Twists and Turns for Banana Passionfruit
Stem-boring moth. Image: Victoria Barney
In New Zealand the term ‘banana passionfruit’ is used to refer to a group of vigorous vines that grow up to 10 m tall and originate from the high Andes of South America. The species of concern all have edible fruits and belong in the same sub- genus, Tacsonia. The three most problematic species, and the regions where they are most commonly found, are: Passiflora tripartita var. mollissima (Wellington, Nelson and Marlborough), P.tripartita var. azuayensis (Wellington, Canterbury and Otago), and P. tarminiana (northern North Island). Less widespread species, but also of concern since they are spreading, include P. mixta (Auckland and Marlborough), P. pinnatistipula (Canterbury and Otago), P. ´rosea (Banks Peninsula), and P. antioquiensis (Auckland and Marlborough).
Banana passionfruit species have naturalised in New Zealand relatively recently, and in a short period of time have become serious weeds of native bush (particularly forest margins and gaps), roadsides and waterways, especially in moist, frost-free, lowland and coastal areas of New Zealand. Banana passionfruit is spread locally by rats, possums, pigs and birds feeding on the fruits, and humans can be responsible for long-distance dispersal. Control is difficult using conventional methods. Cutting the vines and applying herbicide treatment is time consuming and can cause damage to non-target plants, and the plants can quickly regrow from small fragments.
The first efforts to explore biocontrol for banana passionfruit in New Zealand began with a feasibility study in 1999. This study identified the need to find agents that could reduce the vigour and spread of weedy passionfruit species without harming our native passionfruit, kōhia (P. tetrandra) or the commercially grown black passionfruit (P. edulis). “Fortunately kōhia and black passionfruit are in different sub-genera, Tetrapathaea and Passiflora respectively, to the weedy species,” explained Hugh Gourlay, who has been leading this project. Also a number of potential agents had been identified for Hawai‘i, where banana poka, as it is known there, is also a problem weed. But the project has proven much more challenging than ever expected, for a number of reasons.
A fungus (Septoria sp.), which has been used with some success in Hawai‘i, was tested and ruled out in 2005, since it was able to attack black passionfruit. Attention then turned to two moth species. The first of these, the passionfruit moth Glyphodes perelegans (formerly Pyrausta perelegans), is reported to be a major pest of banana passionfruit in its native range, with its larvae destroying a high proportion of flower buds, shoot tips, and young fruits. Female passionfruit moths lay eggs on the foliage, and the resulting larvae burrow into the shoot tips. Larger larvae tunnel into the flower buds and fruits. This moth has been released in Hawai‘i but has not become abundant there, which is believed to be due to heavy attack by parasitoids. Parasitism was expected to be less of an issue in New Zealand since the main parasitoids that attack the moth in Hawai‘i are not present here, and there is no ecological equivalent for the passionfruit moth in New Zealand, which also reduces the risk. The other moth is a stem borer (Odonna passiflorae), about which relatively little was known as it is not widespread in its native range, and so the passionfruit moth was prioritised for study.
With the help of collaborators in Colombia (Victoria Barney and Martha Rojas), several shipments of the passionfruit moth were sourced for testing in the early 2000s. The process of gaining permits and shipping the moths to New Zealand proved to be fraught and uncertain. Finally, in 2006 a healthy shipment was received when Vicki delivered it by hand. Hugh was also able to bring back another shipment in 2010. However, the moth proved challenging to test, producing inconsistent results, and it appeared to be one of those species that becomes confused under confined conditions and unable to discriminate between hosts and non-hosts in the usual way. It seemed that a more natural field trial in Colombia would be the only way forward, and again our Colombian collaborators agreed to help.
There were many challenges in successfully shipping and establishing kōhia plants in South America, with only four plants eventually establishing successfully at the site. They have also grown slowly, and after nearly a decade have not yet produced flowers. However, on the bright side there has been no sign of them being attacked by the passionfruit moth. Conveniently, the field site turned out to also have stem-boring moths present, which provided the opportunity to see if they would attack kōhia (which they haven’t).
“In 2018 the risk of relying on just four kōhia plants to conclusively prove the passionfruit moth was safe felt like too risky a strategy,” explained Hugh. The plants could potentially be lost in a severe storm, and also over the years there have been times when the site has been unsafe to visit due to political tensions, which means continued access cannot be guaranteed. So a decision was made to attempt further indoor testing in New Zealand using larger cages and natural light. Hugh Gourlay travelled to Colombia once again in November 2018 and brought back a shipment of the passionfruit moth. Disappointingly, this time the moths failed to lay fertilised eggs. We reviewed the possible reasons for this failure, but no obvious cause could be determined (see table).
“At that point we also reviewed priorities for the project in light of the results emerging from the field trial,” explained Hugh. The stem-boring moth has killed many large P. tarminina plants at the trial site, and replacement plants are now beginning to suffer the same fate, while kōhia remains unharmed. It is now becoming clear that the stem-boring moth is much more damaging than the passionfruit moth and is potentially a more useful agent. More work is needed to understand the life cycle and ecology of the stem-boring moth, and to develop rearing techniques to allow host-range testing to be undertaken. The larvae appear to be long-lived and are likely take a year to complete development. We have now asked our Colombian collaborators to undertake some studies to learn more about this moth, and we hope to be able to import a shipment for host-testing around the middle of 2020.
| Some common reasons moths won’t mate in captivity* | |
|---|---|
| Stress | Some species lose interest in pairing if handled too much. |
| Artificial light | The biological clock of moths is regulated by the day/night cycle. Artificial light can disturb their biological clock, causing desychronised periods of activity or inactivity. |
| Apathy | Refusal to mate is seen in adults that hatch from cocoons shipped over a large distance: the stress and shock from postage/transport can lead to developmentally impaired adults. Apathetic adults can also be produced when caterpillars are raised on an unusual diet that lacks the specific compounds they need to produce pheromones. |
| No ventilation | Males are unable to find the females because the whole room or container is filled with the females’ pheromones. |
| Insufficient temperature/ lighting | Most moths are not picky and will pair in a dark room in front of a window at room temperature. But some have specific needs, such as a high degree of humidity, UV light or specific temperatures. |
| Intensity of pheromone | Some moths have populations that live in very low densities and find each other using more concentrated or stronger pheromones. These species are prone to ‘sensory overload’ if not given a lot of space and ventilation. |
If the stem-boring moth proves to be a suitable agent for New Zealand, some consideration has been given as to what might best complement it. The stem borer has the potential to kill large, mature vines, but younger, smaller plants might still manage to flower and make fruits before being attacked. So currently the top prospect for a second agent appears to be a fly (Dasiops caustonae), which damages the seeds inside pods. This fly was discovered by a PhD student, Charlotte
Causton, in the 1990s. She rated its potential highly but it has not been studied since. It is anticipated that further studies of the fly will be undertaken as funds for the project permit. For now a decision has been made to put further work with the passionfruit moth on hold, but this could be revisited in the future if the stem-boring moth and the fly prove to be unsuitable.
This project is funded by the National Biocontrol Collective.
CONTACT
Hugh Gourlay – gourlayh@landcareresearch.co.nz
or from May until September
Lynley Hayes – hayesl@landcarerearch.co.nz