A Sudden Defoliated Evergreen [突然秃頂的常綠樹]

It was extremely hot in most time of this August, especially when we found that the shade originally provided by the large banyan trees had gone suddenly.  This happened in various parts in Hong Kong according to some reports over the social media.  Witnesses felt worries about the fate of the trees.  According to the reports those affected trees were mostly Ficus benjamina (Weeping Fig; 垂葉榕) although F. microcarpa (Chinese Banyan; 細葉榕) were occasionally spotted to have similar problems.

Ficus tree row defoliated

The two tree species are very common in Hong Kong and are also housed by many of our parks and gardens.  With closer inspection, some gardeners may be able to identify caterpillars on trees or on the ground mixed with the dropped leaves.  The defoliation of these trees appears to be quite sudden – it took only one or just more than a week from full tree canopy to completely bare crowns.  In some severe cases all trees of the whole area had defoliated, leaving the branches only.  Some people were even afraid of the tree stability and the associated public safety.

Defoliated Ficus benjamina

Premature fallen leaves with chewing marks

In fact, the severity of this phenomenon has lasted (or increased to some extent) for at least several years but it appeared that it was (or was one of) the worst this year.  For evergreen trees like Weeping Fig they do renew their leaves throughout the year normally (in contrast to deciduous species which drop off all/most leaves within a short period of time) but in a gradual manner.  Such excessive defoliation is therefore extraordinary.

When you find any of the above Ficus trees (or even a few more other species) having this problem, the trees are probably affected by an outbreak of a pest namely, Phauda flammans 燄色榕蛾/朱紅毛斑蛾.  It is the leaf-feeding larvae of this moth which can chew off all leaves of a mature tree.  The pest is native to South China to South Asia and they have three generations a year.  The caterpillars may first appear in late June but the bigger and longer outbreak usually appears in Aug till October.  The larvae are relatively small with yellow and reddish strips, usually appeared sticky on the surface.  Before pupating, the larvae climb down the trunk and pupate between roots or underground.

Larva of moth creeping down the trunk

Adult of Phauda flammans
(source www.hkwildlife.net)

With severe infestation (leaves all consumed), the affected trees surely will suffer for a period of time.  Their resources and food storage are wasted due to the pest.  For healthy trees without much stress, the impact of merely leaf consumption is limited and the trees can recover themselves.  This would only affect the growth rate of the tree, but not tree stability.  However, if the impact happens repeatedly within a short period of time, stored foods of the trees may also be depleted and the health compromised.

Unless the situation is really bad, it is not recommended to spray pesticide (onto the large surface of tree crown) trying to eradicate the larvae.  Such attempt may also harm or kill beneficial organisms such as parasitic wasp, or even birds that feed on the larvae.  One suggested way to control the scale of next outbreak is to remove the larvae that creep down the trunk or that try pupating in the surface soil.  Providing a better growing condition for the trees such as adding mulch will also help tree recovery.  Over-fertilization (esp. nitrogen) would favor appetite of pests in general and should be avoided.

Weeping Fig Tree recovering from the infestation (photos taken with 10 days apart)