Utilising scientific data for banyan pest control

In mid-May, on my routine walk down Tai Chung Kiu Road outside Hong Kong Heritage Museum, I noticed that the leaves of a banyan tree had almost completely disappeared. At first, I was wondering if drilling works earlier had damaged its main root system. Upon closer inspection, I spotted brownish debris and chewed leaves underneath the tree with moth larvae crawling on the trunk—evidence that the tree was in the initial stage of Phauda flammans larvae infestation.

Since 2020, Phauda flammans have been threatening Ficus spp. trees and shrubs as defoliator in Hong Kong. Their larvae only feed on the leaves and young petioles of banyan trees, such as Chinese Banyan, Weeping Fig, Big-leaved Fig and Mountain Fig. Although this moth species is widely distributed in South China and Southeast Asia since the early 20th century, no major outbreaks were recorded in Hong Kong before 2019-20. Initially discovered in Tin Shui Wai and Yuen Long, Phauda flammans have been breeding on banyan trees in urban area for the past 2-3 years. And by the first half of this year, only Hong Kong Island, East Kowloon, Tseung Kwan O, South Lantau and other outlying islands have been left untouched by pest outbreaks. Phauda flammans are highly fertile, producing an average of 3-4 generations of offspring per year. If no measures are taken to prevent their spread, it will sooner or later affect the high-risk century-old banyan trees and the iconic stonewall trees on Hong Kong Island.

Some people may ask, "Everything in the world reinforces and counteracts each other, don’t Phauda flammanshave their natural predators too?” After years of observation by horticulturists and arborists, the number of local predators, such as Gotra octocincta(parasitic wasps) and Oecophylla smaragdina (weaver ants), is far from effective in controlling Phauda flammans population. In addition, their toxicity prevents them from being eaten by birds. Some may argue that banyan trees are very vigorous and resilient, so why the alarmist talk? Why go to all the trouble and expense of exterminating pests at taxpayers’ expense when the banyan trees are able to sustain and regenerate themselves?

I would like to raise a few potential concerns and considerations here:

1. Many of the old banyan trees (such as the ones on Park Lane Shopper's Boulevard) are currently infected with brown root rot disease and listed on the Register of Old & Valuable Trees. Will they succumb to tree decline when faced with recurring pest infestation in the future?

2. Old banyan trees and stonewall trees along roadsides with heavy pedestrian and vehicular traffic tend to have poorer regeneration capacity. Repeated infestations by multiple generations of larvae can cause defoliation and reduce the trees’ ability to replenish food reserves, leading to dieback. Tree management personnel may increase the magnitude and frequency of pruning and crown reduction as a result. Under such vicious cycle, will the capacity for tree regeneration be gradually depleted?

3. According to my research study, Chinese Banyan accounts for approximately 6% of tree number in Hong Kong’s public green space, which is considered the most common banyan tree species. If the government and concerned sectors fail to implement systematic pest control strategies, old and declining banyan trees will die gradually, followed by other banyan trees planted in unfavourable growing environment.

4. The first to be affected are the old (or veteran) banyan trees in older districts. Their wide canopies are effective in mitigating urban heat island effect and facilitating carbon and nitrogen cycles. They also provide essential breeding grounds and habitats for mutualism species such as fig wasp, which in turn affect the food sources of their predators.

5.It is nature's law that infested and diseased banyan trees would gradually decline without intervention or treatment, but the general public has deep connection and collective memory for these iconic banyan trees. Therefore, we can conduct a public opinion survey with "Yes/No treatment” cost initiatives for the public to express whether it is worthwhile to allocate resources for this purpose. It is worth noting that the cost of non-treatment includes tree removal and replanting, while the loss of respective ecosystem services is difficult to quantify.

Simulation models for pest management scenarios

In a review paper of tree pests in urban areas around the world, 49 studies demonstrated that tree pests had significantly weakened the trees and accelerated their mortality[1]. To scientifically assess the impact of Phauda flammans, we can deploy mathematical simulation models to estimate the decline and mortality of Ficus trees, as well as to compare the input required for pest control management under different weather conditions and scenarios in the future.

As pest infestations continue to spread, the chances of using chemical pesticides for pest control become higher. It is likely that highly toxic substances would creep into the urban ecosystem, bringing harm to the environment and humans. To prevent this from happening, the government must develop integrated pest control guidelines, prioritise physical and biological pest control strategies, and strictly prohibit the use of pesticides with high toxicity, so as to protect the ecological environment and the health of humans and animals.