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.
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| 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.
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| Defoliated Ficus benjamina |
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| 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.
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| Larva of moth creeping down the trunk |
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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.
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| Weeping Fig Tree recovering from the infestation (photos taken with 10 days apart) |
