Fun but not Pest [是生趣而不是問題]

 [The original article was written for the Hong Kong Gardening Society]

As gardeners we value our plants most and may be quite alert on any ‘alien’ appeared on them, so does an arborist.  However, plants live with other creatures in the nature for millions of years, so for most of the time, we can just enjoy observing the amazing relationship between them rather than getting too panic.

Fruit trees are common in gardens.  Some of the species like Citrus spp. (金橘屬; e.g. mandarin orange, lemon, pomelo) and their relatives in Rutaceae (芸香科) are quite attractive to insects as much as to human.  Their leaves are particularly preferred by many species of swallowtails (i.e. butterflies from family Papilionidae 鳳蝶科), the species from which are usually quite big in size (with attractive wing tails in most cases) – e.g. lime butterfly / Papilio demoleus 達摩鳳蝶 (no wing tail).  The butterflies lay their eggs and entrust the life of their offspring to the trees.  The tree leaves then feed the caterpillars and the visited plants sometimes may look unhealthy without many intact leaves – this is when their presence is aware of.  Nervous ones may worry about the health of the affected plants and may look for way to treat the ‘pests’.

 

Mandarins (C. reticulata or C. erythrosa) are host plants of many swallowtails

Mandarin fed by lime butterfly caterpillar

In fact, not all species of butterflies or moths would outbreak like a pandemic (e.g. by moth Phauda flammans 燄色榕蛾 - refer to another article - or other sucking insects) that creates serious defoliation and may kill trees.  For swallowtails their number of eggs laid on a particular plant are usually limited.  As long as your trees are healthy with sufficient food storage, they can resprout quickly after the feast of the caterpillars, even though most of the leaves are eaten.

There is no clear boundary for the term ‘pest’.  Observing butterfly life-cycle or just flying butterflies does add interest to our gardens – for example the colourful lime butterfly likes to visit the flowers of Lantana.  Having butterflies in a garden also increase its vitality and ecological function, when urban development has destroyed much of their habitat.  Caterpillars do feed on plants in one way, but many of the adults also help plant pollination in another.

Picking caterpillar off the plant for observation and controlling the impacted extent of our plant

Having some caterpillars increase gardening fun

 If the number of caterpillars is big, manually picking some of them may help your plants.  Transferring part of them to an observation box with manual feeding would also limit the impacted parts of the host plants, keeping the plants neat while we can enjoy watching the growing process.

Lemon (C. lemon) is also recommended for attracting 'fun' to the garden

Adult of a lime butterfly

  

Applying pesticide is always least preferred as usually it would be an over-reaction to this normal wildlife relationship, not to say that we would harvest the fruits from the same trees!