground.

First, she picked up every single feijoa that fell to the ground, sorting the edible ones into the fruit bowl, and the caterpillar-infested ones into a sealed plastic bag in the rubbish. The next year, she added a pheromone trap, which killed some of the male moths and let her know when activity was high.

The following year, she also hung a Little Bugga solar-powered moth trap in the feijoa tree. It attracts moths to a light and then drowns them in a thin layer of rice bran oil. Its a bit controversial because it kills all moths, Diti says. And its fairly expensive but Ive had the same one for nearly three years now, and I have trapped mostly guava moths. After a while you become very familiar with what a guava moth looks like!

As she increased her arsenal, the number of affected fruits began to diminish. Finally, last summer, she also sprayed the trees with organic neem oil just as the feijoa fruitlets were beginning to form on the ends of the flowers.

In April, during the first lockdown, she went out every day, raking up the fruit as usual. She looked and looked for signs of guava moth in the fruit and there were none. There were barely any moths caught in the pheromone traps, either. Ive been using just about every technique. Maybe its the sum total of babying my feijoa tree. Its a bit pathetic really! What Im hoping is that eventually I dont have to do any of those things. Id really like to know whether I can actually stop for one year as a trial, just to see.

A good control system can ensure your feijoas are free of guava moth damage.

Chhagan says it is possible to eliminate guava moth from your garden. Its harder in the city, or if you have a guava moth-infested tree next door, but as soon as you are a little bit more isolated, or collaborate with your neighbours, were finding that if you have a good control system, then you have a pretty good chance of not getting it again.

Hill-Denee is doing all the right things, Chhagan says. Killing as many adults as possible with pheromone and solar traps helps. Picking up the fallen fruits is crucial to disrupt the life cycle any larvae still in the fruit can no longer pupate right under the tree. Just make sure you dont compost them, she says; feeding them to cows or other farm animals is an option. Fencing chickens in around the fruit trees could help to kill the grubs as they pupate in the soil.

Additionally, you could try covering smaller trees with a fine netting when fruitlets appear or writing to your local council to suggest they fund more research.

Most importantly, Chhagan says, know your tree, and know your crop. Get out there and take a look and see if youre noticing any damage.

If you are, and the numbers in the traps are increasing, Chhagan suggests giving neem oil a go. Anecdotally, she has heard of home gardeners having success with neem oil, and now she wants to test that scientifically. This season, she plans to test neem oil alongside two possible insecticide regimes to see which is most effective at reducing fruit damage.

On the side, she also hopes to do a small study to see which feijoa varieties are more susceptible or resilient to guava moth attack. Results should be available by this time next year.

As for Hill-Denee, she plans to keep up her four-pronged guava moth regime for another year. Time will tell, in April, as to whether Ive been effective for a second year in a row.


Our Partner