Insect pests of trees may be divided for convenience into categories, namely leaf-eating sag-sucking, gall-forming and conductive tissue-feeding.
These destroy the leaves either by direct eating, skeletonising or mining between the leaf surfaces Mostly defoliation is slight, but when large numbers of insects are involved a tree may be complete denuded of foliage. Most trees can withstand one or two such defoliations, but if their food resend of starch are used it is possible for some trees to die. There are trees which cannot withstand one severe defoliation.
More importantly, defoliation removes water by transpiration. If transpiration is affected a tree is not moving its minerals quickly and this in turn affects the flow of manufactured food to various parts of the tree. Thus a tree becomes debilitated and susceptible to the gests of the conductive tissue.
Trees are under constant attack during the spring and summer by longicorns, weevils and jewel beetles, but trees which are healthy can withstand these. They do this by producing gums and resins which they move to the point of injury, thus engulfing the young larvae in these materials. Resin can be readily seen in many pines which have been injured. Kino may be seen in Eucalyptus species and tree’s food manufacturing units and also it cannot lose
It is therefore desirable to hale any incipient attack before extensive defoliation occurs. Sapwood treatment makes this possible for defoliations can be predicted in many instances.
A side effect of leaf-eaters, and one that is encountered in many suburbs of Sydney, involves staining of patios, swimming pools and other structures from the faeces of the insects.
These reduce the vigour of trees, causing premature leaf fall, and in some cases transmit disease organisms capable of killing the tree. They produce the same result as the leaf-eaters by destroying the leaves or reducing their efficiency. In this manner they open the way to conductive tissue-feeding insects.
Sometimes scales, aphids and leaf hoppers are attended by ants to obtain a sugary secretion; theexcess of this falls on to the leaves, where a black sooty mould develops.
Often considerable distortion of the leaves and stems occurs. These insects appear to be more abundant on trees which are growing in bad sites, Fertilizer treatments which incorporate minor or trace elements frequently assist trees to reduce these infestations. They are very difficult to control even by using systemic insecticides. Sapwood treatment is effective during the early developmental stages only.
CONDUCTIVE TISSUE-FEEDING INSECTS
These are opportunists which gain entry at points of injury. The building up of soil around the base of trees, gas leaks, effluent from septic tanks, insect attack of leaves, bad site quality, placement building near trees, extended dry periods, often render trees susceptible to attack.
Most conductive tissue-feeding insects are weevils, longicorns and jewel beetles. Their larvae feed
under the bark causing the same effect as ringbarking. These insects usually pupate in the sapwood for protection.
Once attack has been initiated the larvae extend their activities, making large and extensive scars which may be cleaned out and painted with a bitumen emulsion sealer.
Sapwood treatment is not effective against conductive tissue pests unless used at concentrations which would damage the foliage and possibly kill the tree.
Fertilizer treatments are desirable in restoring a tree’s vigour. Mixed fertilizers containing trace elements are good general purpose formulations and many of these are available on the market under various trade names.
It is important to realise that under natural conditions leaf litter returns to the soil most of the mineral material as well as organic matter. In situations where the litter is not returned to the soil, fertilizer treatments are desirable. Mineral deficiencies are more likely surrounded by lawns or other plants and these more successfully compete with tree roots. At least two applications of fertilizer per year are desirable.