Phryneta spinator (Fig tree borer)

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Ecdysozoa > Panarthropoda > Tritocerebra > Arthopoda > Mandibulata > Atelocerata > Panhexapoda > Hexapoda > Insecta (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!

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.

The hatched 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. 

Pupa. 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. 

Distribution

Brain (1929) recorded the distribution of this species as follows: 

"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 Town. 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. 

Natural enemies

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.

Host plants

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:

• Cupressaceae: Cupressus lusitanica and C. horizontalis (feed on bark).
• Meliaceae: Syringa Melia azedarach (feed on bark).
• Moraceae: fig trees Ficus spp. (feed on bark and eat rind of unripe figs).
• Rosaceae: Pyrus spp. (e.g. apples and pears), Prunus spp. (e.g. apricot, nectarine, peach, plum) (feed on bark).
• Salicaceae: Willow Salix mucronata (= S. capensis). Other Salix species (e.g. Weeping willow Salix babylonica)? (Feed on bark.)
• Vitaceae: Grapevine Vitis vinifera (feed on bark).

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.

Control

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 roots. 

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 suggestion.

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.

References

• 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)]

Text and photos by Hamish Robertson