Phryneta spinator (Fig tree borer)
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(insects) > Dicondyla > Pterygota > Metapterygota > Neoptera > Eumetabola >
Holometabola > Coleoptera >
Polyphaga >Family: Cerambycidae
This large indigenous beetle of about 35 mm in length has
become a serious pest of fig trees in Cape Town. The larvae bore into the wood
and in severe cases can eventually kill the tree, particularly if it is old and
lacking in vigour. It can also be a pest of Willow trees. The large size, long
antennae, mottled appearance and the prominent spines on each side of the thorax
make this species easily recognisable. The life cycle (see above) takes slightly
over three years!
Mature larva in tunnel
of wood. [image
by H. Robertson ©].
Close up of mature
[image by H. Robertson ©].
(c 35 mm long). This stage does not bore into wood but they do feed on
bark of trees and eat the rind of unripe figs. They can live as long as
207 days (c 7 months). The male is smaller than the female.
[image by H. Robertson ©].
Each egg is
laid in summer in a T-shaped slit that the female makes near the base of a
branch. In Pretoria, Gunn (1919) recorded oviposition from mid-November
through to mid-March. The beetles are not usually nocturnal and egg laying
seems to only occur during the day. Eggs take 10-18 days to develop.
larva initially feeds on the bark and then later enters the
wood and chews its way
through the wood, forming tunnels as it does so. This diet of wood is not
particularly nutritious and so it needs to eat a lot of wood in order to
grow. Larvae are cannabalistic so tend to space themselves out in the
wood, avoiding contact with larvae in other tunnels. The larva packs
excrement ('frass') and castings behind it as it burrows and pushes some
of them through small holes in the bark so that one can often find small
heaps of reddish wood fragments at the base of the tree. Larval
development takes from 2.7-2.8 years.
When fully grown, the larvae tunnel to just beneath the bark where they
moult into a pupa. By pupating under the bark, they can make an easy exit
when they emerge as adults. The pupa is enclosed in a chamber consisting
of calcium carbonate from the Malpighian tubules together with silk and
gum. The tunnel leading from pupation chamber is blocked with frass and
wood chips. The pupal development period lasts 89-99 days.
The adult form develops inside the pupa and
on maturity emerges. The female then needs to mate with a male and find
suitable sites on host plants for laying her eggs.
Brain (1929) recorded the distribution of this species as
"Phryneta spinator is a native insect of
Africa which is commonly distributed over most of the Union [i.e. South Africa] except
the Winter Rainfall area [my italics], and which has been recorded
from Southern Rhodesia [= Zimbabwe], the Belgian Congo [=DRC], British East
Africa [=Kenya] and Nyasaland [=Malawi]."
Gunn (1919) stated that is was distributed
"throughout the Union" which seems to contradict Brain's statement
that it is not found in the winter rainfall area. If Brain was correct in his
statement, then sometime between the 1920's and now it has spread into Cape
Dr Vin Whitehead knew of this
species from the 1950's when he remembers Wellington fruit growers placing
protective barriers round their fig trees. However, it is puzzling that in the
South African Museum collection, the first Phryneta spinator specimen for
the Western Cape dates back to only 1990 for a specimen collected in Sea Point
in the Cape Peninsula. Based on public enquiries at the S.A. Museum, there can
be no doubt that this beetle has become an increasing problem in Cape Town fig
trees through the 1990's and at present it is probably the main insect that
members of the public enquire about. So it seems that either P. spinator
has reached Cape Town only relatively recently or conditions have changed
to favour its expansion in Cape Town.
Surprisingly there have been no natural enemies recorded
for this species other than a parasitic fungus in the genus Isaria. The
habit of the larvae of packing excrement and wood fragments behind it as it
burrows might explain the absence of parasitoids and other natural enemies
because they have difficulty in reaching the larvae.
Feeding by adults. Gunn (1919): "A
considerable amount of damage is ...caused by the beetles gnawing the bark of
willow, fig, apple, apricot, nectarine, peach, pear and plum trees, and also
grap-vines. The beetles are also very fond of the rind of unripe figs, and
occasionally extensive damage to the crop is done." Further on he notes
that "Mr T.R. Sim, Pietermaritzburg, has found the adults feeding upon the
bark of syringa (Melia azedarach), Cupressus lusitanica and Cupressus
horizontalis. Extensive damage, however, was not done to the trees." So
adults have been found to damage the following plants:
Feeding by larvae. Larvae bore into wood of
the following species of trees.
Rosaceae: Occasionally feeds in wood of Pear Pyrus
communis and Peach Prunus sp.
Salicaceae: The native host plant is the Willow Salix
mucronata (= S. capensis) but it also attacks the introduced Weeping willow Salix babylonica.
Moraceae: Domestic fig Ficus carica. It
is unclear whether it attacks native Ficus species or not.
Controlling these beetles is not easy because the larvae
feed deep inside the wood. The following procedure for control can be
effective if done properly:
Destruction of larvae. According to Gunn (1919),
larvae should be cut out with a sharp knife. "The burrows can be easily
located by the presence of course sawdust-like material which has been thrown
out, and in the absence of such material by the broken and sunken condition of
the bark. Only the point of the knife should be inserted in the bark, and cross
cuts should be avoided. The reason why only vertical slits should be made when
digging out the larvae is that a callus readily forms on the sides of such
slits, and in time, if no further injury is caused, new tissue grows over the
wound. After the larva has been located it can be destroyed either by means of a
knife or by inserting a strong flexible piece of wire into the burrow. If the
larva has burrowed deeply into parts below the surface of the soil, it is more
difficult to destroy it. For this purpose, the wire should be thrust deeply into
the burrow until it comes into contact with the larva. The slightest touch of
the wire is sufficient for its destruction." Gunn goes on to recommend
using paraffin oil or carbon bisulphide [highly flammable - not recommended]
pored into the burrow in cases where the burrow extends far into the
There were no aerosol insecticides in Gunn's day. An
alternative to his method of killing the larvae would be to locate the tunnels
(as he advises) and then spray an insecticide with fumigant properties down the
hole. Fit a thin tube to the nozzle to direct the spray. After spraying, close
the entrance of the tunnel with cotton wool or clay to prolong the life of the
fumigant for as long as possible. I have not tried this procedure - it is just a
After the larvae have been destroyed, any damage you made
to the tree should be painted over with a pruning sealant.
Mechanical protectors. The purpose of mechanical
protectors is to prevent the beetles from depositing eggs. It is essential to
first kill the larvae already in the tree. The best type of mechanical protector
is simply a piece of fine-meshed wire netting loosely enclosing the base of the
tree. According to Gunn (1919): "The netting should rest upon the roots and
be tied around the trunk with strong twine at both top and bottom. Paper
fasteners are satisfactory to hold the overlapping ends at the side in close
contact. It is considered necessary that the netting between the tied parts
should not touch the bark, but should stand an inch or two from it so that no
beetle may have a chance to deposit eggs through the meshes. Soil should be
heaped several inches high around the lower part of the protector. ... As the
beetles usually begin to emerge about the first week of November and lay eggs
throughout the summer, it is essential that protectors for fig trees should be
put in place during October and kept in good condition until the end of April.
They may safely be taken off the trees for the winter." Stems need to be
protected to a height of at least 60 cm.
Cultural prevention. A single-stemmed tree is
preferable to one that is multistemmed because barriers can be more easily
applied and it also seems that infestations are higher in multi-stemmed trees.
Avoid letting weeds grow round the base of the tree in summer because this
provides shelter for the female when she is laying eggs and might also prevent
damage from being seen.
Annecke, D.P. & Moran, V.C. 1982. Insects and mites of
cultivated plants in South Africa. Butterworths, Durban.
Gunn, D. 1919. The fig and willow borer (Phryneta
spinator). Union of South Africa, Department of Agriculture, Bulletin No. 6,
1919, pp. 1-22.
Winstanley, J.K. n.d. The fig tree borer, Phryneta
spinator F. (Cerambycidae: Coleoptera). Pests and Diseases of South
African Forests and Timber: Pamphlet 273. [mainly a summary of the
information in Gunn (1919)]