Can we beat guava moth?
The latest research and practical tips to protect your vulnerable crops.
Five years ago, guava moths flapped into Diti Hill-Denee’s garden in Leigh, north of Auckland, and laid their eggs among her fruit trees. That first year was a shock, Diti says. The lemons and the feijoas were riddled with tiny caterpillars. “It was a mess. I wasn’t 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.
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 member of the Myrtaceae plant family that includes pohutukawa,¯ eucalyptus and feijoas. In Australia, they’re 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 that’s probably wishful thinking. “I don’t 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? Andrew 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.
They looked for natural enemies in New Zealand, but didn’t 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 didn’t definitively reduce fruit damage. To really have an impact, you’d need to kill the females before they lay any eggs — or stop the males from finding them. As part of his doctoral research, Andrew – a chemist – has been trying to do just that. “There are so many conversations
Last season, Asha Chhagan’s team tested the fake pheromones and the female attractants in some Auckland feijoa orchards, but the results were inconclusive.
going on in the environment, and we are only just scratching the surface of what’s going on,” he says. “I’m looking at those chemical conversations and seeing what tools we can develop.”
Andrew 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 Andrew 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.)
At the same time, he’s 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 that’s more attractive than the feijoa fruit. That is a more complex task — feijoas give off somewhere between 50 and 100 chemical compounds. Andrew 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 it’s the feijoa’s bad luck that it happens to smell like the magenta lilly pilly? “It’s 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, Andrew sliced off a female guava moth’s 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. Last season, Asha Chhagan’s 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 Andrew’s compounds, but it’s 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 don’t know about the moth’s 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. “We’ve got multiple generations overlapping,” says Asha. “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 don’t know how far they can fly – whether a neighbour’s infested tree at the end of your street could infect yours. (It’s important to remember that “it’s not just flight when it comes to the spread,” Asha says, “it’s also people taking infected fruit with them – the human factor is involved in the distribution as well.”)
Some of these questions could potentially be answered in the lab, but Asha’s team have been unable to get guava moths to produce fertile eggs in captivity. “We haven’t been able to pinpoint exactly what factor we’re missing. We’ve 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 wellestablished and could easily reintroduce 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 Asha’s team. But the money is about to run out. Asha 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 doesn’t affect New Zealand’s big-business produce such as apples, grapes or kiwifruit, and it’s 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. 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.
“It’s a bit controversial because it kills all moths,” Diti says. “And it’s fairly expensive – but I’ve 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.
“I’ve been using just about every technique. Maybe it’s the sum total of babying my feijoa tree. It’s a bit pathetic really! What I’m hoping is that eventually I don’t have to do any of those things. I’d really like to know whether I can actually stop for one year as a trial, just to see.”
Asha says it is possible to eliminate guava moth from your garden.
It’s 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, “we’re finding that if you have a good control system, then you have a pretty good chance of not getting it again.”
Diti is doing all the right things, Asha 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 don’t 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. (In the feijoa’s natural habitat in Brazil, traditional communities put their pigs in under the trees to stop infestations of another pest, a type of fruit fly – and they say it makes the pork taste good, too.)
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, Asha says,
“know your tree, and know your crop. Get out there and take a look and see if you’re noticing any damage.” If you are, and the numbers in the traps are increasing, Asha suggests giving neem oil a go.
Anecdotally, Asha 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 Diti, she plans to keep up her four-pronged guava moth regime for another year. “Time will tell, in April, as to whether I’ve been effective for a second year in a row.”