Phauda flammans Outbreak and Global Warming

It is autumn now. A few months ago in spring and summer, trees in Hong Kong's urban areas were supposed to be lush and green. But you may well notice that the banyan trees are no longer like the lush scenes of the past, with branches and leaves thinning out and some crowns even going bald. How did this happen?

Small-fruited Fig suffering from wilting branches and defoliation caused by Phauda flammans attacks (Photo source: Ming Pao)

Since 2020, Hong Kong has been falling victim to considerable Phauda flammans infestations from April to October. [1][2]Their larvae are oligophagous, targeting the leaves of banyan trees and shrubs for food. Their large numbers resulted in heavy feeding on foliage, leading to wilting branches and defoliation.

Life Cycle of Phauda flammans

Normally, overwintering larvae would become active during early spring (around March to April) and develop into adults. They start mating within 3 to 4 days after fledging, and lay eggs on the following day. By day 13 to 14, their eggs would hatch into larvae and start feeding on leaves. After 40 to 45 days of growth, the larvae will enter the pupal stage, which lasts for 11 to 20 days before hatching into adults. That means a new generation of larvae will start feeding again in July, ravaging the trees. [3]

The annual life history of Phauda flammans in 2021 (Photo source: Mr. Kevin Yuen)

Phauda flammans and the World’s Climate

In the past, the last generation of Phauda flammans would lay dormant in October each year in crevices of tree trunks or on mud surfaces, waiting to emerge from their cocoons when it gets warmer the following year. However, owing to global warming, the dormancy period has been delayed, causing overwintering pupae and larvae to become active earlier. This results in the increase in the number of generations from 3-4 to 5, making it difficult for banyan trees to recover by early spring.

As banyan trees are evergreen trees, a Phauda flammans attack triggers massive defoliation and seriously hampers nutrient production efficiency. Trees need to regrow leaves to replace the lost ones, a process that consumes a large amount of energy reserves. If this happens continuously throughout the year, it will significantly impact tree health. To counteract insect infestation, trees need to regrow all their leaves within the same season. This leads to physiological change, with newly grown leaves containing less chlorophyll. Ultimately, this becomes a vicious cycle, impacting energy reserves and nutrient production efficiency. Under such circumstances, the immune system, root development and xylem density of trees worsen, causing the trees to wither due to the lack of leaves for photosynthesis. [4]

Moreover, the increase in extreme weather conditions will enhance the population growth of Phauda flammans. For example, global warming leads to extreme rainfall pattern, where heavy rains get heavier, and small to moderate rains get lighter. Under normal circumstances, the rainfall can wash Phauda flammans out of tree canopies. But under new climatic patterns, the shorter rainfall durations create less pressure on Phauda flammans. [5]

At the same time, rising temperatures will make the Phauda flammans larvae grow more actively. According to the Hong Kong Observatory, the average temperature was 21.3°C this March, while the average minimum and maximum temperatures were 19.4°C and 24.2°C respectively. These figures were 1.8°C, 1.8°C and 2.3°C higher than their respective normal values. The optimal growing temperature of Phauda flammans larvae is between 22.2°C and 28.7°C. Even if the optimal temperature is not reached, the increase in temperature may lead to earlier shedding of cocoon, earlier start for overwintering generation, and a shorter growth period. Besides, the rise of fall and winter temperatures would delay their dormancy period. Taking last year’s figure as an example, the average temperature of November was 23.4℃, which was 1.2℃ higher than normal. Only a temperature below 22.2℃ will make them less active or even go dormant. The temperature increase has made the whole population more active and numerous, which has greatly increased the pressure on trees. [6][7]

Average daily temperatures recorded at the Hong Kong Observatory in March 2023 (Photo source: Hong Kong Observatory)

Average daily temperatures recorded at the Hong Kong Observatory in November 2022 (Photo source: Hong Kong Observatory)

Global warming is not only causing extreme weather conditions, but also a significant increase in pests. Let us all work together for the trees and climate, reduce carbon emissions and save the planet!